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authorChristopher James Halse Rogers <christopher.halse.rogers@canonical.com>2013-09-10 11:36:45 +0530
committerSumit Semwal <sumit.semwal@linaro.org>2013-09-10 11:36:45 +0530
commit19e8697ba45e7bcdb04f2adf6110fbf4882863e5 (patch)
tree249547a50a16d33e2c3c2dea96a592b2b823dc2b /Documentation/dma-buf-sharing.txt
parent9022e24e8946400d53719a761815069c3183e2bd (diff)
downloadlinux-aarch64-19e8697ba45e7bcdb04f2adf6110fbf4882863e5.tar.gz
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 <christopher.halse.rogers@canonical.com> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> Tested-by: Daniel Vetter <daniel.vetter@ffwll.ch> Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Diffstat (limited to 'Documentation/dma-buf-sharing.txt')
-rw-r--r--Documentation/dma-buf-sharing.txt12
1 files changed, 12 insertions, 0 deletions
diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
index e31a2a9d2b07..505e71172ae7 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
-------------------