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

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Message-ID: <0dbfa509-8c82-7470-c18b-24ab5c92dc4b@xs4all.nl>
Date:   Fri, 27 Nov 2020 13:59:58 +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: [PATCH] media: vb2: always set buffer cache sync hints

On 27/11/2020 10:41, 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.
> 
> Reported-by: Tomasz Figa <tfiga@...omium.org>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> ---
>  drivers/media/common/videobuf2/videobuf2-core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> index 5499013cf82e..14a26888a892 100644
> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> @@ -414,6 +414,8 @@ 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;
> +		vb->need_cache_sync_on_prepare = 1;
> +		vb->need_cache_sync_on_finish = 1;

I think this needs a comment, basically explaining what you said in
the commit log. It's not obvious from the code that this is a
workaround.

Regards,

	Hans

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