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

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Message-ID: <CAAFQd5D=Lj2XzUMK1LpOXbK3nTdNCFKRkfj3yfpV9mguDkcvdw@mail.gmail.com>
Date:   Sat, 20 Oct 2018 19:24:20 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Linux Media Mailing List <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil@...all.nl>,
        Pawel Osciak <posciak@...omium.org>,
        Alexandre Courbot <acourbot@...omium.org>, kamil@...as.org,
        a.hajda@...sung.com, Kyungmin Park <kyungmin.park@...sung.com>,
        jtp.park@...sung.com, Philipp Zabel <p.zabel@...gutronix.de>,
        Tiffany Lin (林慧珊) 
        <tiffany.lin@...iatek.com>,
        Andrew-CT Chen (陳智迪) 
        <andrew-ct.chen@...iatek.com>, todor.tomov@...aro.org,
        nicolas@...fresne.ca,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        dave.stevenson@...pberrypi.org,
        Ezequiel Garcia <ezequiel@...labora.com>
Subject: Re: [PATCH 1/2] media: docs-rst: Document memory-to-memory video
 decoder interface

On Thu, Oct 18, 2018 at 7:03 PM Tomasz Figa <tfiga@...omium.org> wrote:
>
> Hi Laurent,
>
> On Wed, Oct 17, 2018 at 10:34 PM Laurent Pinchart
> <laurent.pinchart@...asonboard.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