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