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From:   Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Date:   Wed, 26 Aug 2020 01:07:52 -0300
Message-ID: <CAAEAJfBeJnSjqfyhosM_6jP4C+wQ7UVmt=oG_O0w--sAf0=0PQ@mail.gmail.com>
Subject: Re: [PATCH 1/2] media: v4l2-mem2mem: consider OUTPUT queue first when polling
To:     Alexandre Courbot <gnurou@gmail.com>
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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Hi Alexandre,

On Tue, 25 Aug 2020 at 11:56, Alexandre Courbot <gnurou@gmail.com> wrote:
>
> If poll() is called on a m2m device with the EPOLLOUT event after the
> last buffer of the CAPTURE queue is dequeued, any buffer available on
> OUTPUT queue will never be signaled because v4l2_m2m_poll_for_data()
> starts by checking whether dst_q->last_buffer_dequeued is set and
> returns EPOLLIN in this case, without looking at the state of the OUTPUT
> queue.
>
> Fix this by checking the state of the OUTPUT queue before considering
> that early-return case.
>
> This also has the side-effect of bringing the two blocks of code dealing
> with the CAPTURE queue next to one another, and saves us one spin
> lock/unlock cycle, for what it's worth.
>
> Signed-off-by: Alexandre Courbot <gnurou@gmail.com>

Change looks good to me.

Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>

Do you think it qualifies for -stable? The issue has been
here since the dawn of time.

Thanks,
Ezequiel

> ---
>  drivers/media/v4l2-core/v4l2-mem2mem.c | 23 +++++++++++------------
>  1 file changed, 11 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/media/v4l2-core/v4l2-mem2mem.c b/drivers/media/v4l2-core/v4l2-mem2mem.c
> index 95a8f2dc5341d..0d0192119af20 100644
> --- a/drivers/media/v4l2-core/v4l2-mem2mem.c
> +++ b/drivers/media/v4l2-core/v4l2-mem2mem.c
> @@ -862,6 +862,15 @@ static __poll_t v4l2_m2m_poll_for_data(struct file *file,
>              list_empty(&dst_q->queued_list)))
>                 return EPOLLERR;
>
> +       spin_lock_irqsave(&src_q->done_lock, flags);
> +       if (!list_empty(&src_q->done_list))
> +               src_vb = list_first_entry(&src_q->done_list, struct vb2_buffer,
> +                                               done_entry);
> +       if (src_vb && (src_vb->state == VB2_BUF_STATE_DONE
> +                       || src_vb->state == VB2_BUF_STATE_ERROR))
> +               rc |= EPOLLOUT | EPOLLWRNORM;
> +       spin_unlock_irqrestore(&src_q->done_lock, flags);
> +
>         spin_lock_irqsave(&dst_q->done_lock, flags);
>         if (list_empty(&dst_q->done_list)) {
>                 /*
> @@ -870,21 +879,11 @@ static __poll_t v4l2_m2m_poll_for_data(struct file *file,
>                  */
>                 if (dst_q->last_buffer_dequeued) {
>                         spin_unlock_irqrestore(&dst_q->done_lock, flags);
> -                       return EPOLLIN | EPOLLRDNORM;
> +                       rc |= EPOLLIN | EPOLLRDNORM;
> +                       return rc;
>                 }
>         }
> -       spin_unlock_irqrestore(&dst_q->done_lock, flags);
>
> -       spin_lock_irqsave(&src_q->done_lock, flags);
> -       if (!list_empty(&src_q->done_list))
> -               src_vb = list_first_entry(&src_q->done_list, struct vb2_buffer,
> -                                               done_entry);
> -       if (src_vb && (src_vb->state == VB2_BUF_STATE_DONE
> -                       || src_vb->state == VB2_BUF_STATE_ERROR))
> -               rc |= EPOLLOUT | EPOLLWRNORM;
> -       spin_unlock_irqrestore(&src_q->done_lock, flags);
> -
> -       spin_lock_irqsave(&dst_q->done_lock, flags);
>         if (!list_empty(&dst_q->done_list))
>                 dst_vb = list_first_entry(&dst_q->done_list, struct vb2_buffer,
>                                                 done_entry);
> --
> 2.28.0
>