[PATCH] RFC: dma-buf: Add an API for importing and exporting sync files

[PATCH] RFC: dma-buf: Add an API for importing and exporting sync files

[Jason Ekstrand]

Explicit synchronization is the future.  At least, that seems to be what
most userspace APIs are agreeing on at this point.  However, most of our
Linux APIs (both userspace and kernel UAPI) are currently built around
implicit synchronization with dma-buf.  While work is ongoing to change
many of the userspace APIs and protocols to an explicit synchronization
model, switching over piecemeal is difficult due to the number of
potential components involved.  On the kernel side, many drivers use
dma-buf including GPU (3D/compute), display, v4l, and others.  In
userspace, we have X11, several Wayland compositors, 3D drivers, compute
drivers (OpenCL etc.), media encode/decode, and the list goes on.

This patch provides a path forward by allowing userspace to manually
manage the fences attached to a dma-buf.  Alternatively, one can think
of this as making dma-buf's implicit synchronization simply a carrier
for an explicit fence.  This is accomplished by adding two IOCTLs to
dma-buf for importing and exporting a sync file to/from the dma-buf.
This way a userspace component which is uses explicit synchronization,
such as a Vulkan driver, can manually set the write fence on a buffer
before handing it off to an implicitly synchronized component such as a
Wayland compositor or video encoder.  In this way, each of the different
components can be upgraded to an explicit synchronization model one at a
time as long as the userspace pieces connecting them are aware of it and
import/export fences at the right times.

There is a potential race condition with this API if userspace is not
careful.  A typical use case for implicit synchronization is to wait for
the dma-buf to be ready, use it, and then signal it for some other
component.  Because a sync_file cannot be created until it is guaranteed
to complete in finite time, userspace can only signal the dma-buf after
it has already submitted the work which uses it to the kernel and has
received a sync_file back.  There is no way to atomically submit a
wait-use-signal operation.  This is not, however, really a problem with
this API so much as it is a problem with explicit synchronization
itself.  The way this is typically handled is to have very explicit
ownership transfer points in the API or protocol which ensure that only
one component is using it at any given time.  Both X11 (via the PRESENT
extension) and Wayland provide such ownership transfer points via
explicit present and idle messages.

The decision was intentionally made in this patch to make the import and
export operations IOCTLs on the dma-buf itself rather than as a DRM
IOCTL.  This makes it the import/export operation universal across all
components which use dma-buf including GPU, display, v4l, and others.
It also means that a userspace component can do the import/export
without access to the DRM fd which may be tricky to get in cases where
the client communicates with DRM via a userspace API such as OpenGL or
Vulkan.  At a future date we may choose to add direct import/export APIs
to components such as drm_syncobj to avoid allocating a file descriptor
and going through two ioctls.  However, that seems to be something of a
micro-optimization as import/export operations are likely to happen at a
rate of a few per frame of rendered or decoded video.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
---

This is marked as an RFC because I intend it to start a discussion about
how to solve a problem.  The current patch compiles but that's it for now.
I'll be writing IGT tests and Vulkan driver patches which exercise it over
the next couple of days.  In the mean time, feel free to tell me why you
think this is a great and/or terrible idea. :-)

--Jason


 drivers/dma-buf/dma-buf.c    | 115 +++++++++++++++++++++++++++++++++++
 include/uapi/linux/dma-buf.h |  13 +++-
 2 files changed, 126 insertions(+), 2 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index d4097856c86b..3845b87e209e 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -20,6 +20,7 @@
 #include <linux/debugfs.h>
 #include <linux/module.h>
 #include <linux/seq_file.h>
+#include <linux/sync_file.h>
 #include <linux/poll.h>
 #include <linux/dma-resv.h>
 #include <linux/mm.h>
@@ -348,6 +349,114 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
 	return ret;
 }
 
+static long dma_buf_wait_sync_file(struct dma_buf *dmabuf,
+				   const void __user *user_data)
+{
+	struct dma_buf_sync_file arg;
+	struct dma_fence *fence;
+
+	if (copy_from_user(&arg, user_data, sizeof(arg)))
+		return -EFAULT;
+
+	if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
+		return -EINVAL;
+
+	fence = sync_file_get_fence(arg.fd);
+	if (!fence)
+		return -EINVAL;
+
+	if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
+		dma_resv_add_excl_fence(dmabuf->resv, fence);
+	} else {
+		dma_resv_add_shared_fence(dmabuf->resv, fence);
+	}
+
+	return 0;
+}
+
+static long dma_buf_signal_sync_file(struct dma_buf *dmabuf,
+				     void __user *user_data)
+{
+	struct dma_buf_sync_file arg;
+	struct dma_fence *fence = NULL;
+	struct sync_file *sync_file;
+	int fd, ret;
+
+	if (copy_from_user(&arg, user_data, sizeof(arg)))
+		return -EFAULT;
+
+	if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
+		return -EINVAL;
+
+	fd = get_unused_fd_flags(O_CLOEXEC);
+	if (fd < 0)
+		return fd;
+
+	if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
+		/* We need to include both the exclusive fence and all of
+		 * the shared fences in our fence.
+		 */
+		struct dma_fence **fences = NULL;
+		unsigned i, num_fences = 0;
+
+		ret = dma_resv_get_fences_rcu(dmabuf->resv, NULL,
+					      &num_fences, &fences);
+		if (ret)
+			goto err_put_fd;
+
+		if (num_fences == 0) {
+			fence = dma_fence_get_stub();
+		} else if (num_fences == 1) {
+			fence = fences[0];
+			kfree(fences);
+		} else {
+			struct dma_fence_array *fence_arr;
+
+			fence_arr = dma_fence_array_create(num_fences, fences,
+							   dma_fence_context_alloc(1),
+							   1, false);
+			if (!fence_arr) {
+				for (i = 0; i < num_fences; i++)
+					dma_fence_put(fences[i]);
+				kfree(fences);
+				ret = -ENOMEM;
+				goto err_put_fd;
+			}
+
+			/* The fence array now owns fences_arr and our
+			 * references to each of the individual fences.  We
+			 * only own a reference to the one array fence.
+			 */
+			fence = &fence_arr->base;
+		}
+	} else {
+		fence = dma_resv_get_excl_rcu(dmabuf->resv);
+		if (!fence)
+			fence = dma_fence_get_stub();
+	}
+
+	sync_file = sync_file_create(fence);
+
+	dma_fence_put(fence);
+
+	if (!sync_file) {
+		ret = -EINVAL;
+		goto err_put_fd;
+	}
+
+	fd_install(fd, sync_file->file);
+
+	arg.fd = fd;
+	if (copy_to_user(user_data, &arg, sizeof(arg)))
+		return -EFAULT;
+
+	return 0;
+
+err_put_fd:
+	put_unused_fd(fd);
+	return ret;
+}
+
 static long dma_buf_ioctl(struct file *file,
 			  unsigned int cmd, unsigned long arg)
 {
@@ -390,6 +499,12 @@ static long dma_buf_ioctl(struct file *file,
 	case DMA_BUF_SET_NAME:
 		return dma_buf_set_name(dmabuf, (const char __user *)arg);
 
+	case DMA_BUF_IOCTL_WAIT_SYNC_FILE:
+		return dma_buf_wait_sync_file(dmabuf, (const void __user *)arg);
+
+	case DMA_BUF_IOCTL_SIGNAL_SYNC_FILE:
+		return dma_buf_signal_sync_file(dmabuf, (void __user *)arg);
+
 	default:
 		return -ENOTTY;
 	}
diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
index dbc7092e04b5..825b9a913c89 100644
--- a/include/uapi/linux/dma-buf.h
+++ b/include/uapi/linux/dma-buf.h
@@ -37,8 +37,17 @@ struct dma_buf_sync {
 
 #define DMA_BUF_NAME_LEN	32
 
+struct dma_buf_sync_file {
+	__u32 flags;
+	__s32 fd;
+};
+
+#define DMA_BUF_SYNC_FILE_SYNC_WRITE	(1 << 0)
+
 #define DMA_BUF_BASE		'b'
-#define DMA_BUF_IOCTL_SYNC	_IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
-#define DMA_BUF_SET_NAME	_IOW(DMA_BUF_BASE, 1, const char *)
+#define DMA_BUF_IOCTL_SYNC	    _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
+#define DMA_BUF_SET_NAME	    _IOW(DMA_BUF_BASE, 1, const char *)
+#define DMA_BUF_IOCTL_WAIT_SYNC_FILE	_IOW(DMA_BUF_BASE, 2, struct dma_buf_sync)
+#define DMA_BUF_IOCTL_SIGNAL_SYNC_FILE	_IOW(DMA_BUF_BASE, 3, struct dma_buf_sync)
 
 #endif
-- 
2.24.1

Re: [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files

[Christian König]

Hi Jason,

Am 26.02.20 um 00:58 schrieb Jason Ekstrand:
> Explicit synchronization is the future.  At least, that seems to be what
> most userspace APIs are agreeing on at this point.  However, most of our
> Linux APIs (both userspace and kernel UAPI) are currently built around
> implicit synchronization with dma-buf.  While work is ongoing to change
> many of the userspace APIs and protocols to an explicit synchronization
> model, switching over piecemeal is difficult due to the number of
> potential components involved.  On the kernel side, many drivers use
> dma-buf including GPU (3D/compute), display, v4l, and others.  In
> userspace, we have X11, several Wayland compositors, 3D drivers, compute
> drivers (OpenCL etc.), media encode/decode, and the list goes on.
>
> This patch provides a path forward by allowing userspace to manually
> manage the fences attached to a dma-buf.  Alternatively, one can think
> of this as making dma-buf's implicit synchronization simply a carrier
> for an explicit fence.  This is accomplished by adding two IOCTLs to
> dma-buf for importing and exporting a sync file to/from the dma-buf.
> This way a userspace component which is uses explicit synchronization,
> such as a Vulkan driver, can manually set the write fence on a buffer
> before handing it off to an implicitly synchronized component such as a
> Wayland compositor or video encoder.  In this way, each of the different
> components can be upgraded to an explicit synchronization model one at a
> time as long as the userspace pieces connecting them are aware of it and
> import/export fences at the right times.
>
> There is a potential race condition with this API if userspace is not
> careful.  A typical use case for implicit synchronization is to wait for
> the dma-buf to be ready, use it, and then signal it for some other
> component.  Because a sync_file cannot be created until it is guaranteed
> to complete in finite time, userspace can only signal the dma-buf after
> it has already submitted the work which uses it to the kernel and has
> received a sync_file back.  There is no way to atomically submit a
> wait-use-signal operation.  This is not, however, really a problem with
> this API so much as it is a problem with explicit synchronization
> itself.  The way this is typically handled is to have very explicit
> ownership transfer points in the API or protocol which ensure that only
> one component is using it at any given time.  Both X11 (via the PRESENT
> extension) and Wayland provide such ownership transfer points via
> explicit present and idle messages.
>
> The decision was intentionally made in this patch to make the import and
> export operations IOCTLs on the dma-buf itself rather than as a DRM
> IOCTL.  This makes it the import/export operation universal across all
> components which use dma-buf including GPU, display, v4l, and others.
> It also means that a userspace component can do the import/export
> without access to the DRM fd which may be tricky to get in cases where
> the client communicates with DRM via a userspace API such as OpenGL or
> Vulkan.  At a future date we may choose to add direct import/export APIs
> to components such as drm_syncobj to avoid allocating a file descriptor
> and going through two ioctls.  However, that seems to be something of a
> micro-optimization as import/export operations are likely to happen at a
> rate of a few per frame of rendered or decoded video.
>
> Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> ---
>
> This is marked as an RFC because I intend it to start a discussion about
> how to solve a problem.  The current patch compiles but that's it for now.
> I'll be writing IGT tests and Vulkan driver patches which exercise it over
> the next couple of days.  In the mean time, feel free to tell me why you
> think this is a great and/or terrible idea. :-)

For the exporting part I think it is an absolutely great idea because it 
simplifies compatibility with explicit sync quite a bit.

But for the importing part it is a clear NAK at the moment. See we can't 
allow userspace to mess with DMA-buf fences in that way because it rips 
open a security hole you can push an elephant through.

Just imagine that you access some DMA-buf with a shader and that 
operation is presented as a fence on the DMA-bufs reservation object. 
And now you can go ahead and replace that fence and free up the memory.

Tricking the Linux kernel into allocating page tables in that freed 
memory is trivial and that's basically it you can overwrite page tables 
with your shader and gain access to all of system memory :)

What we could do is to always make sure that the added fences will 
complete later than the already existing ones, but that is also rather 
tricky to get right. I wouldn't do that if we don't have a rather big 
use case for this.

Regards,
Christian.

>
> --Jason
[SNIP]

Re: [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files

[Daniel Vetter]

On Wed, Feb 26, 2020 at 10:16:05AM +0100, Christian König wrote:
> Hi Jason,
> 
> Am 26.02.20 um 00:58 schrieb Jason Ekstrand:
> > Explicit synchronization is the future.  At least, that seems to be what
> > most userspace APIs are agreeing on at this point.  However, most of our
> > Linux APIs (both userspace and kernel UAPI) are currently built around
> > implicit synchronization with dma-buf.  While work is ongoing to change
> > many of the userspace APIs and protocols to an explicit synchronization
> > model, switching over piecemeal is difficult due to the number of
> > potential components involved.  On the kernel side, many drivers use
> > dma-buf including GPU (3D/compute), display, v4l, and others.  In
> > userspace, we have X11, several Wayland compositors, 3D drivers, compute
> > drivers (OpenCL etc.), media encode/decode, and the list goes on.
> > 
> > This patch provides a path forward by allowing userspace to manually
> > manage the fences attached to a dma-buf.  Alternatively, one can think
> > of this as making dma-buf's implicit synchronization simply a carrier
> > for an explicit fence.  This is accomplished by adding two IOCTLs to
> > dma-buf for importing and exporting a sync file to/from the dma-buf.
> > This way a userspace component which is uses explicit synchronization,
> > such as a Vulkan driver, can manually set the write fence on a buffer
> > before handing it off to an implicitly synchronized component such as a
> > Wayland compositor or video encoder.  In this way, each of the different
> > components can be upgraded to an explicit synchronization model one at a
> > time as long as the userspace pieces connecting them are aware of it and
> > import/export fences at the right times.
> > 
> > There is a potential race condition with this API if userspace is not
> > careful.  A typical use case for implicit synchronization is to wait for
> > the dma-buf to be ready, use it, and then signal it for some other
> > component.  Because a sync_file cannot be created until it is guaranteed
> > to complete in finite time, userspace can only signal the dma-buf after
> > it has already submitted the work which uses it to the kernel and has
> > received a sync_file back.  There is no way to atomically submit a
> > wait-use-signal operation.  This is not, however, really a problem with
> > this API so much as it is a problem with explicit synchronization
> > itself.  The way this is typically handled is to have very explicit
> > ownership transfer points in the API or protocol which ensure that only
> > one component is using it at any given time.  Both X11 (via the PRESENT
> > extension) and Wayland provide such ownership transfer points via
> > explicit present and idle messages.
> > 
> > The decision was intentionally made in this patch to make the import and
> > export operations IOCTLs on the dma-buf itself rather than as a DRM
> > IOCTL.  This makes it the import/export operation universal across all
> > components which use dma-buf including GPU, display, v4l, and others.
> > It also means that a userspace component can do the import/export
> > without access to the DRM fd which may be tricky to get in cases where
> > the client communicates with DRM via a userspace API such as OpenGL or
> > Vulkan.  At a future date we may choose to add direct import/export APIs
> > to components such as drm_syncobj to avoid allocating a file descriptor
> > and going through two ioctls.  However, that seems to be something of a
> > micro-optimization as import/export operations are likely to happen at a
> > rate of a few per frame of rendered or decoded video.
> > 
> > Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> > ---
> > 
> > This is marked as an RFC because I intend it to start a discussion about
> > how to solve a problem.  The current patch compiles but that's it for now.
> > I'll be writing IGT tests and Vulkan driver patches which exercise it over
> > the next couple of days.  In the mean time, feel free to tell me why you
> > think this is a great and/or terrible idea. :-)
> 
> For the exporting part I think it is an absolutely great idea because it
> simplifies compatibility with explicit sync quite a bit.
> 
> But for the importing part it is a clear NAK at the moment. See we can't
> allow userspace to mess with DMA-buf fences in that way because it rips open
> a security hole you can push an elephant through.
> 
> Just imagine that you access some DMA-buf with a shader and that operation
> is presented as a fence on the DMA-bufs reservation object. And now you can
> go ahead and replace that fence and free up the memory.
> 
> Tricking the Linux kernel into allocating page tables in that freed memory
> is trivial and that's basically it you can overwrite page tables with your
> shader and gain access to all of system memory :)
> 
> What we could do is to always make sure that the added fences will complete
> later than the already existing ones, but that is also rather tricky to get
> right. I wouldn't do that if we don't have a rather big use case for this.

I think the main use-case for adding a fence is adding a write fence for
vk winsys buffers, which run without any sync at all. So essentially what
we'd do is promote one of the read fences which are already attached to be
the write fence.

But yeah making sure we don't break any of the dma_resv guarantees about
how these fences works is going to be somewhat tricky. Probably can reuse
a big chunk of the fence container work we've done for syncobj timelines,
since they have some of the same issues of having to chain fences to not
break the world.
-Daniel

> 
> Regards,
> Christian.
> 
> > 
> > --Jason
> [SNIP]

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files

[Jason Ekstrand]

On Wed, Feb 26, 2020 at 4:05 AM Daniel Vetter <daniel@ffwll.ch> wrote:
>
> On Wed, Feb 26, 2020 at 10:16:05AM +0100, Christian König wrote:
> > Hi Jason,
> >
> > Am 26.02.20 um 00:58 schrieb Jason Ekstrand:
> > > Explicit synchronization is the future.  At least, that seems to be what
> > > most userspace APIs are agreeing on at this point.  However, most of our
> > > Linux APIs (both userspace and kernel UAPI) are currently built around
> > > implicit synchronization with dma-buf.  While work is ongoing to change
> > > many of the userspace APIs and protocols to an explicit synchronization
> > > model, switching over piecemeal is difficult due to the number of
> > > potential components involved.  On the kernel side, many drivers use
> > > dma-buf including GPU (3D/compute), display, v4l, and others.  In
> > > userspace, we have X11, several Wayland compositors, 3D drivers, compute
> > > drivers (OpenCL etc.), media encode/decode, and the list goes on.
> > >
> > > This patch provides a path forward by allowing userspace to manually
> > > manage the fences attached to a dma-buf.  Alternatively, one can think
> > > of this as making dma-buf's implicit synchronization simply a carrier
> > > for an explicit fence.  This is accomplished by adding two IOCTLs to
> > > dma-buf for importing and exporting a sync file to/from the dma-buf.
> > > This way a userspace component which is uses explicit synchronization,
> > > such as a Vulkan driver, can manually set the write fence on a buffer
> > > before handing it off to an implicitly synchronized component such as a
> > > Wayland compositor or video encoder.  In this way, each of the different
> > > components can be upgraded to an explicit synchronization model one at a
> > > time as long as the userspace pieces connecting them are aware of it and
> > > import/export fences at the right times.
> > >
> > > There is a potential race condition with this API if userspace is not
> > > careful.  A typical use case for implicit synchronization is to wait for
> > > the dma-buf to be ready, use it, and then signal it for some other
> > > component.  Because a sync_file cannot be created until it is guaranteed
> > > to complete in finite time, userspace can only signal the dma-buf after
> > > it has already submitted the work which uses it to the kernel and has
> > > received a sync_file back.  There is no way to atomically submit a
> > > wait-use-signal operation.  This is not, however, really a problem with
> > > this API so much as it is a problem with explicit synchronization
> > > itself.  The way this is typically handled is to have very explicit
> > > ownership transfer points in the API or protocol which ensure that only
> > > one component is using it at any given time.  Both X11 (via the PRESENT
> > > extension) and Wayland provide such ownership transfer points via
> > > explicit present and idle messages.
> > >
> > > The decision was intentionally made in this patch to make the import and
> > > export operations IOCTLs on the dma-buf itself rather than as a DRM
> > > IOCTL.  This makes it the import/export operation universal across all
> > > components which use dma-buf including GPU, display, v4l, and others.
> > > It also means that a userspace component can do the import/export
> > > without access to the DRM fd which may be tricky to get in cases where
> > > the client communicates with DRM via a userspace API such as OpenGL or
> > > Vulkan.  At a future date we may choose to add direct import/export APIs
> > > to components such as drm_syncobj to avoid allocating a file descriptor
> > > and going through two ioctls.  However, that seems to be something of a
> > > micro-optimization as import/export operations are likely to happen at a
> > > rate of a few per frame of rendered or decoded video.
> > >
> > > Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> > > ---
> > >
> > > This is marked as an RFC because I intend it to start a discussion about
> > > how to solve a problem.  The current patch compiles but that's it for now.
> > > I'll be writing IGT tests and Vulkan driver patches which exercise it over
> > > the next couple of days.  In the mean time, feel free to tell me why you
> > > think this is a great and/or terrible idea. :-)
> >
> > For the exporting part I think it is an absolutely great idea because it
> > simplifies compatibility with explicit sync quite a bit.

