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From:   Alexandre Courbot <gnurou@gmail.com>
Date:   Wed, 26 Aug 2020 20:25:38 +0900
Message-ID: <CAAVeFu+0UqVwoUmOXm38E-vikaQZ4Graaz_9ERWozNAeTFkrPQ@mail.gmail.com>
Subject: Re: [PATCH 0/2] media: v4l2-mem2mem: fix poll() bug
To:     Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Cc:     Mauro Carvalho Chehab <mchehab@kernel.org>,
        Hans Verkuil <hverkuil-cisco@xs4all.nl>,
        Nicolas Dufresne <nicolas.dufresne@collabora.com>,
        linux-media <linux-media@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, Aug 26, 2020 at 7:10 AM Ezequiel Garcia
<ezequiel@vanguardiasur.com.ar> wrote:
>
> Hello Alex,
>
> Thanks for the patch.
>
> On Tue, Aug 25, 2020, 11:56 AM Alexandre Courbot <gnurou@gmail.com> wrote:
> >
> > This addresses a very corner case that probably nobody ever encounters,
> > but I have hit it when playing with vicoded so here is a tentative fix.
> >
>
> I'll try to make a more complete review soon, but meanwhile
> I was thinking if it was possible to include a little kselftest program
> for this issue, something CIs can pick-up and test corner cases like this,
> making sure we don't regress on the issue.
>
> (Or alternatively, v4l2-compliance?)

I am not very familiar with kselftest, but IIUC the conditions that
lead to this issue are easier to reproduce using v4l2-compliance,
where we can have a user-space driving the queues. It would also have
the benefit to exercise all drivers. I'll think about adding such a
test, thanks for the suggestion!

>
> Thanks!
> Ezequiel
>
> > Patch 1/2 addresses the issue that when the last buffer of a m2m device
> > has been dequeued, any attempt to poll with EPOLLOUT will result in only
> > EPOLLIN being returned, even if OUTPUT buffers are still pending. The
> > issue stems from the fact that the last buffer check if done first, and
> > returns immediately if true.
> >
> > Patch 2/2 builds on the first one to (hopefully) clean up the code a bit
> > and make the function flow easier to follow. Functionally speaking it is
> > supposed to be a no-op and it can safely be dropped if the former code
> > is preferred - the actual fix is in 1/2.
> >
> > Alexandre Courbot (2):
> >   media: v4l2-mem2mem: consider OUTPUT queue first when polling
> >   media: v4l2-mem2mem: simplify poll logic a bit
> >
> >  drivers/media/v4l2-core/v4l2-mem2mem.c | 42 +++++++++++---------------
> >  1 file changed, 18 insertions(+), 24 deletions(-)
> >
> > --
> > 2.28.0
> >