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

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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@xxxxxxxxxxxxx>
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Tested-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
---
  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)?
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