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

From: Daniel Vetter
Date: Thu Aug 15 2013 - 07:59:49 EST

On Mon, Aug 05, 2013 at 04:22:00PM +1000, Christopher James Halse Rogers wrote:
> 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.
> Signed-off-by: Christopher James Halse Rogers <>

Yeah, loosk good to me and rather useful, so (with the dma-buf docs
improved as suggested below):

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

I've also written some small prime tests in igt, so also:

Tested-by: Daniel Vetter <daniel.vetter@xxxxxxxx>

> ---
> I've run into a point in the radeon DRM userspace where I need the
> size of a dma-buf. I could add a radeon-specific mechanism to get that,
> but this seems like something that would be more generally useful.
> I'm not entirely sure about supporting both SEEK_END and SEEK_CUR; this
> is somewhat of an abuse of lseek, as seeking obviously doesn't make sense.
> It's the obivous idiom for getting the size of what's on the other end of a
> file descriptor, though.
> I didn't notice anywhere to document this; Documentation/dma-buf-api didn't
> seem like the right place. Is there somewhere I've overlooked?

I think adding a section about various other userspace interfaces exposed
below the mmap support section would be good. Feel free to squash in the
belo diff for v2.

Cheers, Daniel

diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
index 0b23261..b3a8aa2 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 allowe
+ 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 llsee.
Miscellaneous notes
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 -
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