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

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Date:   Fri, 27 Nov 2020 15:56:47 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Tomasz Figa <tfiga@...omium.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        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 15:30, Sergey Senozhatsky wrote:
> On (20/11/27 13:59), Hans Verkuil wrote:
>>
>> 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.
> 
> Something like this?

Yes.

BTW, wouldn't it be sufficient to change this code to:

	if (!q->allow_cache_hints && q->memory != VB2_MEMORY_DMABUF) {
		vb->need_cache_sync_on_prepare = 1;
		vb->need_cache_sync_on_finish = 1;
	}

Or am I missing something?

DVB drivers do not set this flag, and even for DVB cache sync shouldn't be
needed for DMABUF, right?

Regards,

	Hans

> 
> ---
> 
> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> index 5499013cf82e..21b2b0ae3629 100644
> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> @@ -414,6 +414,16 @@ 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 always call ->prepare()/->finish()
> +                * cache sync/flush on vb2 buffers. 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.
> +                */
> +               vb->need_cache_sync_on_prepare = 1;
> +               vb->need_cache_sync_on_finish = 1;
>                 for (plane = 0; plane < num_planes; ++plane) {
>                         vb->planes[plane].length = plane_sizes[plane];
>                         vb->planes[plane].min_length = plane_sizes[plane];
>