linux-kernel - Re: [PATCHv3] media: vb2: always set buffer cache sync hints

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Date:   Mon, 30 Nov 2020 11:02:31 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCHv3] media: vb2: always set buffer cache sync hints

On 29/11/2020 04:15, Sergey Senozhatsky wrote:
> We need to always set ->need_cache_sync_on_prepare and
> ->need_cache_sync_on_finish when we initialize vb2 buffer.
> 
> Currently these flags are set/adjusted only in V4L2's
> vb2_queue_or_prepare_buf(), which means that for the code
> paths that don't use V4L2 vb2 will always tell videobuf2
> core to skip ->prepare() and ->finish() cache syncs/flushes.
> 
> This is a quick solution that should do the trick. The
> proper fix, however, is much more complicated and requires
> a rather big videobuf2 refactoring - we need to move cache
> sync/flush decision making out of core videobuf2 to the
> allocators.
> 
> Fixes: f5f5fa73fbfb ("media: videobuf2: handle V4L2 buffer cache flags")
> Reported-by: Tomasz Figa <tfiga@...omium.org>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> ---
>  drivers/media/common/videobuf2/videobuf2-core.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> v3: Improved code comment and dropped queue allow_cache_hints check (Tomasz)
> v2: Added a comment and set cache sync flags only for specific buffers (Hans)
> 
> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> index 5499013cf82e..3f11fc5b5d9a 100644
> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> @@ -414,6 +414,20 @@ static int __vb2_queue_alloc(struct vb2_queue *q, enum vb2_memory memory,
>  		vb->index = q->num_buffers + buffer;
>  		vb->type = q->type;
>  		vb->memory = memory;
> +		/*
> +		 * A workaround fix. We need to set these flags here so that
> +		 * videobuf2 core will call ->prepare()/->finish() cache
> +		 * sync/flush on vb2 buffers when appropriate. Otherwise, for
> +		 * backends that don't rely on V4L2 (perhaps dvb) these flags
> +		 * will always be false and, hence, videobuf2 core will skip
> +		 * cache sync/flush operations. However, we can avoid explicit
> +		 * ->prepare() and ->finish() cache sync for DMABUF buffers,
> +		 * because DMA exporter takes care of it.
> +		 */
> +		if (q->memory != VB2_MEMORY_DMABUF) {
> +			vb->need_cache_sync_on_prepare = 1;
> +			vb->need_cache_sync_on_finish = 1;
> +		}

Is this a work-around fix? Isn't this just a bug fix? It seems reasonable
to always set this when allocating buffers for a queue. And for v4l2 these
values can be changed if supported by the driver (allow_cache_hints is set).

So the comment would be:

	/*
	 * We need to set these flags here so that the videobuf2 core
	 * will call ->prepare()/->finish() cache sync/flush on vb2
	 * buffers when appropriate. However, we can avoid explicit
	 * ->prepare() and ->finish() cache sync for DMABUF buffers,
	 * because DMA exporter takes care of it.
	 */

The commit message would need to be modified as well.

Regards,

	Hans

>  		for (plane = 0; plane < num_planes; ++plane) {
>  			vb->planes[plane].length = plane_sizes[plane];
>  			vb->planes[plane].min_length = plane_sizes[plane];
>