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[Linaro-mm-sig] dma-buf: Expose buffer size to userspace

Message ID 20130815115932.GC776@phenom.ffwll.local
State New
Headers show

Commit Message

Daniel Vetter Aug. 15, 2013, 11:59 a.m. UTC 
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 <christopher.halse.rogers@canonical.com>

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

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

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

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

> ---
> 
> 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

Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
diff mbox

Patch

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