On Mon, May 09, 2022 at 07:54:19AM +0200, Christian König wrote:
> Reviewed-by: Christian König <christian.koenig@amd.com> for the series.
>
> I assume you have the userspace part ready as well? If yes let's push this
> to drm-misc-next asap.

Hopefully I'm not too late, but I think all my review has also been
addressed. On the series:

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

>
> Christian.
>
> Am 06.05.22 um 20:02 schrieb Jason Ekstrand:
> > Modern userspace APIs like Vulkan are built on an explicit
> > synchronization model.  This doesn't always play nicely with the
> > implicit synchronization used in the kernel and assumed by X11 and
> > Wayland.  The client -> compositor half of the synchronization isn't too
> > bad, at least on intel, because we can control whether or not i915
> > synchronizes on the buffer and whether or not it's considered written.
> >
> > The harder part is the compositor -> client synchronization when we get
> > the buffer back from the compositor.  We're required to be able to
> > provide the client with a VkSemaphore and VkFence representing the point
> > in time where the window system (compositor and/or display) finished
> > using the buffer.  With current APIs, it's very hard to do this in such
> > a way that we don't get confused by the Vulkan driver's access of the
> > buffer.  In particular, once we tell the kernel that we're rendering to
> > the buffer again, any CPU waits on the buffer or GPU dependencies will
> > wait on some of the client rendering and not just the compositor.
> >
> > This new IOCTL solves this problem by allowing us to get a snapshot of
> > the implicit synchronization state of a given dma-buf in the form of a
> > sync file.  It's effectively the same as a poll() or I915_GEM_WAIT only,
> > instead of CPU waiting directly, it encapsulates the wait operation, at
> > the current moment in time, in a sync_file so we can check/wait on it
> > later.  As long as the Vulkan driver does the sync_file export from the
> > dma-buf before we re-introduce it for rendering, it will only contain
> > fences from the compositor or display.  This allows to accurately turn
> > it into a VkFence or VkSemaphore without any over-synchronization.
> >
> > By making this an ioctl on the dma-buf itself, it allows this new
> > functionality to be used in an entirely driver-agnostic way without
> > having access to a DRM fd. This makes it ideal for use in driver-generic
> > code in Mesa or in a client such as a compositor where the DRM fd may be
> > hard to reach.
> >
> > 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
> >
> > v6 (Jason Ekstrand):
> >   - Drop the sync_file import as it was all-around sketchy and not nearly
> >     as useful as import.
> >   - Re-introduce READ/WRITE flag support for export
> >   - Rework the commit message
> >
> > v7 (Jason Ekstrand):
> >   - Require at least one sync flag
> >   - Fix a refcounting bug: dma_resv_get_excl() doesn't take a reference
> >   - Use _rcu helpers since we're accessing the dma_resv read-only
> >
> > v8 (Jason Ekstrand):
> >   - Return -ENOMEM if the sync_file_create fails
> >   - Predicate support on IS_ENABLED(CONFIG_SYNC_FILE)
> >
> > v9 (Jason Ekstrand):
> >   - Add documentation for the new ioctl
> >
> > v10 (Jason Ekstrand):
> >   - Go back to dma_buf_sync_file as the ioctl struct name
> >
> > v11 (Daniel Vetter):
> >   - Go back to dma_buf_export_sync_file as the ioctl struct name
> >   - Better kerneldoc describing what the read/write flags do
> >
> > v12 (Christian König):
> >   - Document why we chose to make it an ioctl on dma-buf
> >
> > v13 (Jason Ekstrand):
> >   - Rebase on Christian König's fence rework
> >
> > v14 (Daniel Vetter & Christian König):
> >   - Use dma_rev_usage_rw to get the properly inverted usage to pass to
> >     dma_resv_get_singleton()
> >   - Clean up the sync_file and fd if copy_to_user() fails
> >
> > Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> > Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
> > Signed-off-by: Jason Ekstrand <jason.ekstrand@collabora.com>
> > Acked-by: Simon Ser <contact@emersion.fr>
> > Acked-by: Christian König <christian.koenig@amd.com>
> > Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> > Cc: Sumit Semwal <sumit.semwal@linaro.org>
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > ---
> >   drivers/dma-buf/dma-buf.c    | 67 ++++++++++++++++++++++++++++++++++++
> >   include/uapi/linux/dma-buf.h | 35 +++++++++++++++++++
> >   2 files changed, 102 insertions(+)
> >
> > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > index 79795857be3e..6ff54f7e6119 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>
> > @@ -192,6 +193,9 @@ static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