Unfortunately, it only helps half of explicit sync and not the half
that's hard to deal with from Vulkan. :-/

> > But for the importing part it is a clear NAK at the moment. See we can't
> > allow userspace to mess with DMA-buf fences in that way because it rips open
> > a security hole you can push an elephant through.

Oh, sure, I'm 100% sure I did that part wrong.  Why else would I send
the patch but to have someone who actually knows what they're doing
tell me how to do it correctly? :-P

> > Just imagine that you access some DMA-buf with a shader and that operation
> > is presented as a fence on the DMA-bufs reservation object. And now you can
> > go ahead and replace that fence and free up the memory.
> >
> > Tricking the Linux kernel into allocating page tables in that freed memory
> > is trivial and that's basically it you can overwrite page tables with your
> > shader and gain access to all of system memory :)
> >
> > What we could do is to always make sure that the added fences will complete
> > later than the already existing ones, but that is also rather tricky to get
> > right. I wouldn't do that if we don't have a rather big use case for this.

Right.  I thought about that but I'm still learning how dma_resv
works.  It'd be easy enough to make a fence array that contains both
the old fence and the new fence and replace the old fence with that.
What I don't know is the proper way to replace the exclusive fence
safely.  Some sort of atomic_cpxchg loop, perhaps?  I presume there's
some way of doing it properly because DRM drivers are doing it all the
time.

> I think the main use-case for adding a fence is adding a write fence for
> vk winsys buffers, which run without any sync at all. So essentially what
> we'd do is promote one of the read fences which are already attached to be
> the write fence.

Correct.  We're effectively doing an import in ANV today but we're
doing it with a dummy execbuf which claims to write the BO and has a
batch that's just MI_BATCH_BUFFER_END.

> But yeah making sure we don't break any of the dma_resv guarantees about
> how these fences works is going to be somewhat tricky. Probably can reuse
> a big chunk of the fence container work we've done for syncobj timelines,
> since they have some of the same issues of having to chain fences to not
> break the world.

Happy to not break the world.  I just don't know how yet. :-)

--Jason

Re: [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files

[Bas Nieuwenhuizen]

On Wed, Feb 26, 2020 at 4:29 PM Jason Ekstrand <jason@jlekstrand.net> wrote:
>
> On Wed, Feb 26, 2020 at 4:05 AM Daniel Vetter <daniel@ffwll.ch> wrote:
> >
> > On Wed, Feb 26, 2020 at 10:16:05AM +0100, Christian König wrote:
> > > Hi Jason,
> > >
> > > Am 26.02.20 um 00:58 schrieb Jason Ekstrand:
> > > > Explicit synchronization is the future.  At least, that seems to be what
> > > > most userspace APIs are agreeing on at this point.  However, most of our
> > > > Linux APIs (both userspace and kernel UAPI) are currently built around
> > > > implicit synchronization with dma-buf.  While work is ongoing to change
> > > > many of the userspace APIs and protocols to an explicit synchronization
> > > > model, switching over piecemeal is difficult due to the number of
> > > > potential components involved.  On the kernel side, many drivers use
> > > > dma-buf including GPU (3D/compute), display, v4l, and others.  In
> > > > userspace, we have X11, several Wayland compositors, 3D drivers, compute
> > > > drivers (OpenCL etc.), media encode/decode, and the list goes on.
> > > >
> > > > This patch provides a path forward by allowing userspace to manually
> > > > manage the fences attached to a dma-buf.  Alternatively, one can think
> > > > of this as making dma-buf's implicit synchronization simply a carrier
> > > > for an explicit fence.  This is accomplished by adding two IOCTLs to
> > > > dma-buf for importing and exporting a sync file to/from the dma-buf.
> > > > This way a userspace component which is uses explicit synchronization,
> > > > such as a Vulkan driver, can manually set the write fence on a buffer
> > > > before handing it off to an implicitly synchronized component such as a
> > > > Wayland compositor or video encoder.  In this way, each of the different
> > > > components can be upgraded to an explicit synchronization model one at a
> > > > time as long as the userspace pieces connecting them are aware of it and
> > > > import/export fences at the right times.
> > > >
> > > > There is a potential race condition with this API if userspace is not
> > > > careful.  A typical use case for implicit synchronization is to wait for
> > > > the dma-buf to be ready, use it, and then signal it for some other
> > > > component.  Because a sync_file cannot be created until it is guaranteed
> > > > to complete in finite time, userspace can only signal the dma-buf after
> > > > it has already submitted the work which uses it to the kernel and has
> > > > received a sync_file back.  There is no way to atomically submit a
> > > > wait-use-signal operation.  This is not, however, really a problem with
> > > > this API so much as it is a problem with explicit synchronization
> > > > itself.  The way this is typically handled is to have very explicit
> > > > ownership transfer points in the API or protocol which ensure that only
> > > > one component is using it at any given time.  Both X11 (via the PRESENT
> > > > extension) and Wayland provide such ownership transfer points via
> > > > explicit present and idle messages.
> > > >
> > > > The decision was intentionally made in this patch to make the import and
> > > > export operations IOCTLs on the dma-buf itself rather than as a DRM
> > > > IOCTL.  This makes it the import/export operation universal across all
> > > > components which use dma-buf including GPU, display, v4l, and others.
> > > > It also means that a userspace component can do the import/export
> > > > without access to the DRM fd which may be tricky to get in cases where
> > > > the client communicates with DRM via a userspace API such as OpenGL or
> > > > Vulkan.  At a future date we may choose to add direct import/export APIs
> > > > to components such as drm_syncobj to avoid allocating a file descriptor
> > > > and going through two ioctls.  However, that seems to be something of a
> > > > micro-optimization as import/export operations are likely to happen at a
> > > > rate of a few per frame of rendered or decoded video.
> > > >
> > > > Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> > > > ---
> > > >
> > > > This is marked as an RFC because I intend it to start a discussion about
> > > > how to solve a problem.  The current patch compiles but that's it for now.
> > > > I'll be writing IGT tests and Vulkan driver patches which exercise it over
> > > > the next couple of days.  In the mean time, feel free to tell me why you
> > > > think this is a great and/or terrible idea. :-)
> > >
> > > For the exporting part I think it is an absolutely great idea because it
> > > simplifies compatibility with explicit sync quite a bit.
>
> Unfortunately, it only helps half of explicit sync and not the half
> that's hard to deal with from Vulkan. :-/
>
> > > But for the importing part it is a clear NAK at the moment. See we can't
> > > allow userspace to mess with DMA-buf fences in that way because it rips open
> > > a security hole you can push an elephant through.
>
> Oh, sure, I'm 100% sure I did that part wrong.  Why else would I send
> the patch but to have someone who actually knows what they're doing
> tell me how to do it correctly? :-P
>
> > > Just imagine that you access some DMA-buf with a shader and that operation
> > > is presented as a fence on the DMA-bufs reservation object. And now you can
> > > go ahead and replace that fence and free up the memory.
> > >
> > > Tricking the Linux kernel into allocating page tables in that freed memory
> > > is trivial and that's basically it you can overwrite page tables with your
> > > shader and gain access to all of system memory :)
> > >
> > > What we could do is to always make sure that the added fences will complete
> > > later than the already existing ones, but that is also rather tricky to get
> > > right. I wouldn't do that if we don't have a rather big use case for this.
>
> Right.  I thought about that but I'm still learning how dma_resv
> works.  It'd be easy enough to make a fence array that contains both
> the old fence and the new fence and replace the old fence with that.
> What I don't know is the proper way to replace the exclusive fence
> safely.  Some sort of atomic_cpxchg loop, perhaps?  I presume there's
> some way of doing it properly because DRM drivers are doing it all the
> time.

I think for an exclusive fence you may need to create a fence array
that includes the existing exclusive and shared fences in the dma_resv
combined with the added fence.

However, I'm not sure what the best way is to do garbage collection on
that so that we don't get an impossibly list of fence arrays. (Note
the dma_resv has a lock that needs to be taken before adding an
exclusive fence, might be useful). Some code that does a thing like
this is __dma_resv_make_exclusive in
drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c

The other piece of the puzzle is that on the submit path this would
need something to ignore implicit fences. And there semantically the
question comes up whether it is safe for a driver to ignore exclusive
fences from another driver. (and then we have amdgpu which has its own
rules on exclusiveness of its shared fences based on the context. e.g.
the current option to ignore implicit fences for a buffer still syncs
on exclusive fences on the buffer).









>
> > I think the main use-case for adding a fence is adding a write fence for
> > vk winsys buffers, which run without any sync at all. So essentially what
> > we'd do is promote one of the read fences which are already attached to be
> > the write fence.
>
> Correct.  We're effectively doing an import in ANV today but we're
> doing it with a dummy execbuf which claims to write the BO and has a
> batch that's just MI_BATCH_BUFFER_END.
>
> > But yeah making sure we don't break any of the dma_resv guarantees about
> > how these fences works is going to be somewhat tricky. Probably can reuse
> > a big chunk of the fence container work we've done for syncobj timelines,
> > since they have some of the same issues of having to chain fences to not
> > break the world.
>
> Happy to not break the world.  I just don't know how yet. :-)
>
> --Jason

[PATCH] RFC: dma-buf: Add an API for importing and exporting sync files (v2)

[Jason Ekstrand]

Explicit synchronization is the future.  At least, that seems to be what
most userspace APIs are agreeing on at this point.  However, most of our
Linux APIs (both userspace and kernel UAPI) are currently built around
implicit synchronization with dma-buf.  While work is ongoing to change
many of the userspace APIs and protocols to an explicit synchronization
model, switching over piecemeal is difficult due to the number of
potential components involved.  On the kernel side, many drivers use
dma-buf including GPU (3D/compute), display, v4l, and others.  In
userspace, we have X11, several Wayland compositors, 3D drivers, compute
drivers (OpenCL etc.), media encode/decode, and the list goes on.

This patch provides a path forward by allowing userspace to manually
manage the fences attached to a dma-buf.  Alternatively, one can think
of this as making dma-buf's implicit synchronization simply a carrier
for an explicit fence.  This is accomplished by adding two IOCTLs to
dma-buf for importing and exporting a sync file to/from the dma-buf.
This way a userspace component which is uses explicit synchronization,
such as a Vulkan driver, can manually set the write fence on a buffer
before handing it off to an implicitly synchronized component such as a
Wayland compositor or video encoder.  In this way, each of the different
components can be upgraded to an explicit synchronization model one at a
time as long as the userspace pieces connecting them are aware of it and
import/export fences at the right times.

There is a potential race condition with this API if userspace is not
careful.  A typical use case for implicit synchronization is to wait for
the dma-buf to be ready, use it, and then signal it for some other
component.  Because a sync_file cannot be created until it is guaranteed
to complete in finite time, userspace can only signal the dma-buf after
it has already submitted the work which uses it to the kernel and has
received a sync_file back.  There is no way to atomically submit a
wait-use-signal operation.  This is not, however, really a problem with
this API so much as it is a problem with explicit synchronization
itself.  The way this is typically handled is to have very explicit
ownership transfer points in the API or protocol which ensure that only
one component is using it at any given time.  Both X11 (via the PRESENT
extension) and Wayland provide such ownership transfer points via
explicit present and idle messages.

The decision was intentionally made in this patch to make the import and
export operations IOCTLs on the dma-buf itself rather than as a DRM
IOCTL.  This makes it the import/export operation universal across all
components which use dma-buf including GPU, display, v4l, and others.
It also means that a userspace component can do the import/export
without access to the DRM fd which may be tricky to get in cases where
the client communicates with DRM via a userspace API such as OpenGL or
Vulkan.  At a future date we may choose to add direct import/export APIs
to components such as drm_syncobj to avoid allocating a file descriptor
and going through two ioctls.  However, that seems to be something of a
micro-optimization as import/export operations are likely to happen at a
rate of a few per frame of rendered or decoded video.

v2 (Jason Ekstrand):
 - Use a wrapper dma_fence_array of all fences including the new one
   when importing an exclusive fence.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
---
 drivers/dma-buf/dma-buf.c    | 162 +++++++++++++++++++++++++++++++++++
 include/uapi/linux/dma-buf.h |  13 ++-
 2 files changed, 173 insertions(+), 2 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index d4097856c86b..162f90e8896b 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -20,6 +20,7 @@
 #include <linux/debugfs.h>
 #include <linux/module.h>
 #include <linux/seq_file.h>
+#include <linux/sync_file.h>
 #include <linux/poll.h>
 #include <linux/dma-resv.h>
 #include <linux/mm.h>
@@ -348,6 +349,161 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
 	return ret;
 }
 
+/* This function takes a ref to add_fence on success.  The caller still
+ * owns its ref and has to dma_fence_put it.
+ */
+static struct dma_fence *dma_buf_get_unified_fence(struct dma_buf *dmabuf,
+						   struct dma_fence *add_fence)
+{
+	struct dma_fence **fences = NULL;
+	struct dma_fence_array *array;
+	unsigned i, num_fences = 0;
+	int ret;
+
+	ret = dma_resv_get_fences_rcu(dmabuf->resv, NULL,
+				      &num_fences, &fences);
+	if (ret)
+		return NULL; /* ret can only be 0 or -ENOMEM */
+
+	if (num_fences == 0) {
+		if (add_fence) {
+			return add_fence;
+		} else {
+			return dma_fence_get_stub();
+		}
+	} else if (num_fences == 1 && (!add_fence || add_fence == fences[0])) {
+		struct dma_fence *fence = fences[0];
+		kfree(fences);
+		return fence;
+	}
+
+	if (add_fence) {
+		struct dma_fence **nfences;
+		size_t sz;
+
+		/* Get a ref to add_fence so that we have a ref to every
+		 * fence we are going to put in the array.
+		 */
+		dma_fence_get(add_fence);
+
+		sz = (num_fences + 1) * sizeof(*fences);
+		nfences = krealloc(fences, sz, GFP_NOWAIT | __GFP_NOWARN);
+		if (!nfences)
+			goto err_put_fences;
+
+		nfences[num_fences++] = add_fence;
+	}
+
+	array = dma_fence_array_create(num_fences, fences,
+				       dma_fence_context_alloc(1),
+				       1, false);
+	if (!array)
+		goto err_put_fences;
+
+	/* The fence array now owns fences_arr and our references to each
+	 * of the individual fences.  We only own a reference to the one
+	 * array fence.
+	 */
+
+	return &array->base;
+
+err_put_fences:
+	for (i = 0; i < num_fences; i++)
+		dma_fence_put(fences[0]);
+	dma_fence_put(add_fence);
+	kfree(fences);
+	return NULL;
+}
+
+static long dma_buf_wait_sync_file(struct dma_buf *dmabuf,
+				   const void __user *user_data)
+{
+	struct dma_buf_sync_file arg;
+	struct dma_fence *fence, *unified_fence;
+	int ret;
+
+	if (copy_from_user(&arg, user_data, sizeof(arg)))
+		return -EFAULT;
+
+	if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
+		return -EINVAL;
+
+	fence = sync_file_get_fence(arg.fd);
+	if (!fence)
+		return -EINVAL;
+
+	if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
+		dma_resv_lock(dmabuf->resv, NULL);
+		unified_fence = dma_buf_get_unified_fence(dmabuf, fence);
+		if (unified_fence)
+			dma_resv_add_excl_fence(dmabuf->resv, fence);
+		else
+			ret = -ENOMEM;
+		dma_resv_unlock(dmabuf->resv);
+	} else {
+		dma_resv_add_shared_fence(dmabuf->resv, fence);
+	}
+
+	dma_fence_put(fence);
+
+	return ret;
+}
+
+static long dma_buf_signal_sync_file(struct dma_buf *dmabuf,
+				     void __user *user_data)
+{
+	struct dma_buf_sync_file arg;
+	struct dma_fence *fence = NULL;
+	struct sync_file *sync_file;
+	int fd, ret;
+
+	if (copy_from_user(&arg, user_data, sizeof(arg)))
+		return -EFAULT;
+
+	if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
+		return -EINVAL;
+
+	fd = get_unused_fd_flags(O_CLOEXEC);
+	if (fd < 0)
+		return fd;
+
+	if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
+		/* We need to include both the exclusive fence and all of
+		 * the shared fences in our fence.
+		 */
+		fence = dma_buf_get_unified_fence(dmabuf, NULL);
+		if (!fence) {
+			ret = -ENOMEM;
+			goto err_put_fd;
+		}
+	} else {
+		fence = dma_resv_get_excl_rcu(dmabuf->resv);
+		if (!fence)
+			fence = dma_fence_get_stub();
+	}
+
+	sync_file = sync_file_create(fence);
+
+	dma_fence_put(fence);
+
+	if (!sync_file) {
+		ret = -EINVAL;
+		goto err_put_fd;
+	}
+
+	fd_install(fd, sync_file->file);
+
+	arg.fd = fd;
+	if (copy_to_user(user_data, &arg, sizeof(arg)))
+		return -EFAULT;
+
+	return 0;
+
+err_put_fd:
+	put_unused_fd(fd);
+	return ret;
+}
+
 static long dma_buf_ioctl(struct file *file,
 			  unsigned int cmd, unsigned long arg)
 {
@@ -390,6 +546,12 @@ static long dma_buf_ioctl(struct file *file,
 	case DMA_BUF_SET_NAME:
 		return dma_buf_set_name(dmabuf, (const char __user *)arg);
 
+	case DMA_BUF_IOCTL_WAIT_SYNC_FILE:
+		return dma_buf_wait_sync_file(dmabuf, (const void __user *)arg);
+
+	case DMA_BUF_IOCTL_SIGNAL_SYNC_FILE:
+		return dma_buf_signal_sync_file(dmabuf, (void __user *)arg);
+
 	default:
 		return -ENOTTY;
 	}
diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
index dbc7092e04b5..825b9a913c89 100644
--- a/include/uapi/linux/dma-buf.h
+++ b/include/uapi/linux/dma-buf.h
@@ -37,8 +37,17 @@ struct dma_buf_sync {
 
 #define DMA_BUF_NAME_LEN	32
 
+struct dma_buf_sync_file {
+	__u32 flags;
+	__s32 fd;
+};
+
+#define DMA_BUF_SYNC_FILE_SYNC_WRITE	(1 << 0)
+
 #define DMA_BUF_BASE		'b'
-#define DMA_BUF_IOCTL_SYNC	_IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
-#define DMA_BUF_SET_NAME	_IOW(DMA_BUF_BASE, 1, const char *)
+#define DMA_BUF_IOCTL_SYNC	    _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
+#define DMA_BUF_SET_NAME	    _IOW(DMA_BUF_BASE, 1, const char *)
+#define DMA_BUF_IOCTL_WAIT_SYNC_FILE	_IOW(DMA_BUF_BASE, 2, struct dma_buf_sync)
+#define DMA_BUF_IOCTL_SIGNAL_SYNC_FILE	_IOW(DMA_BUF_BASE, 3, struct dma_buf_sync)
 
 #endif
-- 
2.24.1

Re: [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files

[Christian König]

Am 26.02.20 um 17:46 schrieb Bas Nieuwenhuizen:
> On Wed, Feb 26, 2020 at 4:29 PM Jason Ekstrand <jason@jlekstrand.net> wrote:
>> On Wed, Feb 26, 2020 at 4:05 AM Daniel Vetter <daniel@ffwll.ch> wrote:
>>> On Wed, Feb 26, 2020 at 10:16:05AM +0100, Christian König wrote:
>>> [SNIP]
>>>> Just imagine that you access some DMA-buf with a shader and that operation
>>>> is presented as a fence on the DMA-bufs reservation object. And now you can
>>>> go ahead and replace that fence and free up the memory.
>>>>
>>>> Tricking the Linux kernel into allocating page tables in that freed memory
>>>> is trivial and that's basically it you can overwrite page tables with your
>>>> shader and gain access to all of system memory :)
>>>>
>>>> What we could do is to always make sure that the added fences will complete
>>>> later than the already existing ones, but that is also rather tricky to get
>>>> right. I wouldn't do that if we don't have a rather big use case for this.
>> Right.  I thought about that but I'm still learning how dma_resv
>> works.  It'd be easy enough to make a fence array that contains both
>> the old fence and the new fence and replace the old fence with that.
>> What I don't know is the proper way to replace the exclusive fence
>> safely.  Some sort of atomic_cpxchg loop, perhaps?  I presume there's
>> some way of doing it properly because DRM drivers are doing it all the
>> time.

