From 19e8697ba45e7bcdb04f2adf6110fbf4882863e5 Mon Sep 17 00:00:00 2001 From: Christopher James Halse Rogers Date: Tue, 10 Sep 2013 11:36:45 +0530 Subject: dma-buf: Expose buffer size to userspace (v2) 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 Reviewed-by: Daniel Vetter Tested-by: Daniel Vetter Signed-off-by: Sumit Semwal --- Documentation/dma-buf-sharing.txt | 12 ++++++++++++ 1 file changed, 12 insertions(+) (limited to 'Documentation') diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt index e31a2a9d2b0..505e71172ae 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 ------------------- -- cgit v1.2.3