Re: [RFC 00/10] V4L2 explicit synchronization support

Gustavo Padovan
2017-06-09T06:25:46Z

2017-06-08 Shuah Khan <shuahkhan@gmail.com>:

> Hi Gustavo,
>
> On Thu, Jun 8, 2017 at 2:17 PM, Mauro Carvalho Chehab
> <mchehab@osg.samsung.com> wrote:
> > Hi Gustavo,
> >
> > Em Wed, 24 May 2017 21:31:01 -0300
> > Gustavo Padovan <gustavo@padovan.org> escreveu:
> >
> >> Hi all,
> >>
> >> I've been working on the v2 of this series, but I think I hit a blocker
> >> when trying to cover the case where the driver asks to requeue the
> >> buffer. It is related to the out-fence side.
> >>
> >> In the current implementation we return on QBUF an out-fence fd that is not
> >> tied to any buffer, because we don't know the queueing order until the
> >> buffer is queued to the driver. Then when the buffer is queued we use
> >> the BUF_QUEUED event to notify userspace of the index of the buffer,
> >> so now userspace knows the buffer associated to the out-fence fd
> >> received earlier.
> >>
> >> Userspace goes ahead and send a DRM Atomic Request to the kernel to
> >> display that buffer on the screen once the fence signals. If it is
> >> a nonblocking request the fence waiting is past the check phase, thus
> >> it isn't allowed to fail anymore.
> >>
> >> But now, what happens if the V4L2 driver calls buffer_done() asking
> >> to requeue the buffer. That means the operation failed and can't
> >> signal the fence, starving the DRM side.
> >>
> >> We need to fix that. The only way I can see is to guarantee ordering of
> >> buffers when out-fences are used. Ordering is something that HAL3 needs
> >> to so maybe there is more than one reason to do it like this. I'm not
> >> a V4L2 expert, so I don't know all the consequences of such a change.
> >>
> >> Any other ideas?
> >>
> >> The current patchset is at:
> >>
> >> https://git.kernel.org/pub/scm/linux/kernel/git/padovan/linux.git/log/?h=v4l2-fences
>
> Do you plan to send the v2 out? I did a quick review and have a few comments.
>
> [media] vb2: split out queueing from vb_core_qbuf()
>
> It changes the sequence a bit.
>
> /* Fill buffer information for the userspace */
> if (pb)
> call_void_bufop(q, fill_user_buffer, vb, pb);
>
> With the changes - user information is filled before __enqueue_in_driver(vb);

Without my changes it also fills it before __enqueue_in_driver() when
start_streaming wasn't called yet. So I don't think it really matters.

>
> Anyway, it might be a good idea to send the v2 out for review and we can review
> patches in detail. I am hoping to test your patch series on odroid-xu4
> next week.
> Could you please add me to the thread as well as include me when you send
> v2 and subsequent versions.

I will send a v2 as soon as I can, but from Thursday next week until
the 25th I'll be on vacation.

Gustavo