> >    * Note that this only signals the completion of the respective fences, i.e. the
> >    * DMA transfers are complete. Cache flushing and any other necessary
> >    * preparations before CPU access can begin still need to happen.
> > + *
> > + * As an alternative to poll(), the set of fences on DMA buffer can be
> > + * exported as a &sync_file using &dma_buf_sync_file_export.
> >    */
> >   static void dma_buf_poll_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
> > @@ -326,6 +330,64 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
> >     return 0;
> >   }
> > +#if IS_ENABLED(CONFIG_SYNC_FILE)
> > +static long dma_buf_export_sync_file(struct dma_buf *dmabuf,
> > +                                void __user *user_data)
> > +{
> > +   struct dma_buf_export_sync_file arg;
> > +   enum dma_resv_usage usage;
> > +   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 & ~DMA_BUF_SYNC_RW)
> > +           return -EINVAL;
> > +
> > +   if ((arg.flags & DMA_BUF_SYNC_RW) == 0)
> > +           return -EINVAL;
> > +
> > +   fd = get_unused_fd_flags(O_CLOEXEC);
> > +   if (fd < 0)
> > +           return fd;
> > +
> > +   usage = dma_resv_usage_rw(arg.flags & DMA_BUF_SYNC_WRITE);
> > +   ret = dma_resv_get_singleton(dmabuf->resv, usage, &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 = -ENOMEM;
> > +           goto err_put_fd;
> > +   }
> > +
> > +   arg.fd = fd;
> > +   if (copy_to_user(user_data, &arg, sizeof(arg))) {
> > +           ret = -EFAULT;
> > +           goto err_put_file;
> > +   }
> > +
> > +   fd_install(fd, sync_file->file);
> > +
> > +   return 0;
> > +
> > +err_put_file:
> > +   fput(sync_file->file);
> > +err_put_fd:
> > +   put_unused_fd(fd);
> > +   return ret;
> > +}
> > +#endif
> > +
> >   static long dma_buf_ioctl(struct file *file,
> >                       unsigned int cmd, unsigned long arg)
> >   {
> > @@ -369,6 +431,11 @@ static long dma_buf_ioctl(struct file *file,
> >     case DMA_BUF_SET_NAME_B:
> >             return dma_buf_set_name(dmabuf, (const char __user *)arg);
> > +#if IS_ENABLED(CONFIG_SYNC_FILE)
> > +   case DMA_BUF_IOCTL_EXPORT_SYNC_FILE:
> > +           return dma_buf_export_sync_file(dmabuf, (void __user *)arg);
> > +#endif
> > +
> >     default:
> >             return -ENOTTY;
> >     }
> > diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
> > index 8e4a2ca0bcbf..46f1e3e98b02 100644
> > --- a/include/uapi/linux/dma-buf.h
> > +++ b/include/uapi/linux/dma-buf.h
> > @@ -85,6 +85,40 @@ struct dma_buf_sync {
> >   #define DMA_BUF_NAME_LEN  32
> > +/**
> > + * struct dma_buf_export_sync_file - Get a sync_file from a dma-buf
> > + *
> > + * Userspace can perform a DMA_BUF_IOCTL_EXPORT_SYNC_FILE to retrieve the
> > + * current set of fences on a dma-buf file descriptor as a sync_file.  CPU
> > + * waits via poll() or other driver-specific mechanisms typically wait on
> > + * whatever fences are on the dma-buf at the time the wait begins.  This
> > + * is similar except that it takes a snapshot of the current fences on the
> > + * dma-buf for waiting later instead of waiting immediately.  This is
> > + * useful for modern graphics APIs such as Vulkan which assume an explicit
> > + * synchronization model but still need to inter-operate with dma-buf.
> > + */
> > +struct dma_buf_export_sync_file {
> > +   /**
> > +    * @flags: Read/write flags
> > +    *
> > +    * Must be DMA_BUF_SYNC_READ, DMA_BUF_SYNC_WRITE, or both.
> > +    *
> > +    * If DMA_BUF_SYNC_READ is set and DMA_BUF_SYNC_WRITE is not set,
> > +    * the returned sync file waits on any writers of the dma-buf to
> > +    * complete.  Waiting on the returned sync file is equivalent to
> > +    * poll() with POLLIN.
> > +    *
> > +    * If DMA_BUF_SYNC_WRITE is set, the returned sync file waits on
> > +    * any users of the dma-buf (read or write) to complete.  Waiting
> > +    * on the returned sync file is equivalent to poll() with POLLOUT.
> > +    * If both DMA_BUF_SYNC_WRITE and DMA_BUF_SYNC_READ are set, this
> > +    * is equivalent to just DMA_BUF_SYNC_WRITE.
> > +    */
> > +   __u32 flags;
> > +   /** @fd: Returned sync file descriptor */
> > +   __s32 fd;
> > +};
> > +
> >   #define DMA_BUF_BASE              'b'
> >   #define DMA_BUF_IOCTL_SYNC        _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
> > @@ -94,5 +128,6 @@ struct dma_buf_sync {
> >   #define DMA_BUF_SET_NAME  _IOW(DMA_BUF_BASE, 1, const char *)
> >   #define DMA_BUF_SET_NAME_A        _IOW(DMA_BUF_BASE, 1, u32)
> >   #define DMA_BUF_SET_NAME_B        _IOW(DMA_BUF_BASE, 1, u64)
> > +#define DMA_BUF_IOCTL_EXPORT_SYNC_FILE     _IOWR(DMA_BUF_BASE, 2, struct dma_buf_export_sync_file)
> >   #endif
>

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