First of all you need to grab the lock of the dma_resv object or you 
can't replace the exclusive nor the shared ones.

This way you don't need to do a atomic_cmpxchg or anything else and 
still guarantee correct ordering.

> I think for an exclusive fence you may need to create a fence array
> that includes the existing exclusive and shared fences in the dma_resv
> combined with the added fence.

Yes, that at least gives us the correct synchronization.

> However, I'm not sure what the best way is to do garbage collection on
> that so that we don't get an impossibly list of fence arrays.

Exactly yes. That's also the reason why the dma_fence_chain container I 
came up with for the sync timeline stuff has such a rather sophisticated 
garbage collection.

When some of the included fences signal you need to free up the 
array/chain and make sure that the memory for the container can be reused.

>   (Note
> the dma_resv has a lock that needs to be taken before adding an
> exclusive fence, might be useful). Some code that does a thing like
> this is __dma_resv_make_exclusive in
> drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c 

Wanted to move that into dma_resv.c for quite a while since there are 
quite a few other cases where we need this.

Regards,
Christian.

> The other piece of the puzzle is that on the submit path this would
> need something to ignore implicit fences. And there semantically the
> question comes up whether it is safe for a driver to ignore exclusive
> fences from another driver. (and then we have amdgpu which has its own
> rules on exclusiveness of its shared fences based on the context. e.g.
> the current option to ignore implicit fences for a buffer still syncs
> on exclusive fences on the buffer).

[PATCH] RFC: dma-buf: Add an API for importing and exporting sync files (v3)

[Jason Ekstrand]

Explicit synchronization is the future.  At least, that seems to be what
most userspace APIs are agreeing on at this point.  However, most of our
Linux APIs (both userspace and kernel UAPI) are currently built around
implicit synchronization with dma-buf.  While work is ongoing to change
many of the userspace APIs and protocols to an explicit synchronization
model, switching over piecemeal is difficult due to the number of
potential components involved.  On the kernel side, many drivers use
dma-buf including GPU (3D/compute), display, v4l, and others.  In
userspace, we have X11, several Wayland compositors, 3D drivers, compute
drivers (OpenCL etc.), media encode/decode, and the list goes on.

This patch provides a path forward by allowing userspace to manually
manage the fences attached to a dma-buf.  Alternatively, one can think
of this as making dma-buf's implicit synchronization simply a carrier
for an explicit fence.  This is accomplished by adding two IOCTLs to
dma-buf for importing and exporting a sync file to/from the dma-buf.
This way a userspace component which is uses explicit synchronization,
such as a Vulkan driver, can manually set the write fence on a buffer
before handing it off to an implicitly synchronized component such as a
Wayland compositor or video encoder.  In this way, each of the different
components can be upgraded to an explicit synchronization model one at a
time as long as the userspace pieces connecting them are aware of it and
import/export fences at the right times.

There is a potential race condition with this API if userspace is not
careful.  A typical use case for implicit synchronization is to wait for
the dma-buf to be ready, use it, and then signal it for some other
component.  Because a sync_file cannot be created until it is guaranteed
to complete in finite time, userspace can only signal the dma-buf after
it has already submitted the work which uses it to the kernel and has
received a sync_file back.  There is no way to atomically submit a
wait-use-signal operation.  This is not, however, really a problem with
this API so much as it is a problem with explicit synchronization
itself.  The way this is typically handled is to have very explicit
ownership transfer points in the API or protocol which ensure that only
one component is using it at any given time.  Both X11 (via the PRESENT
extension) and Wayland provide such ownership transfer points via
explicit present and idle messages.

The decision was intentionally made in this patch to make the import and
export operations IOCTLs on the dma-buf itself rather than as a DRM
IOCTL.  This makes it the import/export operation universal across all
components which use dma-buf including GPU, display, v4l, and others.
It also means that a userspace component can do the import/export
without access to the DRM fd which may be tricky to get in cases where
the client communicates with DRM via a userspace API such as OpenGL or
Vulkan.  At a future date we may choose to add direct import/export APIs
to components such as drm_syncobj to avoid allocating a file descriptor
and going through two ioctls.  However, that seems to be something of a
micro-optimization as import/export operations are likely to happen at a
rate of a few per frame of rendered or decoded video.

v2 (Jason Ekstrand):
 - Use a wrapper dma_fence_array of all fences including the new one
   when importing an exclusive fence.

v3 (Jason Ekstrand):
 - Lock around setting shared fences as well as exclusive
 - Mark SIGNAL_SYNC_FILE as a read-write ioctl.
 - Initialize ret to 0 in dma_buf_wait_sync_file

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
---
 drivers/dma-buf/dma-buf.c    | 164 +++++++++++++++++++++++++++++++++++
 include/uapi/linux/dma-buf.h |  13 ++-
 2 files changed, 175 insertions(+), 2 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index d4097856c86b..2c4608bae3c2 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -20,6 +20,7 @@
 #include <linux/debugfs.h>
 #include <linux/module.h>
 #include <linux/seq_file.h>
+#include <linux/sync_file.h>
 #include <linux/poll.h>
 #include <linux/dma-resv.h>
 #include <linux/mm.h>
@@ -348,6 +349,163 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
 	return ret;
 }
 
+/* This function takes a ref to add_fence on success.  The caller still
+ * owns its ref and has to dma_fence_put it.
+ */
+static struct dma_fence *dma_buf_get_unified_fence(struct dma_buf *dmabuf,
+						   struct dma_fence *add_fence)
+{
+	struct dma_fence **fences = NULL;
+	struct dma_fence_array *array;
+	unsigned i, num_fences = 0;
+	int ret;
+
+	ret = dma_resv_get_fences_rcu(dmabuf->resv, NULL,
+				      &num_fences, &fences);
+	if (ret)
+		return NULL; /* ret can only be 0 or -ENOMEM */
+
+	if (num_fences == 0) {
+		if (add_fence) {
+			return add_fence;
+		} else {
+			return dma_fence_get_stub();
+		}
+	} else if (num_fences == 1 && (!add_fence || add_fence == fences[0])) {
+		struct dma_fence *fence = fences[0];
+		kfree(fences);
+		return fence;
+	}
+
+	if (add_fence) {
+		struct dma_fence **nfences;
+		size_t sz;
+
+		/* Get a ref to add_fence so that we have a ref to every
+		 * fence we are going to put in the array.
+		 */
+		dma_fence_get(add_fence);
+
+		sz = (num_fences + 1) * sizeof(*fences);
+		nfences = krealloc(fences, sz, GFP_NOWAIT | __GFP_NOWARN);
+		if (!nfences)
+			goto err_put_fences;
+
+		nfences[num_fences++] = add_fence;
+	}
+
+	array = dma_fence_array_create(num_fences, fences,
+				       dma_fence_context_alloc(1),
+				       1, false);
+	if (!array)
+		goto err_put_fences;
+
+	/* The fence array now owns fences_arr and our references to each
+	 * of the individual fences.  We only own a reference to the one
+	 * array fence.
+	 */
+
+	return &array->base;
+
+err_put_fences:
+	for (i = 0; i < num_fences; i++)
+		dma_fence_put(fences[0]);
+	dma_fence_put(add_fence);
+	kfree(fences);
+	return NULL;
+}
+
+static long dma_buf_wait_sync_file(struct dma_buf *dmabuf,
+				   const void __user *user_data)
+{
+	struct dma_buf_sync_file arg;
+	struct dma_fence *fence, *unified_fence;
+	int ret = 0;
+
+	if (copy_from_user(&arg, user_data, sizeof(arg)))
+		return -EFAULT;
+
+	if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
+		return -EINVAL;
+
+	fence = sync_file_get_fence(arg.fd);
+	if (!fence)
+		return -EINVAL;
+
+	dma_resv_lock(dmabuf->resv, NULL);
+
+	if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
+		unified_fence = dma_buf_get_unified_fence(dmabuf, fence);
+		if (unified_fence)
+			dma_resv_add_excl_fence(dmabuf->resv, fence);
+		else
+			ret = -ENOMEM;
+	} else {
+		dma_resv_add_shared_fence(dmabuf->resv, fence);
+	}
+
+	dma_resv_unlock(dmabuf->resv);
+
+	dma_fence_put(fence);
+
+	return ret;
+}
+
+static long dma_buf_signal_sync_file(struct dma_buf *dmabuf,
+				     void __user *user_data)
+{
+	struct dma_buf_sync_file arg;
+	struct dma_fence *fence = NULL;
+	struct sync_file *sync_file;
+	int fd, ret;
+
+	if (copy_from_user(&arg, user_data, sizeof(arg)))
+		return -EFAULT;
+
+	if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
+		return -EINVAL;
+
+	fd = get_unused_fd_flags(O_CLOEXEC);
+	if (fd < 0)
+		return fd;
+
+	if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
+		/* We need to include both the exclusive fence and all of
+		 * the shared fences in our fence.
+		 */
+		fence = dma_buf_get_unified_fence(dmabuf, NULL);
+		if (!fence) {
+			ret = -ENOMEM;
+			goto err_put_fd;
+		}
+	} else {
+		fence = dma_resv_get_excl_rcu(dmabuf->resv);
+		if (!fence)
+			fence = dma_fence_get_stub();
+	}
+
+	sync_file = sync_file_create(fence);
+
+	dma_fence_put(fence);
+
+	if (!sync_file) {
+		ret = -EINVAL;
+		goto err_put_fd;
+	}
+
+	fd_install(fd, sync_file->file);
+
+	arg.fd = fd;
+	if (copy_to_user(user_data, &arg, sizeof(arg)))
+		return -EFAULT;
+
+	return 0;
+
+err_put_fd:
+	put_unused_fd(fd);
+	return ret;
+}
+
 static long dma_buf_ioctl(struct file *file,
 			  unsigned int cmd, unsigned long arg)
 {
@@ -390,6 +548,12 @@ static long dma_buf_ioctl(struct file *file,
 	case DMA_BUF_SET_NAME:
 		return dma_buf_set_name(dmabuf, (const char __user *)arg);
 
+	case DMA_BUF_IOCTL_WAIT_SYNC_FILE:
+		return dma_buf_wait_sync_file(dmabuf, (const void __user *)arg);
+
+	case DMA_BUF_IOCTL_SIGNAL_SYNC_FILE:
+		return dma_buf_signal_sync_file(dmabuf, (void __user *)arg);
+
 	default:
 		return -ENOTTY;
 	}
diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
index dbc7092e04b5..86e07acca90c 100644
--- a/include/uapi/linux/dma-buf.h
+++ b/include/uapi/linux/dma-buf.h
@@ -37,8 +37,17 @@ struct dma_buf_sync {
 
 #define DMA_BUF_NAME_LEN	32
 
+struct dma_buf_sync_file {
+	__u32 flags;
+	__s32 fd;
+};
+
+#define DMA_BUF_SYNC_FILE_SYNC_WRITE	(1 << 0)
+
 #define DMA_BUF_BASE		'b'
-#define DMA_BUF_IOCTL_SYNC	_IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
-#define DMA_BUF_SET_NAME	_IOW(DMA_BUF_BASE, 1, const char *)
+#define DMA_BUF_IOCTL_SYNC	    _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
+#define DMA_BUF_SET_NAME	    _IOW(DMA_BUF_BASE, 1, const char *)
+#define DMA_BUF_IOCTL_WAIT_SYNC_FILE	_IOW(DMA_BUF_BASE, 2, struct dma_buf_sync)
+#define DMA_BUF_IOCTL_SIGNAL_SYNC_FILE	_IOWR(DMA_BUF_BASE, 3, struct dma_buf_sync)
 
 #endif
-- 
2.24.1

Re: [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files (v3)

[Jason Ekstrand]

Vulkan WSI user of the new API:

https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4037

On Tue, Mar 3, 2020 at 1:03 PM Jason Ekstrand <jason@jlekstrand.net> wrote:
>
> Explicit synchronization is the future.  At least, that seems to be what
> most userspace APIs are agreeing on at this point.  However, most of our
> Linux APIs (both userspace and kernel UAPI) are currently built around
> implicit synchronization with dma-buf.  While work is ongoing to change
> many of the userspace APIs and protocols to an explicit synchronization
> model, switching over piecemeal is difficult due to the number of
> potential components involved.  On the kernel side, many drivers use
> dma-buf including GPU (3D/compute), display, v4l, and others.  In
> userspace, we have X11, several Wayland compositors, 3D drivers, compute
> drivers (OpenCL etc.), media encode/decode, and the list goes on.
>
> This patch provides a path forward by allowing userspace to manually
> manage the fences attached to a dma-buf.  Alternatively, one can think
> of this as making dma-buf's implicit synchronization simply a carrier
> for an explicit fence.  This is accomplished by adding two IOCTLs to
> dma-buf for importing and exporting a sync file to/from the dma-buf.
> This way a userspace component which is uses explicit synchronization,
> such as a Vulkan driver, can manually set the write fence on a buffer
> before handing it off to an implicitly synchronized component such as a
> Wayland compositor or video encoder.  In this way, each of the different
> components can be upgraded to an explicit synchronization model one at a
> time as long as the userspace pieces connecting them are aware of it and
> import/export fences at the right times.
>
> There is a potential race condition with this API if userspace is not
> careful.  A typical use case for implicit synchronization is to wait for
> the dma-buf to be ready, use it, and then signal it for some other
> component.  Because a sync_file cannot be created until it is guaranteed
> to complete in finite time, userspace can only signal the dma-buf after
> it has already submitted the work which uses it to the kernel and has
> received a sync_file back.  There is no way to atomically submit a
> wait-use-signal operation.  This is not, however, really a problem with
> this API so much as it is a problem with explicit synchronization
> itself.  The way this is typically handled is to have very explicit
> ownership transfer points in the API or protocol which ensure that only
> one component is using it at any given time.  Both X11 (via the PRESENT
> extension) and Wayland provide such ownership transfer points via
> explicit present and idle messages.
>
> The decision was intentionally made in this patch to make the import and
> export operations IOCTLs on the dma-buf itself rather than as a DRM
> IOCTL.  This makes it the import/export operation universal across all
> components which use dma-buf including GPU, display, v4l, and others.
> It also means that a userspace component can do the import/export
> without access to the DRM fd which may be tricky to get in cases where
> the client communicates with DRM via a userspace API such as OpenGL or
> Vulkan.  At a future date we may choose to add direct import/export APIs
> to components such as drm_syncobj to avoid allocating a file descriptor
> and going through two ioctls.  However, that seems to be something of a
> micro-optimization as import/export operations are likely to happen at a
> rate of a few per frame of rendered or decoded video.
>
> v2 (Jason Ekstrand):
>  - Use a wrapper dma_fence_array of all fences including the new one
>    when importing an exclusive fence.
>
> v3 (Jason Ekstrand):
>  - Lock around setting shared fences as well as exclusive
>  - Mark SIGNAL_SYNC_FILE as a read-write ioctl.
>  - Initialize ret to 0 in dma_buf_wait_sync_file
>
> Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> ---
>  drivers/dma-buf/dma-buf.c    | 164 +++++++++++++++++++++++++++++++++++
>  include/uapi/linux/dma-buf.h |  13 ++-
>  2 files changed, 175 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index d4097856c86b..2c4608bae3c2 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -20,6 +20,7 @@
>  #include <linux/debugfs.h>
>  #include <linux/module.h>
>  #include <linux/seq_file.h>
> +#include <linux/sync_file.h>
>  #include <linux/poll.h>
>  #include <linux/dma-resv.h>
>  #include <linux/mm.h>
> @@ -348,6 +349,163 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
>         return ret;
>  }
>
> +/* This function takes a ref to add_fence on success.  The caller still
> + * owns its ref and has to dma_fence_put it.
> + */
> +static struct dma_fence *dma_buf_get_unified_fence(struct dma_buf *dmabuf,
> +                                                  struct dma_fence *add_fence)
> +{
> +       struct dma_fence **fences = NULL;
> +       struct dma_fence_array *array;
> +       unsigned i, num_fences = 0;
> +       int ret;
> +
> +       ret = dma_resv_get_fences_rcu(dmabuf->resv, NULL,
> +                                     &num_fences, &fences);
> +       if (ret)
> +               return NULL; /* ret can only be 0 or -ENOMEM */
> +
> +       if (num_fences == 0) {
> +               if (add_fence) {
> +                       return add_fence;
> +               } else {
> +                       return dma_fence_get_stub();
> +               }
> +       } else if (num_fences == 1 && (!add_fence || add_fence == fences[0])) {
> +               struct dma_fence *fence = fences[0];
> +               kfree(fences);
> +               return fence;
> +       }
> +
> +       if (add_fence) {
> +               struct dma_fence **nfences;
> +               size_t sz;
> +
> +               /* Get a ref to add_fence so that we have a ref to every
> +                * fence we are going to put in the array.
> +                */
> +               dma_fence_get(add_fence);
> +
> +               sz = (num_fences + 1) * sizeof(*fences);
> +               nfences = krealloc(fences, sz, GFP_NOWAIT | __GFP_NOWARN);
> +               if (!nfences)
> +                       goto err_put_fences;
> +
> +               nfences[num_fences++] = add_fence;
> +       }
> +
> +       array = dma_fence_array_create(num_fences, fences,
> +                                      dma_fence_context_alloc(1),
> +                                      1, false);
> +       if (!array)
> +               goto err_put_fences;
> +
> +       /* The fence array now owns fences_arr and our references to each
> +        * of the individual fences.  We only own a reference to the one
> +        * array fence.
> +        */
> +
> +       return &array->base;
> +
> +err_put_fences:
> +       for (i = 0; i < num_fences; i++)
> +               dma_fence_put(fences[0]);
> +       dma_fence_put(add_fence);
> +       kfree(fences);
> +       return NULL;
> +}
> +
> +static long dma_buf_wait_sync_file(struct dma_buf *dmabuf,
> +                                  const void __user *user_data)
> +{
> +       struct dma_buf_sync_file arg;
> +       struct dma_fence *fence, *unified_fence;
> +       int ret = 0;
> +
> +       if (copy_from_user(&arg, user_data, sizeof(arg)))
> +               return -EFAULT;
> +
> +       if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
> +               return -EINVAL;
> +
> +       fence = sync_file_get_fence(arg.fd);
> +       if (!fence)
> +               return -EINVAL;
> +
> +       dma_resv_lock(dmabuf->resv, NULL);
> +
> +       if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
> +               unified_fence = dma_buf_get_unified_fence(dmabuf, fence);
> +               if (unified_fence)
> +                       dma_resv_add_excl_fence(dmabuf->resv, fence);
> +               else
> +                       ret = -ENOMEM;
> +       } else {
> +               dma_resv_add_shared_fence(dmabuf->resv, fence);
> +       }
> +
> +       dma_resv_unlock(dmabuf->resv);
> +
> +       dma_fence_put(fence);
> +
> +       return ret;
> +}
> +
> +static long dma_buf_signal_sync_file(struct dma_buf *dmabuf,
> +                                    void __user *user_data)
> +{
> +       struct dma_buf_sync_file arg;
> +       struct dma_fence *fence = NULL;
> +       struct sync_file *sync_file;
> +       int fd, ret;
> +
> +       if (copy_from_user(&arg, user_data, sizeof(arg)))
> +               return -EFAULT;
> +
> +       if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
> +               return -EINVAL;
> +
> +       fd = get_unused_fd_flags(O_CLOEXEC);
> +       if (fd < 0)
> +               return fd;
> +
> +       if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
> +               /* We need to include both the exclusive fence and all of
> +                * the shared fences in our fence.
> +                */
> +               fence = dma_buf_get_unified_fence(dmabuf, NULL);
> +               if (!fence) {
> +                       ret = -ENOMEM;
> +                       goto err_put_fd;
> +               }
> +       } else {
> +               fence = dma_resv_get_excl_rcu(dmabuf->resv);
> +               if (!fence)
> +                       fence = dma_fence_get_stub();
> +       }
> +
> +       sync_file = sync_file_create(fence);
> +
> +       dma_fence_put(fence);
> +
> +       if (!sync_file) {
> +               ret = -EINVAL;
> +               goto err_put_fd;
> +       }
> +
> +       fd_install(fd, sync_file->file);
> +
> +       arg.fd = fd;
> +       if (copy_to_user(user_data, &arg, sizeof(arg)))
> +               return -EFAULT;
> +
> +       return 0;
> +
> +err_put_fd:
> +       put_unused_fd(fd);
> +       return ret;
> +}
> +
>  static long dma_buf_ioctl(struct file *file,
>                           unsigned int cmd, unsigned long arg)
>  {
> @@ -390,6 +548,12 @@ static long dma_buf_ioctl(struct file *file,
>         case DMA_BUF_SET_NAME:
>                 return dma_buf_set_name(dmabuf, (const char __user *)arg);
>
> +       case DMA_BUF_IOCTL_WAIT_SYNC_FILE:
> +               return dma_buf_wait_sync_file(dmabuf, (const void __user *)arg);
> +
> +       case DMA_BUF_IOCTL_SIGNAL_SYNC_FILE:
> +               return dma_buf_signal_sync_file(dmabuf, (void __user *)arg);
> +
>         default:
>                 return -ENOTTY;
>         }
> diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
> index dbc7092e04b5..86e07acca90c 100644
> --- a/include/uapi/linux/dma-buf.h
> +++ b/include/uapi/linux/dma-buf.h
> @@ -37,8 +37,17 @@ struct dma_buf_sync {
>
>  #define DMA_BUF_NAME_LEN       32
>
> +struct dma_buf_sync_file {
> +       __u32 flags;
> +       __s32 fd;
> +};
> +
> +#define DMA_BUF_SYNC_FILE_SYNC_WRITE   (1 << 0)
> +
>  #define DMA_BUF_BASE           'b'
> -#define DMA_BUF_IOCTL_SYNC     _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
> -#define DMA_BUF_SET_NAME       _IOW(DMA_BUF_BASE, 1, const char *)
> +#define DMA_BUF_IOCTL_SYNC         _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
> +#define DMA_BUF_SET_NAME           _IOW(DMA_BUF_BASE, 1, const char *)
> +#define DMA_BUF_IOCTL_WAIT_SYNC_FILE   _IOW(DMA_BUF_BASE, 2, struct dma_buf_sync)
> +#define DMA_BUF_IOCTL_SIGNAL_SYNC_FILE _IOWR(DMA_BUF_BASE, 3, struct dma_buf_sync)
>
>  #endif
> --
> 2.24.1
>

