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

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Message-ID: <CAAEAJfCfjzGDOhD2WHYny-wVwL19qc_VA9c3uVNiHxpYdEHsLQ@mail.gmail.com>
Date:   Wed, 26 Aug 2020 11:32:31 -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

On Wed, 26 Aug 2020 at 08:19, Alexandre Courbot <gnurou@...il.com> wrote:
>
> Hi Ezequiel, thanks for the review!
>
> On Wed, Aug 26, 2020 at 1:15 PM Ezequiel Garcia
> <ezequiel@...guardiasur.com.ar> wrote:
> >
> > 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?
>
> I understood this sentence as referring to the system call and not
> just this function, but maybe we can rephrase this as "... make
> user-space wake up immediately"?
>

But is this really about user-space wakeup? I am under the impression
that past poll_wait on both queues, we are already about to return
(and wakeup).

The way I see it, the original commit intention was to skip any
done_list handling, returning immediately on the last buffer condition.

How about just

"""
If the last buffer was dequeued from the capture queue,
signal userspace. DQBUF will return -EPIPE.
"""

?

> >
> > 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
> > >