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

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Date:   Wed, 26 Aug 2020 14:46:03 +0200
From:   Hans Verkuil <hverkuil-cisco@...all.nl>
To:     Alexandre Courbot <gnurou@...il.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Nicolas Dufresne <nicolas.dufresne@...labora.com>
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] media: v4l2-mem2mem: simplify poll logic a bit

On 25/08/2020 16:55, Alexandre Courbot 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.
> 
> 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)

This test is unnecessary: only buffers in state DONE or ERROR can be on the done_list.

> +			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)

Ditto.

Regards,

	Hans

> +			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.
>  		 */
> -		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;
>