Re: [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files

[Jason Ekstrand]

On Thu, Feb 27, 2020 at 2:28 AM Christian König
<christian.koenig@amd.com> wrote:
>
> Am 26.02.20 um 17:46 schrieb Bas Nieuwenhuizen:
> > On Wed, Feb 26, 2020 at 4:29 PM Jason Ekstrand <jason@jlekstrand.net> wrote:
> >> On Wed, Feb 26, 2020 at 4:05 AM Daniel Vetter <daniel@ffwll.ch> wrote:
> >>> On Wed, Feb 26, 2020 at 10:16:05AM +0100, Christian König wrote:
> >>> [SNIP]
> >>>> Just imagine that you access some DMA-buf with a shader and that operation
> >>>> is presented as a fence on the DMA-bufs reservation object. And now you can
> >>>> go ahead and replace that fence and free up the memory.
> >>>>
> >>>> Tricking the Linux kernel into allocating page tables in that freed memory
> >>>> is trivial and that's basically it you can overwrite page tables with your
> >>>> shader and gain access to all of system memory :)
> >>>>
> >>>> What we could do is to always make sure that the added fences will complete
> >>>> later than the already existing ones, but that is also rather tricky to get
> >>>> right. I wouldn't do that if we don't have a rather big use case for this.
> >> Right.  I thought about that but I'm still learning how dma_resv
> >> works.  It'd be easy enough to make a fence array that contains both
> >> the old fence and the new fence and replace the old fence with that.
> >> What I don't know is the proper way to replace the exclusive fence
> >> safely.  Some sort of atomic_cpxchg loop, perhaps?  I presume there's
> >> some way of doing it properly because DRM drivers are doing it all the
> >> time.
>
> First of all you need to grab the lock of the dma_resv object or you
> can't replace the exclusive nor the shared ones.
>
> This way you don't need to do a atomic_cmpxchg or anything else and
> still guarantee correct ordering.

Fixed in v3.

> > I think for an exclusive fence you may need to create a fence array
> > that includes the existing exclusive and shared fences in the dma_resv
> > combined with the added fence.
>
> Yes, that at least gives us the correct synchronization.

Fixed in v2

> > However, I'm not sure what the best way is to do garbage collection on
> > that so that we don't get an impossibly list of fence arrays.
>
> Exactly yes. That's also the reason why the dma_fence_chain container I
> came up with for the sync timeline stuff has such a rather sophisticated
> garbage collection.
>
> When some of the included fences signal you need to free up the
> array/chain and make sure that the memory for the container can be reused.

Currently (as of v2), I'm using dma_fence_array and being careful to
not bother constructing one if there's only one fence in play.  Is
this insufficient?  If so, maybe we should consider improving
dma_fence_array.

> >   (Note
> > the dma_resv has a lock that needs to be taken before adding an
> > exclusive fence, might be useful). Some code that does a thing like
> > this is __dma_resv_make_exclusive in
> > drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>
> Wanted to move that into dma_resv.c for quite a while since there are
> quite a few other cases where we need this.

I've roughly done that.  The primary difference is that my version
takes an optional additional fence to add to the array.  This makes it
a bit more complicated but I think I got it mostly right.

I've also written userspace code which exercises this and it seems to
work.  Hopefully, that will give a better idea of what I'm trying to
accomplish.

https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4037

--Jason

Re: [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files

[Christian König]

Am 03.03.20 um 20:10 schrieb Jason Ekstrand:
> On Thu, Feb 27, 2020 at 2:28 AM Christian König
> <christian.koenig@amd.com> wrote:
>> [SNIP]
>>> However, I'm not sure what the best way is to do garbage collection on
>>> that so that we don't get an impossibly list of fence arrays.
>> Exactly yes. That's also the reason why the dma_fence_chain container I
>> came up with for the sync timeline stuff has such a rather sophisticated
>> garbage collection.
>>
>> When some of the included fences signal you need to free up the
>> array/chain and make sure that the memory for the container can be reused.
> Currently (as of v2), I'm using dma_fence_array and being careful to
> not bother constructing one if there's only one fence in play.  Is
> this insufficient?  If so, maybe we should consider improving
> dma_fence_array.

That still won't work correctly in all cases. See the problem is not 
only optimization, but also avoiding situations where userspace can 
abuse the interface to do nasty things.

For example if userspace just calls that function in a loop you can 
create a long chain of dma_fence_array objects.

If that chain is then suddenly released the recursive dropping of 
references can overwrite the kernel stack.

For reference see what dance is necessary in the dma_fence_chain_release 
function to avoid that:
>         /* Manually unlink the chain as much as possible to avoid 
> recursion
>          * and potential stack overflow.
>          */
>         while ((prev = rcu_dereference_protected(chain->prev, true))) {
....

It took me quite a while to figure out how to do this without causing 
issues. But I don't see how this would be possible for dma_fence_array.

As far as I can see the only real option to implement this would be to 
change the dma_resv object container so that you can add fences without 
overriding existing ones.

For shared fences that can be done relative easily, but I absolutely 
don't see how to do this for exclusive ones without a larger rework.

>>>    (Note
>>> the dma_resv has a lock that needs to be taken before adding an
>>> exclusive fence, might be useful). Some code that does a thing like
>>> this is __dma_resv_make_exclusive in
>>> drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
>> Wanted to move that into dma_resv.c for quite a while since there are
>> quite a few other cases where we need this.
> I've roughly done that.  The primary difference is that my version
> takes an optional additional fence to add to the array.  This makes it
> a bit more complicated but I think I got it mostly right.
>
> I've also written userspace code which exercises this and it seems to
> work.  Hopefully, that will give a better idea of what I'm trying to
> accomplish.

Yes, that is indeed a really nice to have feature.

Regards,
Christian.

Re: [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files

[Jason Ekstrand]

On Wed, Mar 4, 2020 at 2:34 AM Christian König <christian.koenig@amd.com> wrote:
>
> Am 03.03.20 um 20:10 schrieb Jason Ekstrand:
> > On Thu, Feb 27, 2020 at 2:28 AM Christian König
> > <christian.koenig@amd.com> wrote:
> >> [SNIP]
> >>> However, I'm not sure what the best way is to do garbage collection on
> >>> that so that we don't get an impossibly list of fence arrays.
> >> Exactly yes. That's also the reason why the dma_fence_chain container I
> >> came up with for the sync timeline stuff has such a rather sophisticated
> >> garbage collection.
> >>
> >> When some of the included fences signal you need to free up the
> >> array/chain and make sure that the memory for the container can be reused.
> > Currently (as of v2), I'm using dma_fence_array and being careful to
> > not bother constructing one if there's only one fence in play.  Is
> > this insufficient?  If so, maybe we should consider improving
> > dma_fence_array.
>
> That still won't work correctly in all cases. See the problem is not
> only optimization, but also avoiding situations where userspace can
> abuse the interface to do nasty things.
>
> For example if userspace just calls that function in a loop you can
> create a long chain of dma_fence_array objects.
>
> If that chain is then suddenly released the recursive dropping of
> references can overwrite the kernel stack.
>
> For reference see what dance is necessary in the dma_fence_chain_release
> function to avoid that:
> >         /* Manually unlink the chain as much as possible to avoid
> > recursion
> >          * and potential stack overflow.
> >          */
> >         while ((prev = rcu_dereference_protected(chain->prev, true))) {
> ....
>
> It took me quite a while to figure out how to do this without causing
> issues. But I don't see how this would be possible for dma_fence_array.

Ah, I see the issue now!  It hadn't even occurred to me that userspace
could use this to build up an infinite recursion chain.  That's nasty!
 I'll give this some more thought and see if can come up with
something clever.

Here's one thought:  We could make dma_fence_array automatically
collapse any arrays it references and instead directly reference their
fences.  This way, no matter how much the client chains things, they
will never get more than one dma_fence_array.  Of course, the
difficulty here (answering my own question) comes if they ping-pong
back-and-forth between something which constructs a dma_fence_array
and something which constructs a dma_fence_chain to get
array-of-chain-of-array-of-chain-of-...  More thought needed.

> As far as I can see the only real option to implement this would be to
> change the dma_resv object container so that you can add fences without
> overriding existing ones.
>
> For shared fences that can be done relative easily, but I absolutely
> don't see how to do this for exclusive ones without a larger rework.

Fair enough.  Thanks for taking the time to explain the issue.  I'll
give this some more thought.

--Jason


> >>>    (Note
> >>> the dma_resv has a lock that needs to be taken before adding an
> >>> exclusive fence, might be useful). Some code that does a thing like
> >>> this is __dma_resv_make_exclusive in
> >>> drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> >> Wanted to move that into dma_resv.c for quite a while since there are
> >> quite a few other cases where we need this.
> > I've roughly done that.  The primary difference is that my version
> > takes an optional additional fence to add to the array.  This makes it
> > a bit more complicated but I think I got it mostly right.
> >
> > I've also written userspace code which exercises this and it seems to
> > work.  Hopefully, that will give a better idea of what I'm trying to
> > accomplish.
>
> Yes, that is indeed a really nice to have feature.
>
> Regards,
> Christian.

Re: [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files

[Jason Ekstrand]

On Wed, Mar 4, 2020 at 10:27 AM Jason Ekstrand <jason@jlekstrand.net> wrote:
>
> On Wed, Mar 4, 2020 at 2:34 AM Christian König <christian.koenig@amd.com> wrote:
> >
> > Am 03.03.20 um 20:10 schrieb Jason Ekstrand:
> > > On Thu, Feb 27, 2020 at 2:28 AM Christian König
> > > <christian.koenig@amd.com> wrote:
> > >> [SNIP]
> > >>> However, I'm not sure what the best way is to do garbage collection on
> > >>> that so that we don't get an impossibly list of fence arrays.
> > >> Exactly yes. That's also the reason why the dma_fence_chain container I
> > >> came up with for the sync timeline stuff has such a rather sophisticated
> > >> garbage collection.
> > >>
> > >> When some of the included fences signal you need to free up the
> > >> array/chain and make sure that the memory for the container can be reused.
> > > Currently (as of v2), I'm using dma_fence_array and being careful to
> > > not bother constructing one if there's only one fence in play.  Is
> > > this insufficient?  If so, maybe we should consider improving
> > > dma_fence_array.
> >
> > That still won't work correctly in all cases. See the problem is not
> > only optimization, but also avoiding situations where userspace can
> > abuse the interface to do nasty things.
> >
> > For example if userspace just calls that function in a loop you can
> > create a long chain of dma_fence_array objects.
> >
> > If that chain is then suddenly released the recursive dropping of
> > references can overwrite the kernel stack.
> >
> > For reference see what dance is necessary in the dma_fence_chain_release
> > function to avoid that:
> > >         /* Manually unlink the chain as much as possible to avoid
> > > recursion
> > >          * and potential stack overflow.
> > >          */
> > >         while ((prev = rcu_dereference_protected(chain->prev, true))) {
> > ....
> >
> > It took me quite a while to figure out how to do this without causing
> > issues. But I don't see how this would be possible for dma_fence_array.
>
> Ah, I see the issue now!  It hadn't even occurred to me that userspace
> could use this to build up an infinite recursion chain.  That's nasty!
>  I'll give this some more thought and see if can come up with
> something clever.
>
> Here's one thought:  We could make dma_fence_array automatically
> collapse any arrays it references and instead directly reference their
> fences.  This way, no matter how much the client chains things, they
> will never get more than one dma_fence_array.  Of course, the
> difficulty here (answering my own question) comes if they ping-pong
> back-and-forth between something which constructs a dma_fence_array
> and something which constructs a dma_fence_chain to get
> array-of-chain-of-array-of-chain-of-...  More thought needed.

Answering my own questions again...  I think the
array-of-chain-of-array case is also solvable.

For array-of-chain, we can simply add all unsignaled dma_fences in the
chain to the array.  The array won't signal until all of them have
which is exactly the same behavior as if we'd added the chain itself.

For chain-of-array, we can add all unsignaled dma_fences in the array
to the same point in the chain.  There may be some fiddling with the
chain numbering required here but I think we can get it so the chain
won't signal until everything in the array has signaled and we get the
same behavior as if we'd added the dma_fence_array to the chain.

In both cases, we end up with either a single array or a single and
destruction doesn't require recursion.  Thoughts?

--Jason

Re: [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files

[Christian König]

Am 04.03.20 um 17:41 schrieb Jason Ekstrand:
> On Wed, Mar 4, 2020 at 10:27 AM Jason Ekstrand <jason@jlekstrand.net> wrote:
>> On Wed, Mar 4, 2020 at 2:34 AM Christian König <christian.koenig@amd.com> wrote:
>>> Am 03.03.20 um 20:10 schrieb Jason Ekstrand:
>>>> On Thu, Feb 27, 2020 at 2:28 AM Christian König
>>>> <christian.koenig@amd.com> wrote:
>>>> [SNIP]
>>> For reference see what dance is necessary in the dma_fence_chain_release
>>> function to avoid that:
>>>>          /* Manually unlink the chain as much as possible to avoid
>>>> recursion
>>>>           * and potential stack overflow.
>>>>           */
>>>>          while ((prev = rcu_dereference_protected(chain->prev, true))) {
>>> ....
>>>
>>> It took me quite a while to figure out how to do this without causing
>>> issues. But I don't see how this would be possible for dma_fence_array.
>> Ah, I see the issue now!  It hadn't even occurred to me that userspace
>> could use this to build up an infinite recursion chain.  That's nasty!

Yeah, when I first stumbled over it it was like why the heck is my code 
crashing in an interrupt handler?

Realizing that this is stack corruption because of the long chain we 
constructed was quite an enlightenment.

And then it took me even longer to fix it :)

>>   I'll give this some more thought and see if can come up with
>> something clever.
>>
>> Here's one thought:  We could make dma_fence_array automatically
>> collapse any arrays it references and instead directly reference their
>> fences.  This way, no matter how much the client chains things, they
>> will never get more than one dma_fence_array.  Of course, the
>> difficulty here (answering my own question) comes if they ping-pong
>> back-and-forth between something which constructs a dma_fence_array
>> and something which constructs a dma_fence_chain to get
>> array-of-chain-of-array-of-chain-of-...  More thought needed.

Condensing the fences into a larger array can certainly work, yes.

> Answering my own questions again...  I think the
> array-of-chain-of-array case is also solvable.
>
> For array-of-chain, we can simply add all unsignaled dma_fences in the
> chain to the array.  The array won't signal until all of them have
> which is exactly the same behavior as if we'd added the chain itself.

Yeah, that should work. Probably best to implement something like a 
cursor to walk all fences in the data structure.

> For chain-of-array, we can add all unsignaled dma_fences in the array
> to the same point in the chain.  There may be some fiddling with the
> chain numbering required here but I think we can get it so the chain
> won't signal until everything in the array has signaled and we get the
> same behavior as if we'd added the dma_fence_array to the chain.

Well as far as I can see this won't work because it would break the 
semantics of the timeline sync.

But I think I know a different way which should work: A dma_fence_chain 
can still contain a dma_fence_array, only the other way around is 
forbidden. Then we create the cursor functionality in such a way that it 
allows us to deep dive into the data structure and return all containing 
fences one by one.

I can prototype that if you want, shouldn't be more than a few hours of 
hacking anyway.

Regards,
Christian.

>
> In both cases, we end up with either a single array or a single and
> destruction doesn't require recursion.  Thoughts?
>
> --Jason

Re: [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files

[Jason Ekstrand]

On Thu, Mar 5, 2020 at 7:06 AM Christian König <christian.koenig@amd.com> wrote:
>
> Am 04.03.20 um 17:41 schrieb Jason Ekstrand:
> > On Wed, Mar 4, 2020 at 10:27 AM Jason Ekstrand <jason@jlekstrand.net> wrote:
> >> On Wed, Mar 4, 2020 at 2:34 AM Christian König <christian.koenig@amd.com> wrote:
> >>> Am 03.03.20 um 20:10 schrieb Jason Ekstrand:
> >>>> On Thu, Feb 27, 2020 at 2:28 AM Christian König
> >>>> <christian.koenig@amd.com> wrote:
> >>>> [SNIP]
> >>> For reference see what dance is necessary in the dma_fence_chain_release
> >>> function to avoid that:
> >>>>          /* Manually unlink the chain as much as possible to avoid
> >>>> recursion
> >>>>           * and potential stack overflow.
> >>>>           */
> >>>>          while ((prev = rcu_dereference_protected(chain->prev, true))) {
> >>> ....
> >>>
> >>> It took me quite a while to figure out how to do this without causing
> >>> issues. But I don't see how this would be possible for dma_fence_array.
> >> Ah, I see the issue now!  It hadn't even occurred to me that userspace
> >> could use this to build up an infinite recursion chain.  That's nasty!
>
> Yeah, when I first stumbled over it it was like why the heck is my code
> crashing in an interrupt handler?
>
> Realizing that this is stack corruption because of the long chain we
> constructed was quite an enlightenment.
>
> And then it took me even longer to fix it :)

Fun....

