[PATCH] dma-buf: Expose buffer size to userspace (v2)

[PATCH] dma-buf: Expose buffer size to userspace (v2)

[Christopher James Halse Rogers]

Each dma-buf has an associated size and it's reasonable for userspace
to want to know what it is.

Since userspace already has an fd, expose the size using the
size = lseek(fd, SEEK_END, 0); lseek(fd, SEEK_CUR, 0);
idiom.

v2: Added Daniel's sugeested documentation, with minor fixups

Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---
 Documentation/dma-buf-sharing.txt | 12 ++++++++++++
 drivers/base/dma-buf.c            | 28 ++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+)

diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
index 0b23261..849e982 100644
--- a/Documentation/dma-buf-sharing.txt
+++ b/Documentation/dma-buf-sharing.txt
@@ -407,6 +407,18 @@ Being able to mmap an export dma-buf buffer object has 2 main use-cases:
    interesting ways depending upong the exporter (if userspace starts depending
    upon this implicit synchronization).
 
+Other Interfaces Exposed to Userspace on the dma-buf FD
+------------------------------------------------------
+
+- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
+  with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
+  the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
+  llseek operation will report -EINVAL.
+
+  If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
+  cases. Userspace can use this to detect support for discovering the dma-buf
+  size using llseek.
+
 Miscellaneous notes
 -------------------
 
diff --git a/drivers/base/dma-buf.c b/drivers/base/dma-buf.c
index 6687ba7..c33a857 100644
--- a/drivers/base/dma-buf.c
+++ b/drivers/base/dma-buf.c
@@ -77,9 +77,36 @@ static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
 	return dmabuf->ops->mmap(dmabuf, vma);
 }
 
+static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
+{
+	struct dma_buf *dmabuf;
+	loff_t base;
+
+	if (!is_dma_buf_file(file))
+		return -EBADF;
+
+	dmabuf = file->private_data;
+
+	/* only support discovering the end of the buffer,
+	   but also allow SEEK_SET to maintain the idiomatic
+	   SEEK_END(0), SEEK_CUR(0) pattern */
+	if (whence == SEEK_END)
+		base = dmabuf->size;
+	else if (whence == SEEK_SET)
+		base = 0;
+	else
+		return -EINVAL;
+
+	if (offset != 0)
+		return -EINVAL;
+
+	return base + offset;
+}
+
 static const struct file_operations dma_buf_fops = {
 	.release	= dma_buf_release,
 	.mmap		= dma_buf_mmap_internal,
+	.llseek		= dma_buf_llseek,
 };
 
 /*
@@ -133,6 +160,7 @@ struct dma_buf *dma_buf_export_named(void *priv, const struct dma_buf_ops *ops,
 	dmabuf->exp_name = exp_name;
 
 	file = anon_inode_getfile("dmabuf", &dma_buf_fops, dmabuf, flags);
+	file->f_mode |= FMODE_LSEEK;
 
 	dmabuf->file = file;
 
-- 
1.8.3.2

Re: [PATCH] dma-buf: Expose buffer size to userspace (v2)

[Martin Peres]

Hi Christopher,
Le 04/09/2013 05:15, Christopher James Halse Rogers a écrit :
> Each dma-buf has an associated size and it's reasonable for userspace
> to want to know what it is.
>
> Since userspace already has an fd, expose the size using the
> size = lseek(fd, SEEK_END, 0); lseek(fd, SEEK_CUR, 0);
> idiom.
>
> v2: Added Daniel's sugeested documentation, with minor fixups
>
> Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> Tested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
>   Documentation/dma-buf-sharing.txt | 12 ++++++++++++
>   drivers/base/dma-buf.c            | 28 ++++++++++++++++++++++++++++
>   2 files changed, 40 insertions(+)
>
> diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
> index 0b23261..849e982 100644
> --- a/Documentation/dma-buf-sharing.txt
> +++ b/Documentation/dma-buf-sharing.txt
> @@ -407,6 +407,18 @@ Being able to mmap an export dma-buf buffer object has 2 main use-cases:
>      interesting ways depending upong the exporter (if userspace starts depending
>      upon this implicit synchronization).
>   
> +Other Interfaces Exposed to Userspace on the dma-buf FD
> +------------------------------------------------------
> +
> +- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
> +  with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
> +  the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
> +  llseek operation will report -EINVAL.
> +
> +  If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
Shouldn't it be "supported" instead of "support"?

