[PATCH 1/4] media: staging: rkisp1: cap: don't set next buffer from rkisp1_vb2_buf_queue

[Helen Koike]

Hi Dafna,

On 7/14/20 9:38 AM, Dafna Hirschfeld wrote:
> The function 'rkisp1_vb2_buf_queue' sets the next buffer directly
> in case the capture is already streaming but no frame yet arrived
> from the sensor. This is an optimization that tries to avoid
> dropping a frame.
> The call atomic_read(&cap->rkisp1->isp.frame_sequence) is used
> to check if a frame arrived. Reading the 'frame_sequence' should
> be avoided outside irq handlers to avoid race conditions.
> 
> This patch removes this optimization. Dropping of the first
> frames can be avoided if userspace queues the buffers before
> start streaming. If userspace starts queueing buffers
> only after calling 'streamon' he risks frame drops anyway.
> 
> Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld at collabora.com>
> ---
>  drivers/staging/media/rkisp1/rkisp1-capture.c | 13 +------------
>  1 file changed, 1 insertion(+), 12 deletions(-)
> 
> diff --git a/drivers/staging/media/rkisp1/rkisp1-capture.c b/drivers/staging/media/rkisp1/rkisp1-capture.c
> index 793ec884c894..572b0949c81f 100644
> --- a/drivers/staging/media/rkisp1/rkisp1-capture.c
> +++ b/drivers/staging/media/rkisp1/rkisp1-capture.c
> @@ -743,18 +743,7 @@ static void rkisp1_vb2_buf_queue(struct vb2_buffer *vb)
>  		     ispbuf->buff_addr[RKISP1_PLANE_CB]);
>  
>  	spin_lock_irqsave(&cap->buf.lock, flags);
> -
> -	/*
> -	 * If there's no next buffer assigned, queue this buffer directly
> -	 * as the next buffer, and update the memory interface.
> -	 */
> -	if (cap->is_streaming && !cap->buf.next &&
> -	    atomic_read(&cap->rkisp1->isp.frame_sequence) == -1) {
> -		cap->buf.next = ispbuf;
> -		rkisp1_set_next_buf(cap);

Doesn't this mean we'll always drop the first frame? Since the first user buffer will only be configured to
the hardware after receiving the first frame? So the first frame will always go to the dummy buffer, no?

Thanks
Helen

> -	} else {
> -		list_add_tail(&ispbuf->queue, &cap->buf.queue);
> -	}
> +	list_add_tail(&ispbuf->queue, &cap->buf.queue);
>  	spin_unlock_irqrestore(&cap->buf.lock, flags);
>  }
>  
>