> >>   I'll give this some more thought and see if can come up with
> >> something clever.
> >>
> >> Here's one thought:  We could make dma_fence_array automatically
> >> collapse any arrays it references and instead directly reference their
> >> fences.  This way, no matter how much the client chains things, they
> >> will never get more than one dma_fence_array.  Of course, the
> >> difficulty here (answering my own question) comes if they ping-pong
> >> back-and-forth between something which constructs a dma_fence_array
> >> and something which constructs a dma_fence_chain to get
> >> array-of-chain-of-array-of-chain-of-...  More thought needed.
>
> Condensing the fences into a larger array can certainly work, yes.
>
> > Answering my own questions again...  I think the
> > array-of-chain-of-array case is also solvable.
> >
> > For array-of-chain, we can simply add all unsignaled dma_fences in the
> > chain to the array.  The array won't signal until all of them have
> > which is exactly the same behavior as if we'd added the chain itself.
>
> Yeah, that should work. Probably best to implement something like a
> cursor to walk all fences in the data structure.
>
> > For chain-of-array, we can add all unsignaled dma_fences in the array
> > to the same point in the chain.  There may be some fiddling with the
> > chain numbering required here but I think we can get it so the chain
> > won't signal until everything in the array has signaled and we get the
> > same behavior as if we'd added the dma_fence_array to the chain.
>
> Well as far as I can see this won't work because it would break the
> semantics of the timeline sync.

I'm not 100% convinced it has to.  We already have support for the
seqno regressing and we ensure that we still wait for all the fences.
I thought maybe we could use that but I haven't spent enough time
looking at the details to be sure.  I may be missing something.

> But I think I know a different way which should work: A dma_fence_chain
> can still contain a dma_fence_array, only the other way around is
> forbidden. Then we create the cursor functionality in such a way that it
> allows us to deep dive into the data structure and return all containing
> fences one by one.

Agreed.  As long as one container is able to consume the other, it's fine.

> I can prototype that if you want, shouldn't be more than a few hours of
> hacking anyway.

If you'd like to, go for it.  I'd be happy to give it a go as well but
if you already know what you want, it may be easier for you to just
write the patch for the cursor.

Two more questions:

 1. Do you want this collapsing to happen every time we create a
dma_fence_array or should it be a special entrypoint?  Collapsing all
the time likely means doing extra array calculations instead of the
dma_fence_array taking ownership of the array that's passed in.  My
gut says that cost is ok; but my gut doesn't spend much time in kernel
space.

 2. When we do the collapsing, should we call dma_fence_is_signaled()
to avoid adding signaled fences to the array?  It seems like avoiding
adding references to fences that are already signaled would let the
kernel clean them up faster and reduce the likelihood that a fence
will hang around forever because it keeps getting added to arrays with
other unsignaled fences.

--Jason

Re: [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files

[Christian König]

Am 05.03.20 um 16:54 schrieb Jason Ekstrand:
> On Thu, Mar 5, 2020 at 7:06 AM Christian König <christian.koenig@amd.com> wrote:
>> [SNIP]
>> Well as far as I can see this won't work because it would break the
>> semantics of the timeline sync.
> I'm not 100% convinced it has to.  We already have support for the
> seqno regressing and we ensure that we still wait for all the fences.
> I thought maybe we could use that but I haven't spent enough time
> looking at the details to be sure.  I may be missing something.

That won't work. The seqno regression works by punishing userspace for 
doing something stupid and undefined.

Be we can't do that under normal circumstances.

>> I can prototype that if you want, shouldn't be more than a few hours of
>> hacking anyway.
> If you'd like to, go for it.  I'd be happy to give it a go as well but
> if you already know what you want, it may be easier for you to just
> write the patch for the cursor.

Send you two patches for that a few minutes ago. But keep in mind that 
those are completely untested.

> Two more questions:
>
>   1. Do you want this collapsing to happen every time we create a
> dma_fence_array or should it be a special entrypoint?  Collapsing all
> the time likely means doing extra array calculations instead of the
> dma_fence_array taking ownership of the array that's passed in.  My
> gut says that cost is ok; but my gut doesn't spend much time in kernel
> space.

In my prototype implementation that is a dma_resv function you call and 
get either a single fence or a dma_fence_array with the collapsed fences 
in return.

But I wouldn't add that to the general dma_fence_array_init function 
since this is still a rather special case. Well see the patches, they 
should be pretty self explaining.

>   2. When we do the collapsing, should we call dma_fence_is_signaled()
> to avoid adding signaled fences to the array?  It seems like avoiding
> adding references to fences that are already signaled would let the
> kernel clean them up faster and reduce the likelihood that a fence
> will hang around forever because it keeps getting added to arrays with
> other unsignaled fences.

I think so. Can't think of a good reason why we would want to add 
already signaled fences to the array.

Christian.

>
> --Jason

Re: [PATCH] RFC: dma-buf: Add an API for importing and exporting sync files

[Jason Ekstrand]

On Mon, Mar 9, 2020 at 11:21 AM Christian König
<christian.koenig@amd.com> wrote:
>
> Am 05.03.20 um 16:54 schrieb Jason Ekstrand:
> > On Thu, Mar 5, 2020 at 7:06 AM Christian König <christian.koenig@amd.com> wrote:
> >> [SNIP]
> >> Well as far as I can see this won't work because it would break the
> >> semantics of the timeline sync.
> > I'm not 100% convinced it has to.  We already have support for the
> > seqno regressing and we ensure that we still wait for all the fences.
> > I thought maybe we could use that but I haven't spent enough time
> > looking at the details to be sure.  I may be missing something.
>
> That won't work. The seqno regression works by punishing userspace for
> doing something stupid and undefined.
>
> Be we can't do that under normal circumstances.
>
> >> I can prototype that if you want, shouldn't be more than a few hours of
> >> hacking anyway.
> > If you'd like to, go for it.  I'd be happy to give it a go as well but
> > if you already know what you want, it may be easier for you to just
> > write the patch for the cursor.
>
> Send you two patches for that a few minutes ago. But keep in mind that
> those are completely untested.

No worries.  They were full of bugs but I think I've got them sorted
out now.  The v2's I'm about to send seem to work.  I'm going to leave
a Vulkan demo running all night long just to make sure I'm not leaking
memory like mad.

--Jason

> > Two more questions:
> >
> >   1. Do you want this collapsing to happen every time we create a
> > dma_fence_array or should it be a special entrypoint?  Collapsing all
> > the time likely means doing extra array calculations instead of the
> > dma_fence_array taking ownership of the array that's passed in.  My
> > gut says that cost is ok; but my gut doesn't spend much time in kernel
> > space.
>
> In my prototype implementation that is a dma_resv function you call and
> get either a single fence or a dma_fence_array with the collapsed fences
> in return.
>
> But I wouldn't add that to the general dma_fence_array_init function
> since this is still a rather special case. Well see the patches, they
> should be pretty self explaining.
>
> >   2. When we do the collapsing, should we call dma_fence_is_signaled()
> > to avoid adding signaled fences to the array?  It seems like avoiding
> > adding references to fences that are already signaled would let the
> > kernel clean them up faster and reduce the likelihood that a fence
> > will hang around forever because it keeps getting added to arrays with
> > other unsignaled fences.
>
> I think so. Can't think of a good reason why we would want to add
> already signaled fences to the array.
>
> Christian.
>
> >
> > --Jason
>

[PATCH 1/3] dma-buf: add dma_fence_array_for_each (v2)

[Jason Ekstrand]

From: Christian König <ckoenig.leichtzumerken@gmail.com>

Add a helper to iterate over all fences in a dma_fence_array object.

v2 (Jason Ekstrand)
 - Return NULL from dma_fence_array_first if head == NULL.  This matches
   the iterator behavior of dma_fence_chain_for_each in that it iterates
   zero times if head == NULL.
 - Return NULL from dma_fence_array_next if index > array->num_fences.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
---
 drivers/dma-buf/dma-fence-array.c | 27 +++++++++++++++++++++++++++
 include/linux/dma-fence-array.h   | 17 +++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/drivers/dma-buf/dma-fence-array.c b/drivers/dma-buf/dma-fence-array.c
index d3fbd950be94..2ac1afc697d0 100644
--- a/drivers/dma-buf/dma-fence-array.c
+++ b/drivers/dma-buf/dma-fence-array.c
@@ -201,3 +201,30 @@ bool dma_fence_match_context(struct dma_fence *fence, u64 context)
 	return true;
 }
 EXPORT_SYMBOL(dma_fence_match_context);
+
+struct dma_fence *dma_fence_array_first(struct dma_fence *head)
+{
+	struct dma_fence_array *array;
+
+	if (!head)
+		return NULL;
+
+	array = to_dma_fence_array(head);
+	if (!array)
+		return head;
+
+	return array->fences[0];
+}
+EXPORT_SYMBOL(dma_fence_array_first);
+
+struct dma_fence *dma_fence_array_next(struct dma_fence *head,
+				       unsigned int index)
+{
+	struct dma_fence_array *array = to_dma_fence_array(head);
+
+	if (!array || index >= array->num_fences)
+		return NULL;
+
+	return array->fences[index];
+}
+EXPORT_SYMBOL(dma_fence_array_next);
diff --git a/include/linux/dma-fence-array.h b/include/linux/dma-fence-array.h
index 303dd712220f..588ac8089dd6 100644
--- a/include/linux/dma-fence-array.h
+++ b/include/linux/dma-fence-array.h
@@ -74,6 +74,19 @@ to_dma_fence_array(struct dma_fence *fence)
 	return container_of(fence, struct dma_fence_array, base);
 }
 
+/**
+ * dma_fence_array_for_each - iterate over all fences in array
+ * @fence: current fence
+ * @index: index into the array
+ * @head: potential dma_fence_array object
+ *
+ * Test if @array is a dma_fence_array object and if yes iterate over all fences
+ * in the array. If not just iterate over the fence in @array itself.
+ */
+#define dma_fence_array_for_each(fence, index, head)			\
+	for (index = 0, fence = dma_fence_array_first(head); fence;	\
+	     ++(index), fence = dma_fence_array_next(head, index))
+
 struct dma_fence_array *dma_fence_array_create(int num_fences,
 					       struct dma_fence **fences,
 					       u64 context, unsigned seqno,
@@ -81,4 +94,8 @@ struct dma_fence_array *dma_fence_array_create(int num_fences,
 
 bool dma_fence_match_context(struct dma_fence *fence, u64 context);
 
+struct dma_fence *dma_fence_array_first(struct dma_fence *head);
+struct dma_fence *dma_fence_array_next(struct dma_fence *head,
+				       unsigned int index);
+
 #endif /* __LINUX_DMA_FENCE_ARRAY_H */
-- 
2.24.1

[PATCH 2/3] dma-buf: add dma_resv_get_singleton (v2)

[Jason Ekstrand]

From: Christian König <ckoenig.leichtzumerken@gmail.com>

Add a helper function to get a single fence representing
all fences in a dma_resv object.

This fence is either the only one in the object or all not
signaled fences of the object in a flatted out dma_fence_array.

v2 (Jason Ekstrand):
 - Take reference of fences both for creating the dma_fence_array and in
   the case where we return one fence.
 - Handle the case where dma_resv_get_list() returns NULL

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
---
 drivers/dma-buf/dma-resv.c | 118 +++++++++++++++++++++++++++++++++++++
 include/linux/dma-resv.h   |   3 +
 2 files changed, 121 insertions(+)

diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 4264e64788c4..66591d8ab7ef 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -33,6 +33,8 @@
  */
 
 #include <linux/dma-resv.h>
+#include <linux/dma-fence-chain.h>
+#include <linux/dma-fence-array.h>
 #include <linux/export.h>
 #include <linux/sched/mm.h>
 
@@ -47,6 +49,19 @@
  * write-side updates.
  */
 
+/**
+ * dma_fence_deep_dive_for_each - deep dive into the fence containers
+ * @fence: resulting fence
+ * @chain: variable for a dma_fence_chain
+ * @index: index into a dma_fence_array
+ * @head: starting point
+ *
+ * Helper to deep dive into the fence containers for flattening them.
+ */
+#define dma_fence_deep_dive_for_each(fence, chain, index, head)	\
+	dma_fence_chain_for_each(chain, head)			\
+		dma_fence_array_for_each(fence, index, chain)
+
 DEFINE_WD_CLASS(reservation_ww_class);
 EXPORT_SYMBOL(reservation_ww_class);
 
@@ -516,6 +531,109 @@ int dma_resv_get_fences_rcu(struct dma_resv *obj,
 }
 EXPORT_SYMBOL_GPL(dma_resv_get_fences_rcu);
 
+/**
+ * dma_resv_get_singleton - get a single fence for the dma_resv object
+ * @obj: the reservation object
+ * @extra: extra fence to add to the resulting array
+ * @result: resulting dma_fence
+ *
+ * Get a single fence representing all unsignaled fences in the dma_resv object
+ * plus the given extra fence. If we got only one fence return a new
+ * reference to that, otherwise return a dma_fence_array object.
+ *
+ * RETURNS
+ * Returns -NOMEM if allocations fail, zero otherwise.
+ */
+int dma_resv_get_singleton(struct dma_resv *obj, struct dma_fence *extra,
+			   struct dma_fence **result)
+{
+	struct dma_resv_list *fobj = dma_resv_get_list(obj);
+	struct dma_fence *excl = dma_resv_get_excl(obj);
+	struct dma_fence *fence, *chain, **fences;
+	struct dma_fence_array *array;
+	unsigned int num_fences, shared_count;
+	unsigned int i, j;
+
+	num_fences = 0;
+	*result = NULL;
+
+	dma_fence_deep_dive_for_each(fence, chain, i, extra) {
+		if (dma_fence_is_signaled(fence))
+			continue;
+
+		*result = fence;
+		++num_fences;
+	}
+
+	dma_fence_deep_dive_for_each(fence, chain, i, excl) {
+		if (dma_fence_is_signaled(fence))
+			continue;
+
+		*result = fence;
+		++num_fences;
+	}
+
+	shared_count = fobj ? fobj->shared_count : 0;
+	for (i = 0; i < shared_count; ++i) {
+		struct dma_fence *f;
+
+		f = rcu_dereference_protected(fobj->shared[i],
+					      dma_resv_held(obj));
+		dma_fence_deep_dive_for_each(fence, chain, j, f) {
+			if (dma_fence_is_signaled(fence))
+				continue;
+
+			*result = fence;
+			++num_fences;
+		}
+	}
+
+	if (num_fences <= 1) {
+		*result = dma_fence_get(*result);
+		return 0;
+	}
+
+	fences = kmalloc_array(num_fences, sizeof(struct dma_fence*),
+			       GFP_KERNEL);
+	if (!fences)
+		return -ENOMEM;
+
+	num_fences = 0;
+
+	dma_fence_deep_dive_for_each(fence, chain, i, extra)
+		if (!dma_fence_is_signaled(fence))
+			fences[num_fences++] = dma_fence_get(fence);
+
+	dma_fence_deep_dive_for_each(fence, chain, i, excl)
+		if (!dma_fence_is_signaled(fence))
+			fences[num_fences++] = dma_fence_get(fence);
+
+	for (i = 0; i < shared_count; ++i) {
+		struct dma_fence *f;
+
+		f = rcu_dereference_protected(fobj->shared[i],
+					      dma_resv_held(obj));
+		dma_fence_deep_dive_for_each(fence, chain, j, f)
+			if (!dma_fence_is_signaled(fence))
+				fences[num_fences++] = dma_fence_get(fence);
+	}
+
+	array = dma_fence_array_create(num_fences, fences,
+				       dma_fence_context_alloc(1),
+				       1, false);
+	if (!array)
+		goto error_free;
+
+	*result = &array->base;
+	return 0;
+
+error_free:
+	while (num_fences--)
+		dma_fence_put(fences[num_fences]);
+	kfree(fences);
+	return -ENOMEM;
+}
+
 /**
  * dma_resv_wait_timeout_rcu - Wait on reservation's objects
  * shared and/or exclusive fences.
diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index ee50d10f052b..d50e753e4550 100644
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h
@@ -287,6 +287,9 @@ int dma_resv_get_fences_rcu(struct dma_resv *obj,
 
 int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src);
 
+int dma_resv_get_singleton(struct dma_resv *obj, struct dma_fence *extra,
+			   struct dma_fence **result);
+
 long dma_resv_wait_timeout_rcu(struct dma_resv *obj, bool wait_all, bool intr,
 			       unsigned long timeout);
 
-- 
2.24.1

[PATCH 3/3] RFC: dma-buf: Add an API for importing and exporting sync files (v4)

[Jason Ekstrand]

Explicit synchronization is the future.  At least, that seems to be what
most userspace APIs are agreeing on at this point.  However, most of our
Linux APIs (both userspace and kernel UAPI) are currently built around
implicit synchronization with dma-buf.  While work is ongoing to change
many of the userspace APIs and protocols to an explicit synchronization
model, switching over piecemeal is difficult due to the number of
potential components involved.  On the kernel side, many drivers use
dma-buf including GPU (3D/compute), display, v4l, and others.  In
userspace, we have X11, several Wayland compositors, 3D drivers, compute
drivers (OpenCL etc.), media encode/decode, and the list goes on.

This patch provides a path forward by allowing userspace to manually
manage the fences attached to a dma-buf.  Alternatively, one can think
of this as making dma-buf's implicit synchronization simply a carrier
for an explicit fence.  This is accomplished by adding two IOCTLs to
dma-buf for importing and exporting a sync file to/from the dma-buf.
This way a userspace component which is uses explicit synchronization,
such as a Vulkan driver, can manually set the write fence on a buffer
before handing it off to an implicitly synchronized component such as a
Wayland compositor or video encoder.  In this way, each of the different
components can be upgraded to an explicit synchronization model one at a
time as long as the userspace pieces connecting them are aware of it and
import/export fences at the right times.

There is a potential race condition with this API if userspace is not
careful.  A typical use case for implicit synchronization is to wait for
the dma-buf to be ready, use it, and then signal it for some other
component.  Because a sync_file cannot be created until it is guaranteed
to complete in finite time, userspace can only signal the dma-buf after
it has already submitted the work which uses it to the kernel and has
received a sync_file back.  There is no way to atomically submit a
wait-use-signal operation.  This is not, however, really a problem with
this API so much as it is a problem with explicit synchronization
itself.  The way this is typically handled is to have very explicit
ownership transfer points in the API or protocol which ensure that only
one component is using it at any given time.  Both X11 (via the PRESENT
extension) and Wayland provide such ownership transfer points via
explicit present and idle messages.

