linux-kernel - Re: [PATCH 2/2] media: v4l2-mem2mem: simplify poll logic a bit

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Date:   Wed, 26 Aug 2020 01:15:12 -0300
From:   Ezequiel Garcia <ezequiel@...guardiasur.com.ar>
To:     Alexandre Courbot <gnurou@...il.com>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Nicolas Dufresne <nicolas.dufresne@...labora.com>,
        linux-media <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] media: v4l2-mem2mem: simplify poll logic a bit

Hi Alexandre,

On Tue, 25 Aug 2020 at 11:56, Alexandre Courbot <gnurou@...il.com> wrote:
>
> Factorize redundant checks into a single code block, remove the early
> return, and declare variables in their innermost block. Hopefully this
> makes this code a little bit easier to follow.
>

This _definitely_ makes the poll handling more readable.

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

See below a nitpick.

> Signed-off-by: Alexandre Courbot <gnurou@...il.com>
> ---
>  drivers/media/v4l2-core/v4l2-mem2mem.c | 35 +++++++++++---------------
>  1 file changed, 15 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/media/v4l2-core/v4l2-mem2mem.c b/drivers/media/v4l2-core/v4l2-mem2mem.c
> index 0d0192119af20..aeac9707123d0 100644
> --- a/drivers/media/v4l2-core/v4l2-mem2mem.c
> +++ b/drivers/media/v4l2-core/v4l2-mem2mem.c
> @@ -841,7 +841,6 @@ static __poll_t v4l2_m2m_poll_for_data(struct file *file,
>                                        struct poll_table_struct *wait)
>  {
>         struct vb2_queue *src_q, *dst_q;
> -       struct vb2_buffer *src_vb = NULL, *dst_vb = NULL;
>         __poll_t rc = 0;
>         unsigned long flags;
>
> @@ -863,33 +862,29 @@ static __poll_t v4l2_m2m_poll_for_data(struct file *file,
>                 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;
> +       if (!list_empty(&src_q->done_list)) {
> +               struct vb2_buffer *src_vb = list_first_entry(
> +                       &src_q->done_list, struct vb2_buffer, done_entry);
> +               if (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)) {
> +       if (!list_empty(&dst_q->done_list)) {
> +               struct vb2_buffer *dst_vb = list_first_entry(
> +                       &dst_q->done_list, struct vb2_buffer, done_entry);
> +               if (dst_vb->state == VB2_BUF_STATE_DONE ||
> +                   dst_vb->state == VB2_BUF_STATE_ERROR)
> +                       rc |= EPOLLIN | EPOLLRDNORM;
> +       } else if (dst_q->last_buffer_dequeued) {
>                 /*
>                  * If the last buffer was dequeued from the capture queue,
>                  * return immediately. DQBUF will return -EPIPE.
>                  */

The part about "returning immediately" doesn't make
much sense now. Could we rephrase this, keeping the -EPIPE
comment?

Thanks,
Ezequiel

> -               if (dst_q->last_buffer_dequeued) {
> -                       spin_unlock_irqrestore(&dst_q->done_lock, flags);
> -                       rc |= EPOLLIN | EPOLLRDNORM;
> -                       return rc;
> -               }
> -       }
> -
> -       if (!list_empty(&dst_q->done_list))
> -               dst_vb = list_first_entry(&dst_q->done_list, struct vb2_buffer,
> -                                               done_entry);
> -       if (dst_vb && (dst_vb->state == VB2_BUF_STATE_DONE
> -                       || dst_vb->state == VB2_BUF_STATE_ERROR))
>                 rc |= EPOLLIN | EPOLLRDNORM;
> +       }
>         spin_unlock_irqrestore(&dst_q->done_lock, flags);
>
>         return rc;
> --
> 2.28.0
>