Le 04/09/2013 14:05, Daniel Vetter a écrit :
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@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)?
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
ACK, thanks Daniel.
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