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Subject: Re: [PATCH] videobuf2: always re-init queue memory consistency
 attribute
To:     Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc:     Mauro Carvalho Chehab <mchehab@kernel.org>,
        Tomasz Figa <tfiga@chromium.org>, linux-media@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20200609060417.8066-1-sergey.senozhatsky@gmail.com>
From:   Hans Verkuil <hverkuil@xs4all.nl>
Message-ID: <d1ebdb9b-ceea-3582-831d-7cef5134d8f9@xs4all.nl>
Date:   Wed, 10 Jun 2020 11:54:33 +0200
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Hi Sergey,

On 09/06/2020 08:04, Sergey Senozhatsky wrote:
> We need a combination of two factors in order to permit modification
> of queue's memory consistency attribute in set_queue_consistency():
>   (1) queue should allow user-space cache hints
>   (2) queue should be used for MMAP-ed I/O
> 
> Therefore the code in videobuf2 core looks as follows:
> 
> 	q->memory = req->memory;
> 	set_queue_consistency(q, consistent);
> 
> This works when we do something like this (suppose that queue allows
> cache hints)
> 
> 	reqbufs.memory = V4L2_MEMORY_DMABUF;
> 	reqbufs.flags = 0;
> 	doioctl(node, VIDIOC_REQBUFS, &reqbufs);
> 
> 	reqbufs.memory = V4L2_MEMORY_MMAP;
> 	reqbufs.flags = V4L2_FLAG_MEMORY_NON_CONSISTENT;
> 	doioctl(node, VIDIOC_REQBUFS, &reqbufs);
> 
> However, this doesn't work the other way around
> 
> 	reqbufs.memory = V4L2_MEMORY_MMAP;
> 	reqbufs.flags = V4L2_FLAG_MEMORY_NON_CONSISTENT;
> 	doioctl(node, VIDIOC_REQBUFS, &reqbufs);
> 
> 	reqbufs.memory = V4L2_MEMORY_DMABUF;
> 	reqbufs.flags = 0;
> 	doioctl(node, VIDIOC_REQBUFS, &reqbufs);
> 
> because __vb2_queue_free() doesn't clear queue's ->dma_attrs
> once its don't freeing queue buffers, and by the time we call
> set_queue_consistency() the queue is DMABUF so (2) is false
> and we never clear the stale consistency attribute.
> 
> Re-implement set_queue_consistency() - it must always clear
> queue's non-consistency attr and set it only if (1) and (2).
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> ---
>  drivers/media/common/videobuf2/videobuf2-core.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> index 7e081716b8da..37d0186ba330 100644
> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> @@ -704,12 +704,11 @@ EXPORT_SYMBOL(vb2_verify_memory_type);
>  
>  static void set_queue_consistency(struct vb2_queue *q, bool consistent_mem)
>  {
> +	q->dma_attrs &= ~DMA_ATTR_NON_CONSISTENT;
> +
>  	if (!vb2_queue_allows_cache_hints(q))
>  		return;
> -
> -	if (consistent_mem)
> -		q->dma_attrs &= ~DMA_ATTR_NON_CONSISTENT;
> -	else
> +	if (!consistent_mem)
>  		q->dma_attrs |= DMA_ATTR_NON_CONSISTENT;
>  }
>  
> 

Is it OK with you if I fold this patch into the original patch series and make a
new PR for it? I prefer to have a series merged without a bug, rather than fixing
it in a final patch.

Regards,

	Hans