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

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Tomasz,

On Saturday, 20 October 2018 13:24:20 EEST Tomasz Figa wrote:
> On Thu, Oct 18, 2018 at 7:03 PM Tomasz Figa wrote:
> > On Wed, Oct 17, 2018 at 10:34 PM Laurent Pinchart 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?

Not really on the spec side. I think we'll have to implement helper functions 
for drivers to use if we want to ensure a consistent and bug-free behaviour.

-- 
Regards,

Laurent Pinchart