The decision was intentionally made in this patch to make the import and
export operations IOCTLs on the dma-buf itself rather than as a DRM
IOCTL.  This makes it the import/export operation universal across all
components which use dma-buf including GPU, display, v4l, and others.
It also means that a userspace component can do the import/export
without access to the DRM fd which may be tricky to get in cases where
the client communicates with DRM via a userspace API such as OpenGL or
Vulkan.  At a future date we may choose to add direct import/export APIs
to components such as drm_syncobj to avoid allocating a file descriptor
and going through two ioctls.  However, that seems to be something of a
micro-optimization as import/export operations are likely to happen at a
rate of a few per frame of rendered or decoded video.

v2 (Jason Ekstrand):
 - Use a wrapper dma_fence_array of all fences including the new one
   when importing an exclusive fence.

v3 (Jason Ekstrand):
 - Lock around setting shared fences as well as exclusive
 - Mark SIGNAL_SYNC_FILE as a read-write ioctl.
 - Initialize ret to 0 in dma_buf_wait_sync_file

v4 (Jason Ekstrand):
 - Use the new dma_resv_get_singleton helper

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
---
 drivers/dma-buf/dma-buf.c    | 96 ++++++++++++++++++++++++++++++++++++
 include/uapi/linux/dma-buf.h | 13 ++++-
 2 files changed, 107 insertions(+), 2 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index d4097856c86b..09973c689866 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -20,6 +20,7 @@
 #include <linux/debugfs.h>
 #include <linux/module.h>
 #include <linux/seq_file.h>
+#include <linux/sync_file.h>
 #include <linux/poll.h>
 #include <linux/dma-resv.h>
 #include <linux/mm.h>
@@ -348,6 +349,95 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
 	return ret;
 }
 
+static long dma_buf_wait_sync_file(struct dma_buf *dmabuf,
+				   const void __user *user_data)
+{
+	struct dma_buf_sync_file arg;
+	struct dma_fence *fence;
+	int ret = 0;
+
+	if (copy_from_user(&arg, user_data, sizeof(arg)))
+		return -EFAULT;
+
+	if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
+		return -EINVAL;
+
+	fence = sync_file_get_fence(arg.fd);
+	if (!fence)
+		return -EINVAL;
+
+	dma_resv_lock(dmabuf->resv, NULL);
+
+	if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
+		struct dma_fence *singleton = NULL;
+		ret = dma_resv_get_singleton(dmabuf->resv, fence, &singleton);
+		if (!ret && singleton)
+			dma_resv_add_excl_fence(dmabuf->resv, singleton);
+	} else {
+		dma_resv_add_shared_fence(dmabuf->resv, fence);
+	}
+
+	dma_resv_unlock(dmabuf->resv);
+
+	dma_fence_put(fence);
+
+	return ret;
+}
+
+static long dma_buf_signal_sync_file(struct dma_buf *dmabuf,
+				     void __user *user_data)
+{
+	struct dma_buf_sync_file arg;
+	struct dma_fence *fence = NULL;
+	struct sync_file *sync_file;
+	int fd, ret;
+
+	if (copy_from_user(&arg, user_data, sizeof(arg)))
+		return -EFAULT;
+
+	if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
+		return -EINVAL;
+
+	fd = get_unused_fd_flags(O_CLOEXEC);
+	if (fd < 0)
+		return fd;
+
+	if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
+		/* We need to include both the exclusive fence and all of
+		 * the shared fences in our fence.
+		 */
+		ret = dma_resv_get_singleton(dmabuf->resv, NULL, &fence);
+		if (ret)
+			goto err_put_fd;
+	} else {
+		fence = dma_resv_get_excl_rcu(dmabuf->resv);
+	}
+
+	if (!fence)
+		fence = dma_fence_get_stub();
+
+	sync_file = sync_file_create(fence);
+
+	dma_fence_put(fence);
+
+	if (!sync_file) {
+		ret = -EINVAL;
+		goto err_put_fd;
+	}
+
+	fd_install(fd, sync_file->file);
+
+	arg.fd = fd;
+	if (copy_to_user(user_data, &arg, sizeof(arg)))
+		return -EFAULT;
+
+	return 0;
+
+err_put_fd:
+	put_unused_fd(fd);
+	return ret;
+}
+
 static long dma_buf_ioctl(struct file *file,
 			  unsigned int cmd, unsigned long arg)
 {
@@ -390,6 +480,12 @@ static long dma_buf_ioctl(struct file *file,
 	case DMA_BUF_SET_NAME:
 		return dma_buf_set_name(dmabuf, (const char __user *)arg);
 
+	case DMA_BUF_IOCTL_WAIT_SYNC_FILE:
+		return dma_buf_wait_sync_file(dmabuf, (const void __user *)arg);
+
+	case DMA_BUF_IOCTL_SIGNAL_SYNC_FILE:
+		return dma_buf_signal_sync_file(dmabuf, (void __user *)arg);
+
 	default:
 		return -ENOTTY;
 	}
diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
index dbc7092e04b5..86e07acca90c 100644
--- a/include/uapi/linux/dma-buf.h
+++ b/include/uapi/linux/dma-buf.h
@@ -37,8 +37,17 @@ struct dma_buf_sync {
 
 #define DMA_BUF_NAME_LEN	32
 
+struct dma_buf_sync_file {
+	__u32 flags;
+	__s32 fd;
+};
+
+#define DMA_BUF_SYNC_FILE_SYNC_WRITE	(1 << 0)
+
 #define DMA_BUF_BASE		'b'
-#define DMA_BUF_IOCTL_SYNC	_IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
-#define DMA_BUF_SET_NAME	_IOW(DMA_BUF_BASE, 1, const char *)
+#define DMA_BUF_IOCTL_SYNC	    _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
+#define DMA_BUF_SET_NAME	    _IOW(DMA_BUF_BASE, 1, const char *)
+#define DMA_BUF_IOCTL_WAIT_SYNC_FILE	_IOW(DMA_BUF_BASE, 2, struct dma_buf_sync)
+#define DMA_BUF_IOCTL_SIGNAL_SYNC_FILE	_IOWR(DMA_BUF_BASE, 3, struct dma_buf_sync)
 
 #endif
-- 
2.24.1

Re: [PATCH 3/3] RFC: dma-buf: Add an API for importing and exporting sync files (v4)

[Christian König]

Am 11.03.20 um 04:43 schrieb Jason Ekstrand:
> Explicit synchronization is the future.  At least, that seems to be what
> most userspace APIs are agreeing on at this point.  However, most of our
> Linux APIs (both userspace and kernel UAPI) are currently built around
> implicit synchronization with dma-buf.  While work is ongoing to change
> many of the userspace APIs and protocols to an explicit synchronization
> model, switching over piecemeal is difficult due to the number of
> potential components involved.  On the kernel side, many drivers use
> dma-buf including GPU (3D/compute), display, v4l, and others.  In
> userspace, we have X11, several Wayland compositors, 3D drivers, compute
> drivers (OpenCL etc.), media encode/decode, and the list goes on.
>
> This patch provides a path forward by allowing userspace to manually
> manage the fences attached to a dma-buf.  Alternatively, one can think
> of this as making dma-buf's implicit synchronization simply a carrier
> for an explicit fence.  This is accomplished by adding two IOCTLs to
> dma-buf for importing and exporting a sync file to/from the dma-buf.
> This way a userspace component which is uses explicit synchronization,
> such as a Vulkan driver, can manually set the write fence on a buffer
> before handing it off to an implicitly synchronized component such as a
> Wayland compositor or video encoder.  In this way, each of the different
> components can be upgraded to an explicit synchronization model one at a
> time as long as the userspace pieces connecting them are aware of it and
> import/export fences at the right times.
>
> There is a potential race condition with this API if userspace is not
> careful.  A typical use case for implicit synchronization is to wait for
> the dma-buf to be ready, use it, and then signal it for some other
> component.  Because a sync_file cannot be created until it is guaranteed
> to complete in finite time, userspace can only signal the dma-buf after
> it has already submitted the work which uses it to the kernel and has
> received a sync_file back.  There is no way to atomically submit a
> wait-use-signal operation.  This is not, however, really a problem with
> this API so much as it is a problem with explicit synchronization
> itself.  The way this is typically handled is to have very explicit
> ownership transfer points in the API or protocol which ensure that only
> one component is using it at any given time.  Both X11 (via the PRESENT
> extension) and Wayland provide such ownership transfer points via
> explicit present and idle messages.
>
> The decision was intentionally made in this patch to make the import and
> export operations IOCTLs on the dma-buf itself rather than as a DRM
> IOCTL.  This makes it the import/export operation universal across all
> components which use dma-buf including GPU, display, v4l, and others.
> It also means that a userspace component can do the import/export
> without access to the DRM fd which may be tricky to get in cases where
> the client communicates with DRM via a userspace API such as OpenGL or
> Vulkan.  At a future date we may choose to add direct import/export APIs
> to components such as drm_syncobj to avoid allocating a file descriptor
> and going through two ioctls.  However, that seems to be something of a
> micro-optimization as import/export operations are likely to happen at a
> rate of a few per frame of rendered or decoded video.
>
> v2 (Jason Ekstrand):
>   - Use a wrapper dma_fence_array of all fences including the new one
>     when importing an exclusive fence.
>
> v3 (Jason Ekstrand):
>   - Lock around setting shared fences as well as exclusive
>   - Mark SIGNAL_SYNC_FILE as a read-write ioctl.
>   - Initialize ret to 0 in dma_buf_wait_sync_file
>
> v4 (Jason Ekstrand):
>   - Use the new dma_resv_get_singleton helper
>
> Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> ---
>   drivers/dma-buf/dma-buf.c    | 96 ++++++++++++++++++++++++++++++++++++
>   include/uapi/linux/dma-buf.h | 13 ++++-
>   2 files changed, 107 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index d4097856c86b..09973c689866 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -20,6 +20,7 @@
>   #include <linux/debugfs.h>
>   #include <linux/module.h>
>   #include <linux/seq_file.h>
> +#include <linux/sync_file.h>
>   #include <linux/poll.h>
>   #include <linux/dma-resv.h>
>   #include <linux/mm.h>
> @@ -348,6 +349,95 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
>   	return ret;
>   }
>   
> +static long dma_buf_wait_sync_file(struct dma_buf *dmabuf,
> +				   const void __user *user_data)
> +{
> +	struct dma_buf_sync_file arg;
> +	struct dma_fence *fence;
> +	int ret = 0;
> +
> +	if (copy_from_user(&arg, user_data, sizeof(arg)))
> +		return -EFAULT;
> +
> +	if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
> +		return -EINVAL;
> +
> +	fence = sync_file_get_fence(arg.fd);
> +	if (!fence)
> +		return -EINVAL;
> +
> +	dma_resv_lock(dmabuf->resv, NULL);
> +
> +	if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
> +		struct dma_fence *singleton = NULL;
> +		ret = dma_resv_get_singleton(dmabuf->resv, fence, &singleton);
> +		if (!ret && singleton)
> +			dma_resv_add_excl_fence(dmabuf->resv, singleton);
> +	} else {
> +		dma_resv_add_shared_fence(dmabuf->resv, fence);
> +	}

You also need to create a singleton when adding a shared fences.

The problem is that shared fences must always signal after exclusive 
ones and you can't guarantee that for the fence you add here.

Regards,
Christian.

> +
> +	dma_resv_unlock(dmabuf->resv);
> +
> +	dma_fence_put(fence);
> +
> +	return ret;
> +}
> +
> +static long dma_buf_signal_sync_file(struct dma_buf *dmabuf,
> +				     void __user *user_data)
> +{
> +	struct dma_buf_sync_file arg;
> +	struct dma_fence *fence = NULL;
> +	struct sync_file *sync_file;
> +	int fd, ret;
> +
> +	if (copy_from_user(&arg, user_data, sizeof(arg)))
> +		return -EFAULT;
> +
> +	if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
> +		return -EINVAL;
> +
> +	fd = get_unused_fd_flags(O_CLOEXEC);
> +	if (fd < 0)
> +		return fd;
> +
> +	if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
> +		/* We need to include both the exclusive fence and all of
> +		 * the shared fences in our fence.
> +		 */
> +		ret = dma_resv_get_singleton(dmabuf->resv, NULL, &fence);
> +		if (ret)
> +			goto err_put_fd;
> +	} else {
> +		fence = dma_resv_get_excl_rcu(dmabuf->resv);
> +	}
> +
> +	if (!fence)
> +		fence = dma_fence_get_stub();
> +
> +	sync_file = sync_file_create(fence);
> +
> +	dma_fence_put(fence);
> +
> +	if (!sync_file) {
> +		ret = -EINVAL;
> +		goto err_put_fd;
> +	}
> +
> +	fd_install(fd, sync_file->file);
> +
> +	arg.fd = fd;
> +	if (copy_to_user(user_data, &arg, sizeof(arg)))
> +		return -EFAULT;
> +
> +	return 0;
> +
> +err_put_fd:
> +	put_unused_fd(fd);
> +	return ret;
> +}
> +
>   static long dma_buf_ioctl(struct file *file,
>   			  unsigned int cmd, unsigned long arg)
>   {
> @@ -390,6 +480,12 @@ static long dma_buf_ioctl(struct file *file,
>   	case DMA_BUF_SET_NAME:
>   		return dma_buf_set_name(dmabuf, (const char __user *)arg);
>   
> +	case DMA_BUF_IOCTL_WAIT_SYNC_FILE:
> +		return dma_buf_wait_sync_file(dmabuf, (const void __user *)arg);
> +
> +	case DMA_BUF_IOCTL_SIGNAL_SYNC_FILE:
> +		return dma_buf_signal_sync_file(dmabuf, (void __user *)arg);
> +
>   	default:
>   		return -ENOTTY;
>   	}
> diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
> index dbc7092e04b5..86e07acca90c 100644
> --- a/include/uapi/linux/dma-buf.h
> +++ b/include/uapi/linux/dma-buf.h
> @@ -37,8 +37,17 @@ struct dma_buf_sync {
>   
>   #define DMA_BUF_NAME_LEN	32
>   
> +struct dma_buf_sync_file {
> +	__u32 flags;
> +	__s32 fd;
> +};
> +
> +#define DMA_BUF_SYNC_FILE_SYNC_WRITE	(1 << 0)
> +
>   #define DMA_BUF_BASE		'b'
> -#define DMA_BUF_IOCTL_SYNC	_IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
> -#define DMA_BUF_SET_NAME	_IOW(DMA_BUF_BASE, 1, const char *)
> +#define DMA_BUF_IOCTL_SYNC	    _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
> +#define DMA_BUF_SET_NAME	    _IOW(DMA_BUF_BASE, 1, const char *)
> +#define DMA_BUF_IOCTL_WAIT_SYNC_FILE	_IOW(DMA_BUF_BASE, 2, struct dma_buf_sync)
> +#define DMA_BUF_IOCTL_SIGNAL_SYNC_FILE	_IOWR(DMA_BUF_BASE, 3, struct dma_buf_sync)
>   
>   #endif

Re: [PATCH 3/3] RFC: dma-buf: Add an API for importing and exporting sync files (v4)

[Jason Ekstrand]

On Wed, Mar 11, 2020 at 8:18 AM Christian König
<christian.koenig@amd.com> wrote:
>
> Am 11.03.20 um 04:43 schrieb Jason Ekstrand:
> > Explicit synchronization is the future.  At least, that seems to be what
> > most userspace APIs are agreeing on at this point.  However, most of our
> > Linux APIs (both userspace and kernel UAPI) are currently built around
> > implicit synchronization with dma-buf.  While work is ongoing to change
> > many of the userspace APIs and protocols to an explicit synchronization
> > model, switching over piecemeal is difficult due to the number of
> > potential components involved.  On the kernel side, many drivers use
> > dma-buf including GPU (3D/compute), display, v4l, and others.  In
> > userspace, we have X11, several Wayland compositors, 3D drivers, compute
> > drivers (OpenCL etc.), media encode/decode, and the list goes on.
> >
> > This patch provides a path forward by allowing userspace to manually
> > manage the fences attached to a dma-buf.  Alternatively, one can think
> > of this as making dma-buf's implicit synchronization simply a carrier
> > for an explicit fence.  This is accomplished by adding two IOCTLs to
> > dma-buf for importing and exporting a sync file to/from the dma-buf.
> > This way a userspace component which is uses explicit synchronization,
> > such as a Vulkan driver, can manually set the write fence on a buffer
> > before handing it off to an implicitly synchronized component such as a
> > Wayland compositor or video encoder.  In this way, each of the different
> > components can be upgraded to an explicit synchronization model one at a
> > time as long as the userspace pieces connecting them are aware of it and
> > import/export fences at the right times.
> >
> > There is a potential race condition with this API if userspace is not
> > careful.  A typical use case for implicit synchronization is to wait for
> > the dma-buf to be ready, use it, and then signal it for some other
> > component.  Because a sync_file cannot be created until it is guaranteed
> > to complete in finite time, userspace can only signal the dma-buf after
> > it has already submitted the work which uses it to the kernel and has
> > received a sync_file back.  There is no way to atomically submit a
> > wait-use-signal operation.  This is not, however, really a problem with
> > this API so much as it is a problem with explicit synchronization
> > itself.  The way this is typically handled is to have very explicit
> > ownership transfer points in the API or protocol which ensure that only
> > one component is using it at any given time.  Both X11 (via the PRESENT
> > extension) and Wayland provide such ownership transfer points via
> > explicit present and idle messages.
> >
> > The decision was intentionally made in this patch to make the import and
> > export operations IOCTLs on the dma-buf itself rather than as a DRM
> > IOCTL.  This makes it the import/export operation universal across all
> > components which use dma-buf including GPU, display, v4l, and others.
> > It also means that a userspace component can do the import/export
> > without access to the DRM fd which may be tricky to get in cases where
> > the client communicates with DRM via a userspace API such as OpenGL or
> > Vulkan.  At a future date we may choose to add direct import/export APIs
> > to components such as drm_syncobj to avoid allocating a file descriptor
> > and going through two ioctls.  However, that seems to be something of a
> > micro-optimization as import/export operations are likely to happen at a
> > rate of a few per frame of rendered or decoded video.
> >
> > v2 (Jason Ekstrand):
> >   - Use a wrapper dma_fence_array of all fences including the new one
> >     when importing an exclusive fence.
> >
> > v3 (Jason Ekstrand):
> >   - Lock around setting shared fences as well as exclusive
> >   - Mark SIGNAL_SYNC_FILE as a read-write ioctl.
> >   - Initialize ret to 0 in dma_buf_wait_sync_file
> >
> > v4 (Jason Ekstrand):
> >   - Use the new dma_resv_get_singleton helper
> >
> > Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> > ---
> >   drivers/dma-buf/dma-buf.c    | 96 ++++++++++++++++++++++++++++++++++++
> >   include/uapi/linux/dma-buf.h | 13 ++++-
> >   2 files changed, 107 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > index d4097856c86b..09973c689866 100644
> > --- a/drivers/dma-buf/dma-buf.c
> > +++ b/drivers/dma-buf/dma-buf.c
> > @@ -20,6 +20,7 @@
> >   #include <linux/debugfs.h>
> >   #include <linux/module.h>
> >   #include <linux/seq_file.h>
> > +#include <linux/sync_file.h>
> >   #include <linux/poll.h>
> >   #include <linux/dma-resv.h>
> >   #include <linux/mm.h>
> > @@ -348,6 +349,95 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
> >       return ret;
> >   }
> >
> > +static long dma_buf_wait_sync_file(struct dma_buf *dmabuf,
> > +                                const void __user *user_data)
> > +{
> > +     struct dma_buf_sync_file arg;
> > +     struct dma_fence *fence;
> > +     int ret = 0;
> > +
> > +     if (copy_from_user(&arg, user_data, sizeof(arg)))
> > +             return -EFAULT;
> > +
> > +     if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
> > +             return -EINVAL;
> > +
> > +     fence = sync_file_get_fence(arg.fd);
> > +     if (!fence)
> > +             return -EINVAL;
> > +
> > +     dma_resv_lock(dmabuf->resv, NULL);
> > +
> > +     if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
> > +             struct dma_fence *singleton = NULL;
> > +             ret = dma_resv_get_singleton(dmabuf->resv, fence, &singleton);
> > +             if (!ret && singleton)
> > +                     dma_resv_add_excl_fence(dmabuf->resv, singleton);
> > +     } else {
> > +             dma_resv_add_shared_fence(dmabuf->resv, fence);
> > +     }
>
> You also need to create a singleton when adding a shared fences.
>
> The problem is that shared fences must always signal after exclusive
> ones and you can't guarantee that for the fence you add here.

I'm beginning to think that I should just drop the flags and always
wait on all fences and always take what's currently the "write" path.
Otherwise, something's going to get it wrong somewhere.  Thoughts?

Also, Michelle (added to CC) commented on IRC today that amdgpu does
something with implicit sync fences where it sorts out the fences
which affect one queue vs. others.  He thought that stuffing fences in
the dma-buf in this way might cause that to not work.  Thoughts?

