On 28 September 2011 04:07, Clark, Rob <rob@ti.com> wrote:

On Tue, Sep 27, 2011 at 9:19 AM, Daniel Vetter <daniel@ffwll.ch> wrote:
>
>> One other thing: once you call REQBUFS on a V4L device the V4L spec says that
>> the memory should be allocated at that time. Because V4L often needs a lot of
>> memory that behavior makes sense: you know immediately if you can get the memory
>> or not. In addition, that memory is mmap-ed before the DMA is started.
>
> If that is actually a fixed requirement for v4l, that's a good reason for
> mmap support on the dma_buf object. We could hide all the complecity of
> shooting down userspace mmapings on buffer movements from the drivers.
> Can you elaborate a bit on this?

I would hope if we are using V4L2_MEMORY_DMABUF that V4L should not
create a user mapping of the buffer.

Given that V4L2_MEMORY_DMABUF is something new, I think it is fine to
spec out this part of the V4L spec to now require a userspace mapping.

Thanks for your questions and comments - and for clarifying the use cases to Hans. I will create a Documentation doc for explaining it as easily and crisply as Daniel has done.

I will also update the patch based on all your comments - I think a v3 posted here is warranted before I post to upstream lists I guess.

BR,
-R

Thanks and best regards,

Sumit Semwal

Linaro Kernel Engineer - Graphics working group

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