linux-kernel - Re: [RFC][PATCH 13/15] videobuf2: do not sync buffers for DMABUF queues

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Date:   Fri, 10 Jan 2020 11:30:41 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     Sakari Ailus <sakari.ailus@....fi>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Pawel Osciak <posciak@...omium.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 13/15] videobuf2: do not sync buffers for DMABUF
 queues

On 12/17/19 4:20 AM, Sergey Senozhatsky wrote:
> DMA-exporter is supposed to handle cache syncs, so we can
> avoid ->prepare()/->finish() syncs from videobuf2 core for
> DMABUF buffers.
> 
> Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> ---
>  drivers/media/common/videobuf2/videobuf2-v4l2.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/media/common/videobuf2/videobuf2-v4l2.c b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> index 1762849288ae..2b9d3318e6fb 100644
> --- a/drivers/media/common/videobuf2/videobuf2-v4l2.c
> +++ b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> @@ -341,8 +341,22 @@ static void set_buffer_cache_hints(struct vb2_queue *q,
>  				   struct vb2_buffer *vb,
>  				   struct v4l2_buffer *b)
>  {
> -	vb->need_cache_sync_on_prepare = 1;
> +	/*
> +	 * DMA exporter should take care of cache syncs, so we can avoid
> +	 * explicit ->prepare()/->finish() syncs.
> +	 */
> +	if (q->memory == VB2_MEMORY_DMABUF) {
> +		vb->need_cache_sync_on_finish = 0;
> +		vb->need_cache_sync_on_prepare = 0;
> +		return;
> +	}
>  
> +	/*
> +	 * For other ->memory types we always need ->prepare() cache
> +	 * sync. ->finish() cache sync, however, can be avoided when queue
> +	 * direction is TO_DEVICE.
> +	 */
> +	vb->need_cache_sync_on_prepare = 1;

I'm trying to remember: what needs to be done in prepare()
for a capture buffer? I thought that for capture you only
needed to invalidate the cache in finish(), but nothing needs
to be done in the prepare().

Regards,

	Hans

>  	if (q->dma_dir != DMA_TO_DEVICE)
>  		vb->need_cache_sync_on_finish = 1;
>  	else
>