--Jason


> Regards,
> Christian.
>
> > +
> > +     dma_resv_unlock(dmabuf->resv);
> > +
> > +     dma_fence_put(fence);
> > +
> > +     return ret;
> > +}
> > +
> > +static long dma_buf_signal_sync_file(struct dma_buf *dmabuf,
> > +                                  void __user *user_data)
> > +{
> > +     struct dma_buf_sync_file arg;
> > +     struct dma_fence *fence = NULL;
> > +     struct sync_file *sync_file;
> > +     int fd, ret;
> > +
> > +     if (copy_from_user(&arg, user_data, sizeof(arg)))
> > +             return -EFAULT;
> > +
> > +     if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
> > +             return -EINVAL;
> > +
> > +     fd = get_unused_fd_flags(O_CLOEXEC);
> > +     if (fd < 0)
> > +             return fd;
> > +
> > +     if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
> > +             /* We need to include both the exclusive fence and all of
> > +              * the shared fences in our fence.
> > +              */
> > +             ret = dma_resv_get_singleton(dmabuf->resv, NULL, &fence);
> > +             if (ret)
> > +                     goto err_put_fd;
> > +     } else {
> > +             fence = dma_resv_get_excl_rcu(dmabuf->resv);
> > +     }
> > +
> > +     if (!fence)
> > +             fence = dma_fence_get_stub();
> > +
> > +     sync_file = sync_file_create(fence);
> > +
> > +     dma_fence_put(fence);
> > +
> > +     if (!sync_file) {
> > +             ret = -EINVAL;
> > +             goto err_put_fd;
> > +     }
> > +
> > +     fd_install(fd, sync_file->file);
> > +
> > +     arg.fd = fd;
> > +     if (copy_to_user(user_data, &arg, sizeof(arg)))
> > +             return -EFAULT;
> > +
> > +     return 0;
> > +
> > +err_put_fd:
> > +     put_unused_fd(fd);
> > +     return ret;
> > +}
> > +
> >   static long dma_buf_ioctl(struct file *file,
> >                         unsigned int cmd, unsigned long arg)
> >   {
> > @@ -390,6 +480,12 @@ static long dma_buf_ioctl(struct file *file,
> >       case DMA_BUF_SET_NAME:
> >               return dma_buf_set_name(dmabuf, (const char __user *)arg);
> >
> > +     case DMA_BUF_IOCTL_WAIT_SYNC_FILE:
> > +             return dma_buf_wait_sync_file(dmabuf, (const void __user *)arg);
> > +
> > +     case DMA_BUF_IOCTL_SIGNAL_SYNC_FILE:
> > +             return dma_buf_signal_sync_file(dmabuf, (void __user *)arg);
> > +
> >       default:
> >               return -ENOTTY;
> >       }
> > diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
> > index dbc7092e04b5..86e07acca90c 100644
> > --- a/include/uapi/linux/dma-buf.h
> > +++ b/include/uapi/linux/dma-buf.h
> > @@ -37,8 +37,17 @@ struct dma_buf_sync {
> >
> >   #define DMA_BUF_NAME_LEN    32
> >
> > +struct dma_buf_sync_file {
> > +     __u32 flags;
> > +     __s32 fd;
> > +};
> > +
> > +#define DMA_BUF_SYNC_FILE_SYNC_WRITE (1 << 0)
> > +
> >   #define DMA_BUF_BASE                'b'
> > -#define DMA_BUF_IOCTL_SYNC   _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
> > -#define DMA_BUF_SET_NAME     _IOW(DMA_BUF_BASE, 1, const char *)
> > +#define DMA_BUF_IOCTL_SYNC       _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
> > +#define DMA_BUF_SET_NAME         _IOW(DMA_BUF_BASE, 1, const char *)
> > +#define DMA_BUF_IOCTL_WAIT_SYNC_FILE _IOW(DMA_BUF_BASE, 2, struct dma_buf_sync)
> > +#define DMA_BUF_IOCTL_SIGNAL_SYNC_FILE       _IOWR(DMA_BUF_BASE, 3, struct dma_buf_sync)
> >
> >   #endif
>

Re: [PATCH 3/3] RFC: dma-buf: Add an API for importing and exporting sync files (v4)

[Christian König]

Am 12.03.20 um 16:57 schrieb Jason Ekstrand:
> On Wed, Mar 11, 2020 at 8:18 AM Christian König
> <christian.koenig@amd.com> wrote:
>> Am 11.03.20 um 04:43 schrieb Jason Ekstrand:
>>> Explicit synchronization is the future.  At least, that seems to be what
>>> most userspace APIs are agreeing on at this point.  However, most of our
>>> Linux APIs (both userspace and kernel UAPI) are currently built around
>>> implicit synchronization with dma-buf.  While work is ongoing to change
>>> many of the userspace APIs and protocols to an explicit synchronization
>>> model, switching over piecemeal is difficult due to the number of
>>> potential components involved.  On the kernel side, many drivers use
>>> dma-buf including GPU (3D/compute), display, v4l, and others.  In
>>> userspace, we have X11, several Wayland compositors, 3D drivers, compute
>>> drivers (OpenCL etc.), media encode/decode, and the list goes on.
>>>
>>> This patch provides a path forward by allowing userspace to manually
>>> manage the fences attached to a dma-buf.  Alternatively, one can think
>>> of this as making dma-buf's implicit synchronization simply a carrier
>>> for an explicit fence.  This is accomplished by adding two IOCTLs to
>>> dma-buf for importing and exporting a sync file to/from the dma-buf.
>>> This way a userspace component which is uses explicit synchronization,
>>> such as a Vulkan driver, can manually set the write fence on a buffer
>>> before handing it off to an implicitly synchronized component such as a
>>> Wayland compositor or video encoder.  In this way, each of the different
>>> components can be upgraded to an explicit synchronization model one at a
>>> time as long as the userspace pieces connecting them are aware of it and
>>> import/export fences at the right times.
>>>
>>> There is a potential race condition with this API if userspace is not
>>> careful.  A typical use case for implicit synchronization is to wait for
>>> the dma-buf to be ready, use it, and then signal it for some other
>>> component.  Because a sync_file cannot be created until it is guaranteed
>>> to complete in finite time, userspace can only signal the dma-buf after
>>> it has already submitted the work which uses it to the kernel and has
>>> received a sync_file back.  There is no way to atomically submit a
>>> wait-use-signal operation.  This is not, however, really a problem with
>>> this API so much as it is a problem with explicit synchronization
>>> itself.  The way this is typically handled is to have very explicit
>>> ownership transfer points in the API or protocol which ensure that only
>>> one component is using it at any given time.  Both X11 (via the PRESENT
>>> extension) and Wayland provide such ownership transfer points via
>>> explicit present and idle messages.
>>>
>>> The decision was intentionally made in this patch to make the import and
>>> export operations IOCTLs on the dma-buf itself rather than as a DRM
>>> IOCTL.  This makes it the import/export operation universal across all
>>> components which use dma-buf including GPU, display, v4l, and others.
>>> It also means that a userspace component can do the import/export
>>> without access to the DRM fd which may be tricky to get in cases where
>>> the client communicates with DRM via a userspace API such as OpenGL or
>>> Vulkan.  At a future date we may choose to add direct import/export APIs
>>> to components such as drm_syncobj to avoid allocating a file descriptor
>>> and going through two ioctls.  However, that seems to be something of a
>>> micro-optimization as import/export operations are likely to happen at a
>>> rate of a few per frame of rendered or decoded video.
>>>
>>> v2 (Jason Ekstrand):
>>>    - Use a wrapper dma_fence_array of all fences including the new one
>>>      when importing an exclusive fence.
>>>
>>> v3 (Jason Ekstrand):
>>>    - Lock around setting shared fences as well as exclusive
>>>    - Mark SIGNAL_SYNC_FILE as a read-write ioctl.
>>>    - Initialize ret to 0 in dma_buf_wait_sync_file
>>>
>>> v4 (Jason Ekstrand):
>>>    - Use the new dma_resv_get_singleton helper
>>>
>>> Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
>>> ---
>>>    drivers/dma-buf/dma-buf.c    | 96 ++++++++++++++++++++++++++++++++++++
>>>    include/uapi/linux/dma-buf.h | 13 ++++-
>>>    2 files changed, 107 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>>> index d4097856c86b..09973c689866 100644
>>> --- a/drivers/dma-buf/dma-buf.c
>>> +++ b/drivers/dma-buf/dma-buf.c
>>> @@ -20,6 +20,7 @@
>>>    #include <linux/debugfs.h>
>>>    #include <linux/module.h>
>>>    #include <linux/seq_file.h>
>>> +#include <linux/sync_file.h>
>>>    #include <linux/poll.h>
>>>    #include <linux/dma-resv.h>
>>>    #include <linux/mm.h>
>>> @@ -348,6 +349,95 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
>>>        return ret;
>>>    }
>>>
>>> +static long dma_buf_wait_sync_file(struct dma_buf *dmabuf,
>>> +                                const void __user *user_data)
>>> +{
>>> +     struct dma_buf_sync_file arg;
>>> +     struct dma_fence *fence;
>>> +     int ret = 0;
>>> +
>>> +     if (copy_from_user(&arg, user_data, sizeof(arg)))
>>> +             return -EFAULT;
>>> +
>>> +     if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
>>> +             return -EINVAL;
>>> +
>>> +     fence = sync_file_get_fence(arg.fd);
>>> +     if (!fence)
>>> +             return -EINVAL;
>>> +
>>> +     dma_resv_lock(dmabuf->resv, NULL);
>>> +
>>> +     if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
>>> +             struct dma_fence *singleton = NULL;
>>> +             ret = dma_resv_get_singleton(dmabuf->resv, fence, &singleton);
>>> +             if (!ret && singleton)
>>> +                     dma_resv_add_excl_fence(dmabuf->resv, singleton);
>>> +     } else {
>>> +             dma_resv_add_shared_fence(dmabuf->resv, fence);
>>> +     }
>> You also need to create a singleton when adding a shared fences.
>>
>> The problem is that shared fences must always signal after exclusive
>> ones and you can't guarantee that for the fence you add here.
> I'm beginning to think that I should just drop the flags and always
> wait on all fences and always take what's currently the "write" path.
> Otherwise, something's going to get it wrong somewhere.  Thoughts?

If that is sufficient for your use case then that is certainly the more 
defensive (e.g. less dangerous) approach.

> Also, Michelle (added to CC) commented on IRC today that amdgpu does
> something with implicit sync fences where it sorts out the fences
> which affect one queue vs. others.  He thought that stuffing fences in
> the dma-buf in this way might cause that to not work.  Thoughts?

Yes that is correct. What amdgpu does is it ignores all fences from the 
same process.

E.g. when A submits IBs 1, 2 and 3 and then B submits IB 4 then 4 waits 
for 1,2,3, but 1,2,3 can run parallel to each other.

And yes adding anything as explicit sync would break that, but I don't 
think that this is much of a problem.

Regards,
Christian.


>
> --Jason
>
>
>> Regards,
>> Christian.
>>
>>> +
>>> +     dma_resv_unlock(dmabuf->resv);
>>> +
>>> +     dma_fence_put(fence);
>>> +
>>> +     return ret;
>>> +}
>>> +
>>> +static long dma_buf_signal_sync_file(struct dma_buf *dmabuf,
>>> +                                  void __user *user_data)
>>> +{
>>> +     struct dma_buf_sync_file arg;
>>> +     struct dma_fence *fence = NULL;
>>> +     struct sync_file *sync_file;
>>> +     int fd, ret;
>>> +
>>> +     if (copy_from_user(&arg, user_data, sizeof(arg)))
>>> +             return -EFAULT;
>>> +
>>> +     if (arg.flags != 0 && arg.flags != DMA_BUF_SYNC_FILE_SYNC_WRITE)
>>> +             return -EINVAL;
>>> +
>>> +     fd = get_unused_fd_flags(O_CLOEXEC);
>>> +     if (fd < 0)
>>> +             return fd;
>>> +
>>> +     if (arg.flags & DMA_BUF_SYNC_FILE_SYNC_WRITE) {
>>> +             /* We need to include both the exclusive fence and all of
>>> +              * the shared fences in our fence.
>>> +              */
>>> +             ret = dma_resv_get_singleton(dmabuf->resv, NULL, &fence);
>>> +             if (ret)
>>> +                     goto err_put_fd;
>>> +     } else {
>>> +             fence = dma_resv_get_excl_rcu(dmabuf->resv);
>>> +     }
>>> +
>>> +     if (!fence)
>>> +             fence = dma_fence_get_stub();
>>> +
>>> +     sync_file = sync_file_create(fence);
>>> +
>>> +     dma_fence_put(fence);
>>> +
>>> +     if (!sync_file) {
>>> +             ret = -EINVAL;
>>> +             goto err_put_fd;
>>> +     }
>>> +
>>> +     fd_install(fd, sync_file->file);
>>> +
>>> +     arg.fd = fd;
>>> +     if (copy_to_user(user_data, &arg, sizeof(arg)))
>>> +             return -EFAULT;
>>> +
>>> +     return 0;
>>> +
>>> +err_put_fd:
>>> +     put_unused_fd(fd);
>>> +     return ret;
>>> +}
>>> +
>>>    static long dma_buf_ioctl(struct file *file,
>>>                          unsigned int cmd, unsigned long arg)
>>>    {
>>> @@ -390,6 +480,12 @@ static long dma_buf_ioctl(struct file *file,
>>>        case DMA_BUF_SET_NAME:
>>>                return dma_buf_set_name(dmabuf, (const char __user *)arg);
>>>
>>> +     case DMA_BUF_IOCTL_WAIT_SYNC_FILE:
>>> +             return dma_buf_wait_sync_file(dmabuf, (const void __user *)arg);
>>> +
>>> +     case DMA_BUF_IOCTL_SIGNAL_SYNC_FILE:
>>> +             return dma_buf_signal_sync_file(dmabuf, (void __user *)arg);
>>> +
>>>        default:
>>>                return -ENOTTY;
>>>        }
>>> diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
>>> index dbc7092e04b5..86e07acca90c 100644
>>> --- a/include/uapi/linux/dma-buf.h
>>> +++ b/include/uapi/linux/dma-buf.h
>>> @@ -37,8 +37,17 @@ struct dma_buf_sync {
>>>
>>>    #define DMA_BUF_NAME_LEN    32
>>>
>>> +struct dma_buf_sync_file {
>>> +     __u32 flags;
>>> +     __s32 fd;
>>> +};
>>> +
>>> +#define DMA_BUF_SYNC_FILE_SYNC_WRITE (1 << 0)
>>> +
>>>    #define DMA_BUF_BASE                'b'
>>> -#define DMA_BUF_IOCTL_SYNC   _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
>>> -#define DMA_BUF_SET_NAME     _IOW(DMA_BUF_BASE, 1, const char *)
>>> +#define DMA_BUF_IOCTL_SYNC       _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
>>> +#define DMA_BUF_SET_NAME         _IOW(DMA_BUF_BASE, 1, const char *)
>>> +#define DMA_BUF_IOCTL_WAIT_SYNC_FILE _IOW(DMA_BUF_BASE, 2, struct dma_buf_sync)
>>> +#define DMA_BUF_IOCTL_SIGNAL_SYNC_FILE       _IOWR(DMA_BUF_BASE, 3, struct dma_buf_sync)
>>>
>>>    #endif

[PATCH 3/3] RFC: dma-buf: Add an API for importing and exporting sync files (v5)

[Jason Ekstrand]

Explicit synchronization is the future.  At least, that seems to be what
most userspace APIs are agreeing on at this point.  However, most of our
Linux APIs (both userspace and kernel UAPI) are currently built around
implicit synchronization with dma-buf.  While work is ongoing to change
many of the userspace APIs and protocols to an explicit synchronization
model, switching over piecemeal is difficult due to the number of
potential components involved.  On the kernel side, many drivers use
dma-buf including GPU (3D/compute), display, v4l, and others.  In
userspace, we have X11, several Wayland compositors, 3D drivers, compute
drivers (OpenCL etc.), media encode/decode, and the list goes on.

This patch provides a path forward by allowing userspace to manually
manage the fences attached to a dma-buf.  Alternatively, one can think
of this as making dma-buf's implicit synchronization simply a carrier
for an explicit fence.  This is accomplished by adding two IOCTLs to
dma-buf for importing and exporting a sync file to/from the dma-buf.
This way a userspace component which is uses explicit synchronization,
such as a Vulkan driver, can manually set the write fence on a buffer
before handing it off to an implicitly synchronized component such as a
Wayland compositor or video encoder.  In this way, each of the different
components can be upgraded to an explicit synchronization model one at a
time as long as the userspace pieces connecting them are aware of it and
import/export fences at the right times.

There is a potential race condition with this API if userspace is not
careful.  A typical use case for implicit synchronization is to wait for
the dma-buf to be ready, use it, and then signal it for some other
component.  Because a sync_file cannot be created until it is guaranteed
to complete in finite time, userspace can only signal the dma-buf after
it has already submitted the work which uses it to the kernel and has
received a sync_file back.  There is no way to atomically submit a
wait-use-signal operation.  This is not, however, really a problem with
this API so much as it is a problem with explicit synchronization
itself.  The way this is typically handled is to have very explicit
ownership transfer points in the API or protocol which ensure that only
one component is using it at any given time.  Both X11 (via the PRESENT
extension) and Wayland provide such ownership transfer points via
explicit present and idle messages.

The decision was intentionally made in this patch to make the import and
export operations IOCTLs on the dma-buf itself rather than as a DRM
IOCTL.  This makes it the import/export operation universal across all
components which use dma-buf including GPU, display, v4l, and others.
It also means that a userspace component can do the import/export
without access to the DRM fd which may be tricky to get in cases where
the client communicates with DRM via a userspace API such as OpenGL or
Vulkan.  At a future date we may choose to add direct import/export APIs
to components such as drm_syncobj to avoid allocating a file descriptor
and going through two ioctls.  However, that seems to be something of a
micro-optimization as import/export operations are likely to happen at a
rate of a few per frame of rendered or decoded video.

v2 (Jason Ekstrand):
 - Use a wrapper dma_fence_array of all fences including the new one
   when importing an exclusive fence.

v3 (Jason Ekstrand):
 - Lock around setting shared fences as well as exclusive
 - Mark SIGNAL_SYNC_FILE as a read-write ioctl.
 - Initialize ret to 0 in dma_buf_wait_sync_file

v4 (Jason Ekstrand):
 - Use the new dma_resv_get_singleton helper

v5 (Jason Ekstrand):
 - Rename the IOCTLs to import/export rather than wait/signal
 - Drop the WRITE flag and always get/set the exclusive fence

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
---
 drivers/dma-buf/dma-buf.c    | 84 ++++++++++++++++++++++++++++++++++++
 include/uapi/linux/dma-buf.h | 11 ++++-
 2 files changed, 93 insertions(+), 2 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index d4097856c86b..d34d27aa3077 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -20,6 +20,7 @@
 #include <linux/debugfs.h>
 #include <linux/module.h>
 #include <linux/seq_file.h>
+#include <linux/sync_file.h>
 #include <linux/poll.h>
 #include <linux/dma-resv.h>
 #include <linux/mm.h>
@@ -348,6 +349,83 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
 	return ret;
 }
 
+static long dma_buf_import_sync_file(struct dma_buf *dmabuf,
+				     const void __user *user_data)
+{
+	struct dma_buf_sync_file arg;
+	struct dma_fence *fence, *singleton = NULL;
+	int ret = 0;
+
+	if (copy_from_user(&arg, user_data, sizeof(arg)))
+		return -EFAULT;
+
+	if (arg.flags != 0)
+		return -EINVAL;
+
+	fence = sync_file_get_fence(arg.fd);
+	if (!fence)
+		return -EINVAL;
+
+	dma_resv_lock(dmabuf->resv, NULL);
+
+	ret = dma_resv_get_singleton(dmabuf->resv, fence, &singleton);
+	if (!ret && singleton)
+		dma_resv_add_excl_fence(dmabuf->resv, singleton);
+
+	dma_resv_unlock(dmabuf->resv);
+
+	dma_fence_put(fence);
+
+	return ret;
+}
+
+static long dma_buf_export_sync_file(struct dma_buf *dmabuf,
+				     void __user *user_data)
+{
+	struct dma_buf_sync_file arg;
+	struct dma_fence *fence = NULL;
+	struct sync_file *sync_file;
+	int fd, ret;
+
+	if (copy_from_user(&arg, user_data, sizeof(arg)))
+		return -EFAULT;
+
+	if (arg.flags != 0)
+		return -EINVAL;
+
+	fd = get_unused_fd_flags(O_CLOEXEC);
+	if (fd < 0)
+		return fd;
+
+	ret = dma_resv_get_singleton(dmabuf->resv, NULL, &fence);
+	if (ret)
+		goto err_put_fd;
+
+	if (!fence)
+		fence = dma_fence_get_stub();
+
+	sync_file = sync_file_create(fence);
+
+	dma_fence_put(fence);
+
+	if (!sync_file) {
+		ret = -EINVAL;
+		goto err_put_fd;
+	}
+
+	fd_install(fd, sync_file->file);
+
+	arg.fd = fd;
+	if (copy_to_user(user_data, &arg, sizeof(arg)))
+		return -EFAULT;
+
+	return 0;
+
+err_put_fd:
+	put_unused_fd(fd);
+	return ret;
+}
+
 static long dma_buf_ioctl(struct file *file,
 			  unsigned int cmd, unsigned long arg)
 {
@@ -390,6 +468,12 @@ static long dma_buf_ioctl(struct file *file,
 	case DMA_BUF_SET_NAME:
 		return dma_buf_set_name(dmabuf, (const char __user *)arg);
 
+	case DMA_BUF_IOCTL_IMPORT_SYNC_FILE:
+		return dma_buf_import_sync_file(dmabuf, (const void __user *)arg);
+
+	case DMA_BUF_IOCTL_EXPORT_SYNC_FILE:
+		return dma_buf_export_sync_file(dmabuf, (void __user *)arg);
+
 	default:
 		return -ENOTTY;
 	}
diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
index dbc7092e04b5..b746c6459e24 100644
--- a/include/uapi/linux/dma-buf.h
+++ b/include/uapi/linux/dma-buf.h
@@ -37,8 +37,15 @@ struct dma_buf_sync {
 