Anyway, I'm just curious, in which case is it important to know the size?
Do we already have a way to get the dimensions (x, y and stripe)?

Re: [PATCH] dma-buf: Expose buffer size to userspace (v2)

[Daniel Vetter]

On Wed, Sep 04, 2013 at 12:24:27PM +0200, Martin Peres wrote:
> Hi Christopher,
> Le 04/09/2013 05:15, Christopher James Halse Rogers a écrit :
> >Each dma-buf has an associated size and it's reasonable for userspace
> >to want to know what it is.
> >
> >Since userspace already has an fd, expose the size using the
> >size = lseek(fd, SEEK_END, 0); lseek(fd, SEEK_CUR, 0);
> >idiom.
> >
> >v2: Added Daniel's sugeested documentation, with minor fixups
> >
> >Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>
> >Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> >Tested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> >---
> >  Documentation/dma-buf-sharing.txt | 12 ++++++++++++
> >  drivers/base/dma-buf.c            | 28 ++++++++++++++++++++++++++++
> >  2 files changed, 40 insertions(+)
> >
> >diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
> >index 0b23261..849e982 100644
> >--- a/Documentation/dma-buf-sharing.txt
> >+++ b/Documentation/dma-buf-sharing.txt
> >@@ -407,6 +407,18 @@ Being able to mmap an export dma-buf buffer object has 2 main use-cases:
> >     interesting ways depending upong the exporter (if userspace starts depending
> >     upon this implicit synchronization).
> >+Other Interfaces Exposed to Userspace on the dma-buf FD
> >+------------------------------------------------------
> >+
> >+- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
> >+  with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
> >+  the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
> >+  llseek operation will report -EINVAL.
> >+
> >+  If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
> Shouldn't it be "supported" instead of "support"?
> 
> Anyway, I'm just curious, in which case is it important to know the size?
> Do we already have a way to get the dimensions (x, y and stripe)?

Size is an invariant part of a dma-buf. Everything else wrt metadata isn't
tracked in the kernel, so doesn't make much sense to expose it ;-)
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

Re: [PATCH] dma-buf: Expose buffer size to userspace (v2)

[Martin Peres]

Le 04/09/2013 14:05, Daniel Vetter a écrit :
> On Wed, Sep 04, 2013 at 12:24:27PM +0200, Martin Peres wrote:
>> Hi Christopher,
>> Le 04/09/2013 05:15, Christopher James Halse Rogers a écrit :
>>> Each dma-buf has an associated size and it's reasonable for userspace
>>> to want to know what it is.
>>>
>>> Since userspace already has an fd, expose the size using the
>>> size = lseek(fd, SEEK_END, 0); lseek(fd, SEEK_CUR, 0);
>>> idiom.
>>>
>>> v2: Added Daniel's sugeested documentation, with minor fixups
>>>
>>> Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>
>>> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>>> Tested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>>> ---
>>>   Documentation/dma-buf-sharing.txt | 12 ++++++++++++
>>>   drivers/base/dma-buf.c            | 28 ++++++++++++++++++++++++++++
>>>   2 files changed, 40 insertions(+)
>>>
>>> diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
>>> index 0b23261..849e982 100644
>>> --- a/Documentation/dma-buf-sharing.txt
>>> +++ b/Documentation/dma-buf-sharing.txt
>>> @@ -407,6 +407,18 @@ Being able to mmap an export dma-buf buffer object has 2 main use-cases:
>>>      interesting ways depending upong the exporter (if userspace starts depending
>>>      upon this implicit synchronization).
>>> +Other Interfaces Exposed to Userspace on the dma-buf FD
>>> +------------------------------------------------------
>>> +
>>> +- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
>>> +  with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
>>> +  the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
>>> +  llseek operation will report -EINVAL.
>>> +
>>> +  If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
>> Shouldn't it be "supported" instead of "support"?
>>
>> Anyway, I'm just curious, in which case is it important to know the size?
>> Do we already have a way to get the dimensions (x, y and stripe)?
> Size is an invariant part of a dma-buf. Everything else wrt metadata isn't
> tracked in the kernel, so doesn't make much sense to expose it ;-)
> -Daniel
ACK, thanks Daniel.