Re: [PATCH 1/2] media: docs-rst: Document memory-to-memory video decoder interface

[Tomasz Figa <tfiga@chromium.org>]

On Thu, Oct 18, 2018 at 7:03 PM Tomasz Figa <tfiga@chromium.org> wrote:
>
> Hi Laurent,
>
> On Wed, Oct 17, 2018 at 10:34 PM Laurent Pinchart
> <laurent.pinchart@ideasonboard.com> wrote:
> >
> > Hi Tomasz,
> >
> > Thank you for the patch.
>
> Thanks for your comments! Please see my replies inline.
>
> >
> > On Tuesday, 24 July 2018 17:06:20 EEST Tomasz Figa wrote:
[snip]
> > > +4. At this point, decoding is paused and the driver will accept, but not
> > > +   process any newly queued ``OUTPUT`` buffers until the client issues
> > > +   ``V4L2_DEC_CMD_START`` or restarts streaming on any queue.
> > > +
> > > +* Once the drain sequence is initiated, the client needs to drive it to
> > > +  completion, as described by the above steps, unless it aborts the process
> > > +  by issuing :c:func:`VIDIOC_STREAMOFF` on ``OUTPUT`` queue.  The client
> > > +  is not allowed to issue ``V4L2_DEC_CMD_START`` or ``V4L2_DEC_CMD_STOP``
> > > +  again while the drain sequence is in progress and they will fail with
> > > +  -EBUSY error code if attempted.
> >
> > While this seems OK to me, I think drivers will need help to implement all the
> > corner cases correctly without race conditions.
>
> We went through the possible list of corner cases and concluded that
> there is no use in handling them, especially considering how much they
> would complicate both the userspace and the drivers. Not even
> mentioning some hardware, like s5p-mfc, which actually has a dedicated
> flush operation, that needs to complete before the decoder can switch
> back to normal mode.

Actually I misread your comment.

Agreed that the decoder commands are a bit tricky to implement
properly. That's one of the reasons I decided to make the return
-EBUSY while an existing drain is in progress.

Do you have any particular simplification in mind that could avoid
some corner cases?

Best regards,
Tomasz