linux-kernel - Re: [PATCH] media: venus: fix reported size of 0-length buffers

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Message-ID: <CAAFQd5Cw2jmNwHCWYriw4H0TK0uWhVFbgs_RgxXV5npZLWvLbg@mail.gmail.com>
Date:   Fri, 16 Nov 2018 13:08:10 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     nicolas@...fresne.ca
Cc:     Alexandre Courbot <acourbot@...omium.org>,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] media: venus: fix reported size of 0-length buffers

On Fri, Nov 16, 2018 at 1:50 AM Nicolas Dufresne <nicolas@...fresne.ca> wrote:
>
> Le mercredi 14 novembre 2018 à 13:12 +0900, Alexandre Courbot a écrit :
> > On Wed, Nov 14, 2018 at 3:54 AM Nicolas Dufresne <nicolas@...fresne.ca> wrote:
> > >
> > >
> > > Le mar. 13 nov. 2018 04 h 30, Alexandre Courbot <acourbot@...omium.org> a écrit :
> > > > The last buffer is often signaled by an empty buffer with the
> > > > V4L2_BUF_FLAG_LAST flag set. Such buffers were returned with the
> > > > bytesused field set to the full size of the OPB, which leads
> > > > user-space to believe that the buffer actually contains useful data. Fix
> > > > this by passing the number of bytes reported used by the firmware.
> > >
> > > That means the driver does not know on time which one is last. Why not just returned EPIPE to userspace on DQBUF and ovoid this useless roundtrip ?
> >
> > Sorry, I don't understand what you mean. EPIPE is supposed to be
> > returned after a buffer with V4L2_BUF_FLAG_LAST is made available for
> > dequeue. This patch amends the code that prepares this LAST-flagged
> > buffer. How could we avoid a roundtrip in this case?
>
> Maybe it has changed, but when this was introduced, we found that some
> firmware (Exynos MFC) could not know which one is last. Instead, it
> gets an event saying there will be no more buffers.
>

It was never the case with the MFC (firmware/driver) we were using on
Chrome OS and it doesn't seem to be the case for the current upstream
s5p-mfc driver.

> Sending buffers with payload size to 0 just for the sake of setting the
> V4L2_BUF_FLAG_LAST was considered a waste. Specially that after that,
> every polls should return EPIPE. So in the end, we decided the it
> should just unblock the userspace and return EPIPE.
>
> If you look at the related GStreamer code, it completely ignores the
> LAST flag. With fake buffer of size 0, userspace will endup dequeuing
> and throwing away. This is not useful to the process of terminating the
> decoding. To me, this LAST flag is not useful in this context.

Except that -EPIPE is actually signaled by the vb2 core and it happens
after the user space dequeues a buffer with the LAST flag set:
https://elixir.bootlin.com/linux/v4.20-rc2/source/drivers/media/common/videobuf2/videobuf2-core.c#L1634
https://elixir.bootlin.com/linux/v4.20-rc2/source/drivers/media/common/videobuf2/videobuf2-v4l2.c#L555

Best regards,
Tomasz