 #define DMA_BUF_NAME_LEN	32
 
+struct dma_buf_sync_file {
+	__u32 flags;
+	__s32 fd;
+};
+
 #define DMA_BUF_BASE		'b'
-#define DMA_BUF_IOCTL_SYNC	_IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
-#define DMA_BUF_SET_NAME	_IOW(DMA_BUF_BASE, 1, const char *)
+#define DMA_BUF_IOCTL_SYNC	    _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
+#define DMA_BUF_SET_NAME	    _IOW(DMA_BUF_BASE, 1, const char *)
+#define DMA_BUF_IOCTL_IMPORT_SYNC_FILE	_IOW(DMA_BUF_BASE, 2, struct dma_buf_sync)
+#define DMA_BUF_IOCTL_EXPORT_SYNC_FILE	_IOWR(DMA_BUF_BASE, 3, struct dma_buf_sync)
 
 #endif
-- 
2.24.1

Re: [PATCH 3/3] RFC: dma-buf: Add an API for importing and exporting sync files (v5)

[Michel Dänzer]

On 2020-03-17 10:21 p.m., Jason Ekstrand wrote:
> Explicit synchronization is the future.  At least, that seems to be what
> most userspace APIs are agreeing on at this point.  However, most of our
> Linux APIs (both userspace and kernel UAPI) are currently built around
> implicit synchronization with dma-buf.  While work is ongoing to change
> many of the userspace APIs and protocols to an explicit synchronization
> model, switching over piecemeal is difficult due to the number of
> potential components involved.  On the kernel side, many drivers use
> dma-buf including GPU (3D/compute), display, v4l, and others.  In
> userspace, we have X11, several Wayland compositors, 3D drivers, compute
> drivers (OpenCL etc.), media encode/decode, and the list goes on.
> 
> This patch provides a path forward by allowing userspace to manually
> manage the fences attached to a dma-buf.  Alternatively, one can think
> of this as making dma-buf's implicit synchronization simply a carrier
> for an explicit fence.  This is accomplished by adding two IOCTLs to
> dma-buf for importing and exporting a sync file to/from the dma-buf.
> This way a userspace component which is uses explicit synchronization,
> such as a Vulkan driver, can manually set the write fence on a buffer
> before handing it off to an implicitly synchronized component such as a
> Wayland compositor or video encoder.  In this way, each of the different
> components can be upgraded to an explicit synchronization model one at a
> time as long as the userspace pieces connecting them are aware of it and
> import/export fences at the right times.
> 
> There is a potential race condition with this API if userspace is not
> careful.  A typical use case for implicit synchronization is to wait for
> the dma-buf to be ready, use it, and then signal it for some other
> component.  Because a sync_file cannot be created until it is guaranteed
> to complete in finite time, userspace can only signal the dma-buf after
> it has already submitted the work which uses it to the kernel and has
> received a sync_file back.  There is no way to atomically submit a
> wait-use-signal operation.  This is not, however, really a problem with
> this API so much as it is a problem with explicit synchronization
> itself.  The way this is typically handled is to have very explicit
> ownership transfer points in the API or protocol which ensure that only
> one component is using it at any given time.  Both X11 (via the PRESENT
> extension) and Wayland provide such ownership transfer points via
> explicit present and idle messages.
> 
> The decision was intentionally made in this patch to make the import and
> export operations IOCTLs on the dma-buf itself rather than as a DRM
> IOCTL.  This makes it the import/export operation universal across all
> components which use dma-buf including GPU, display, v4l, and others.
> It also means that a userspace component can do the import/export
> without access to the DRM fd which may be tricky to get in cases where
> the client communicates with DRM via a userspace API such as OpenGL or
> Vulkan.  At a future date we may choose to add direct import/export APIs
> to components such as drm_syncobj to avoid allocating a file descriptor
> and going through two ioctls.  However, that seems to be something of a
> micro-optimization as import/export operations are likely to happen at a
> rate of a few per frame of rendered or decoded video.
> 
> v2 (Jason Ekstrand):
>   - Use a wrapper dma_fence_array of all fences including the new one
>     when importing an exclusive fence.
> 
> v3 (Jason Ekstrand):
>   - Lock around setting shared fences as well as exclusive
>   - Mark SIGNAL_SYNC_FILE as a read-write ioctl.
>   - Initialize ret to 0 in dma_buf_wait_sync_file
> 
> v4 (Jason Ekstrand):
>   - Use the new dma_resv_get_singleton helper
> 
> v5 (Jason Ekstrand):
>   - Rename the IOCTLs to import/export rather than wait/signal
>   - Drop the WRITE flag and always get/set the exclusive fence
> 
> Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>

What's the status of this? DMA_BUF_IOCTL_EXPORT_SYNC_FILE would be 
useful for Wayland compositors to wait for client buffers to become 
ready without being prone to getting delayed by later HW access to them, 
so it would be nice to merge that at least (if 
DMA_BUF_IOCTL_IMPORT_SYNC_FILE is still controversial).


-- 
Earthling Michel Dänzer               |               https://redhat.com
Libre software enthusiast             |             Mesa and X developer

Re: [PATCH 3/3] RFC: dma-buf: Add an API for importing and exporting sync files (v5)

[Daniel Vetter]

On Wed, Sep 30, 2020 at 11:39:06AM +0200, Michel Dänzer wrote:
> On 2020-03-17 10:21 p.m., Jason Ekstrand wrote:
> > Explicit synchronization is the future.  At least, that seems to be what
> > most userspace APIs are agreeing on at this point.  However, most of our
> > Linux APIs (both userspace and kernel UAPI) are currently built around
> > implicit synchronization with dma-buf.  While work is ongoing to change
> > many of the userspace APIs and protocols to an explicit synchronization
> > model, switching over piecemeal is difficult due to the number of
> > potential components involved.  On the kernel side, many drivers use
> > dma-buf including GPU (3D/compute), display, v4l, and others.  In
> > userspace, we have X11, several Wayland compositors, 3D drivers, compute
> > drivers (OpenCL etc.), media encode/decode, and the list goes on.
> > 
> > This patch provides a path forward by allowing userspace to manually
> > manage the fences attached to a dma-buf.  Alternatively, one can think
> > of this as making dma-buf's implicit synchronization simply a carrier
> > for an explicit fence.  This is accomplished by adding two IOCTLs to
> > dma-buf for importing and exporting a sync file to/from the dma-buf.
> > This way a userspace component which is uses explicit synchronization,
> > such as a Vulkan driver, can manually set the write fence on a buffer
> > before handing it off to an implicitly synchronized component such as a
> > Wayland compositor or video encoder.  In this way, each of the different
> > components can be upgraded to an explicit synchronization model one at a
> > time as long as the userspace pieces connecting them are aware of it and
> > import/export fences at the right times.
> > 
> > There is a potential race condition with this API if userspace is not
> > careful.  A typical use case for implicit synchronization is to wait for
> > the dma-buf to be ready, use it, and then signal it for some other
> > component.  Because a sync_file cannot be created until it is guaranteed
> > to complete in finite time, userspace can only signal the dma-buf after
> > it has already submitted the work which uses it to the kernel and has
> > received a sync_file back.  There is no way to atomically submit a
> > wait-use-signal operation.  This is not, however, really a problem with
> > this API so much as it is a problem with explicit synchronization
> > itself.  The way this is typically handled is to have very explicit
> > ownership transfer points in the API or protocol which ensure that only
> > one component is using it at any given time.  Both X11 (via the PRESENT
> > extension) and Wayland provide such ownership transfer points via
> > explicit present and idle messages.
> > 
> > The decision was intentionally made in this patch to make the import and
> > export operations IOCTLs on the dma-buf itself rather than as a DRM
> > IOCTL.  This makes it the import/export operation universal across all
> > components which use dma-buf including GPU, display, v4l, and others.
> > It also means that a userspace component can do the import/export
> > without access to the DRM fd which may be tricky to get in cases where
> > the client communicates with DRM via a userspace API such as OpenGL or
> > Vulkan.  At a future date we may choose to add direct import/export APIs
> > to components such as drm_syncobj to avoid allocating a file descriptor
> > and going through two ioctls.  However, that seems to be something of a
> > micro-optimization as import/export operations are likely to happen at a
> > rate of a few per frame of rendered or decoded video.
> > 
> > v2 (Jason Ekstrand):
> >   - Use a wrapper dma_fence_array of all fences including the new one
> >     when importing an exclusive fence.
> > 
> > v3 (Jason Ekstrand):
> >   - Lock around setting shared fences as well as exclusive
> >   - Mark SIGNAL_SYNC_FILE as a read-write ioctl.
> >   - Initialize ret to 0 in dma_buf_wait_sync_file
> > 
> > v4 (Jason Ekstrand):
> >   - Use the new dma_resv_get_singleton helper
> > 
> > v5 (Jason Ekstrand):
> >   - Rename the IOCTLs to import/export rather than wait/signal
> >   - Drop the WRITE flag and always get/set the exclusive fence
> > 
> > Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> 
> What's the status of this? DMA_BUF_IOCTL_EXPORT_SYNC_FILE would be useful
> for Wayland compositors to wait for client buffers to become ready without
> being prone to getting delayed by later HW access to them, so it would be
> nice to merge that at least (if DMA_BUF_IOCTL_IMPORT_SYNC_FILE is still
> controversial).

I think the missing bits are just the usual stuff
- igt testcases
- userspace using the new ioctls
- review of the entire pile

I don't think there's any fundamental objections aside from "no one ever
pushed this over the finish line".

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: [PATCH 3/3] RFC: dma-buf: Add an API for importing and exporting sync files (v5)

[Jason Ekstrand]

On Wed, Sep 30, 2020 at 4:55 AM Daniel Vetter <daniel@ffwll.ch> wrote:
>
> On Wed, Sep 30, 2020 at 11:39:06AM +0200, Michel Dänzer wrote:
> > On 2020-03-17 10:21 p.m., Jason Ekstrand wrote:
> > > Explicit synchronization is the future.  At least, that seems to be what
> > > most userspace APIs are agreeing on at this point.  However, most of our
> > > Linux APIs (both userspace and kernel UAPI) are currently built around
> > > implicit synchronization with dma-buf.  While work is ongoing to change
> > > many of the userspace APIs and protocols to an explicit synchronization
> > > model, switching over piecemeal is difficult due to the number of
> > > potential components involved.  On the kernel side, many drivers use
> > > dma-buf including GPU (3D/compute), display, v4l, and others.  In
> > > userspace, we have X11, several Wayland compositors, 3D drivers, compute
> > > drivers (OpenCL etc.), media encode/decode, and the list goes on.
> > >
> > > This patch provides a path forward by allowing userspace to manually
> > > manage the fences attached to a dma-buf.  Alternatively, one can think
> > > of this as making dma-buf's implicit synchronization simply a carrier
> > > for an explicit fence.  This is accomplished by adding two IOCTLs to
> > > dma-buf for importing and exporting a sync file to/from the dma-buf.
> > > This way a userspace component which is uses explicit synchronization,
> > > such as a Vulkan driver, can manually set the write fence on a buffer
> > > before handing it off to an implicitly synchronized component such as a
> > > Wayland compositor or video encoder.  In this way, each of the different
> > > components can be upgraded to an explicit synchronization model one at a
> > > time as long as the userspace pieces connecting them are aware of it and
> > > import/export fences at the right times.
> > >
> > > There is a potential race condition with this API if userspace is not
> > > careful.  A typical use case for implicit synchronization is to wait for
> > > the dma-buf to be ready, use it, and then signal it for some other
> > > component.  Because a sync_file cannot be created until it is guaranteed
> > > to complete in finite time, userspace can only signal the dma-buf after
> > > it has already submitted the work which uses it to the kernel and has
> > > received a sync_file back.  There is no way to atomically submit a
> > > wait-use-signal operation.  This is not, however, really a problem with
> > > this API so much as it is a problem with explicit synchronization
> > > itself.  The way this is typically handled is to have very explicit
> > > ownership transfer points in the API or protocol which ensure that only
> > > one component is using it at any given time.  Both X11 (via the PRESENT
> > > extension) and Wayland provide such ownership transfer points via
> > > explicit present and idle messages.
> > >
> > > The decision was intentionally made in this patch to make the import and
> > > export operations IOCTLs on the dma-buf itself rather than as a DRM
> > > IOCTL.  This makes it the import/export operation universal across all
> > > components which use dma-buf including GPU, display, v4l, and others.
> > > It also means that a userspace component can do the import/export
> > > without access to the DRM fd which may be tricky to get in cases where
> > > the client communicates with DRM via a userspace API such as OpenGL or
> > > Vulkan.  At a future date we may choose to add direct import/export APIs
> > > to components such as drm_syncobj to avoid allocating a file descriptor
> > > and going through two ioctls.  However, that seems to be something of a
> > > micro-optimization as import/export operations are likely to happen at a
> > > rate of a few per frame of rendered or decoded video.
> > >
> > > v2 (Jason Ekstrand):
> > >   - Use a wrapper dma_fence_array of all fences including the new one
> > >     when importing an exclusive fence.
> > >
> > > v3 (Jason Ekstrand):
> > >   - Lock around setting shared fences as well as exclusive
> > >   - Mark SIGNAL_SYNC_FILE as a read-write ioctl.
> > >   - Initialize ret to 0 in dma_buf_wait_sync_file
> > >
> > > v4 (Jason Ekstrand):
> > >   - Use the new dma_resv_get_singleton helper
> > >
> > > v5 (Jason Ekstrand):
> > >   - Rename the IOCTLs to import/export rather than wait/signal
> > >   - Drop the WRITE flag and always get/set the exclusive fence
> > >
> > > Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> >
> > What's the status of this? DMA_BUF_IOCTL_EXPORT_SYNC_FILE would be useful
> > for Wayland compositors to wait for client buffers to become ready without
> > being prone to getting delayed by later HW access to them, so it would be
> > nice to merge that at least (if DMA_BUF_IOCTL_IMPORT_SYNC_FILE is still
> > controversial).
>
> I think the missing bits are just the usual stuff
> - igt testcases
> - userspace using the new ioctls
> - review of the entire pile
>
> I don't think there's any fundamental objections aside from "no one ever
> pushed this over the finish line".

I just re-upped the patch series and you should have been on the CC
for the cover letter.  The Mesa MR is here:
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4037  I'm
going to try and knock out an IGT test quick but I don't know how much
is really practical to test in that environment besides a basic fuzz
for "does the IOCTL return a sync file".

I've dropped all the sync_file import stuff in the latest version.  It
would have been useful for testing but it's also where all the scary
stuff lives and I'm no longer convinced it solves any real problems
for userspace.

--Jason

Re: [PATCH 3/3] RFC: dma-buf: Add an API for importing and exporting sync files (v5)

[Daniel Vetter]

On Mon, Mar 15, 2021 at 10:11 PM Jason Ekstrand <jason@jlekstrand.net> wrote:
>
> On Wed, Sep 30, 2020 at 4:55 AM Daniel Vetter <daniel@ffwll.ch> wrote:
> >
> > On Wed, Sep 30, 2020 at 11:39:06AM +0200, Michel Dänzer wrote:
> > > On 2020-03-17 10:21 p.m., Jason Ekstrand wrote:
> > > > Explicit synchronization is the future.  At least, that seems to be what
> > > > most userspace APIs are agreeing on at this point.  However, most of our
> > > > Linux APIs (both userspace and kernel UAPI) are currently built around
> > > > implicit synchronization with dma-buf.  While work is ongoing to change
> > > > many of the userspace APIs and protocols to an explicit synchronization
> > > > model, switching over piecemeal is difficult due to the number of
> > > > potential components involved.  On the kernel side, many drivers use
> > > > dma-buf including GPU (3D/compute), display, v4l, and others.  In
> > > > userspace, we have X11, several Wayland compositors, 3D drivers, compute
> > > > drivers (OpenCL etc.), media encode/decode, and the list goes on.
> > > >
> > > > This patch provides a path forward by allowing userspace to manually
> > > > manage the fences attached to a dma-buf.  Alternatively, one can think
> > > > of this as making dma-buf's implicit synchronization simply a carrier
> > > > for an explicit fence.  This is accomplished by adding two IOCTLs to
> > > > dma-buf for importing and exporting a sync file to/from the dma-buf.
> > > > This way a userspace component which is uses explicit synchronization,
> > > > such as a Vulkan driver, can manually set the write fence on a buffer
> > > > before handing it off to an implicitly synchronized component such as a
> > > > Wayland compositor or video encoder.  In this way, each of the different
> > > > components can be upgraded to an explicit synchronization model one at a
> > > > time as long as the userspace pieces connecting them are aware of it and
> > > > import/export fences at the right times.
> > > >
> > > > There is a potential race condition with this API if userspace is not
> > > > careful.  A typical use case for implicit synchronization is to wait for
> > > > the dma-buf to be ready, use it, and then signal it for some other
> > > > component.  Because a sync_file cannot be created until it is guaranteed
> > > > to complete in finite time, userspace can only signal the dma-buf after
> > > > it has already submitted the work which uses it to the kernel and has
> > > > received a sync_file back.  There is no way to atomically submit a
> > > > wait-use-signal operation.  This is not, however, really a problem with
> > > > this API so much as it is a problem with explicit synchronization
> > > > itself.  The way this is typically handled is to have very explicit
> > > > ownership transfer points in the API or protocol which ensure that only
> > > > one component is using it at any given time.  Both X11 (via the PRESENT
> > > > extension) and Wayland provide such ownership transfer points via
> > > > explicit present and idle messages.
> > > >
> > > > The decision was intentionally made in this patch to make the import and
> > > > export operations IOCTLs on the dma-buf itself rather than as a DRM
> > > > IOCTL.  This makes it the import/export operation universal across all
> > > > components which use dma-buf including GPU, display, v4l, and others.
> > > > It also means that a userspace component can do the import/export
> > > > without access to the DRM fd which may be tricky to get in cases where
> > > > the client communicates with DRM via a userspace API such as OpenGL or
> > > > Vulkan.  At a future date we may choose to add direct import/export APIs
> > > > to components such as drm_syncobj to avoid allocating a file descriptor
> > > > and going through two ioctls.  However, that seems to be something of a
> > > > micro-optimization as import/export operations are likely to happen at a
> > > > rate of a few per frame of rendered or decoded video.
> > > >
> > > > v2 (Jason Ekstrand):
> > > >   - Use a wrapper dma_fence_array of all fences including the new one
> > > >     when importing an exclusive fence.
> > > >
> > > > v3 (Jason Ekstrand):
> > > >   - Lock around setting shared fences as well as exclusive
> > > >   - Mark SIGNAL_SYNC_FILE as a read-write ioctl.
> > > >   - Initialize ret to 0 in dma_buf_wait_sync_file
> > > >
> > > > v4 (Jason Ekstrand):
> > > >   - Use the new dma_resv_get_singleton helper
> > > >
> > > > v5 (Jason Ekstrand):
> > > >   - Rename the IOCTLs to import/export rather than wait/signal
> > > >   - Drop the WRITE flag and always get/set the exclusive fence
> > > >
> > > > Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> > >
> > > What's the status of this? DMA_BUF_IOCTL_EXPORT_SYNC_FILE would be useful
> > > for Wayland compositors to wait for client buffers to become ready without
> > > being prone to getting delayed by later HW access to them, so it would be
> > > nice to merge that at least (if DMA_BUF_IOCTL_IMPORT_SYNC_FILE is still
> > > controversial).
> >
> > I think the missing bits are just the usual stuff
> > - igt testcases
> > - userspace using the new ioctls
> > - review of the entire pile
> >
> > I don't think there's any fundamental objections aside from "no one ever
> > pushed this over the finish line".
>
> I just re-upped the patch series and you should have been on the CC
> for the cover letter.  The Mesa MR is here:
> https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4037  I'm
> going to try and knock out an IGT test quick but I don't know how much
> is really practical to test in that environment besides a basic fuzz
> for "does the IOCTL return a sync file".

With vgem you should be able to control the interface fully, since
that allows you to control dma_fences we attach to stuff. Or at least
it has some of the ingredients, and a bunch of igts test it. E.g. we
have one that creates a fence with vgem and then checks (using crc)
that your display driver works correctly with the page flip.

> I've dropped all the sync_file import stuff in the latest version.  It
> would have been useful for testing but it's also where all the scary
> stuff lives and I'm no longer convinced it solves any real problems
> for userspace.

Yeah vgem should allow you to get at least some of that sorted.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch