linux-kernel - Re: [PATCHv3 02/11] videobuf2: handle V4L2 buffer cache flags

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Date:   Thu, 27 Feb 2020 12:58:45 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Tomasz Figa <tfiga@...omium.org>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        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: [PATCHv3 02/11] videobuf2: handle V4L2 buffer cache flags

On 2/26/20 12:15 PM, Sergey Senozhatsky wrote:
> Set video buffer cache management flags corresponding to V4L2 cache
> flags.
> 
> Both ->prepare() and ->finish() cache management hints should be
> passed during this stage (buffer preparation), because there is no
> other way for user-space to skip ->finish() cache flush.
> 
> There are two possible alternative approaches:
> - The first one is to move cache sync from ->finish() to dqbuf().
>   But this breaks some drivers, that need to fix-up buffers before
>   dequeueing them.
> 
> - The second one is to move ->finish() call from ->done() to dqbuf.

It's not clear what the purpose is of describing these alternative approaches.
I'd just drop that. It's a bit confusing.

> 
> Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> ---
>  .../media/common/videobuf2/videobuf2-v4l2.c   | 49 +++++++++++++++++++
>  include/media/videobuf2-core.h                | 11 +++++
>  2 files changed, 60 insertions(+)
> 
> diff --git a/drivers/media/common/videobuf2/videobuf2-v4l2.c b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> index eb5d5db96552..2a604bd7793a 100644
> --- a/drivers/media/common/videobuf2/videobuf2-v4l2.c
> +++ b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> @@ -199,6 +199,15 @@ static int vb2_fill_vb2_v4l2_buffer(struct vb2_buffer *vb, struct v4l2_buffer *b
>  	vbuf->request_fd = -1;
>  	vbuf->is_held = false;
>  
> +	/*
> +	 * Clear buffer cache flags if queue does not support user space hints.
> +	 * That's to indicate to userspace that these flags won't work.
> +	 */
> +	if (!vb2_queue_allows_cache_hints(q)) {
> +		b->flags &= ~V4L2_BUF_FLAG_NO_CACHE_INVALIDATE;
> +		b->flags &= ~V4L2_BUF_FLAG_NO_CACHE_CLEAN;
> +	}
> +
>  	if (V4L2_TYPE_IS_MULTIPLANAR(b->type)) {
>  		switch (b->memory) {
>  		case VB2_MEMORY_USERPTR:
> @@ -337,6 +346,45 @@ static int vb2_fill_vb2_v4l2_buffer(struct vb2_buffer *vb, struct v4l2_buffer *b
>  	return 0;
>  }
>  
> +static void set_buffer_cache_hints(struct vb2_queue *q,
> +				   struct vb2_buffer *vb,
> +				   struct v4l2_buffer *b)
> +{
> +	/*
> +	 * DMA exporter should take care of cache syncs, so we can avoid
> +	 * explicit ->prepare()/->finish() syncs. For other ->memory types
> +	 * we always need ->prepare() or/and ->finish() cache sync.
> +	 */
> +	if (q->memory == VB2_MEMORY_DMABUF) {
> +		vb->need_cache_sync_on_finish = 0;
> +		vb->need_cache_sync_on_prepare = 0;
> +		return;
> +	}
> +
> +	/*
> +	 * Cache sync/invalidation flags are set by default in order to
> +	 * preserve existing behaviour for old apps/drivers.
> +	 */
> +	vb->need_cache_sync_on_prepare = 1;
> +	vb->need_cache_sync_on_finish = 1;
> +
> +	if (!vb2_queue_allows_cache_hints(q))
> +		return;
> +
> +	/*
> +	 * ->finish() cache sync can be avoided when queue direction is
> +	 * TO_DEVICE.
> +	 */
> +	if (q->dma_dir == DMA_TO_DEVICE)
> +		vb->need_cache_sync_on_finish = 0;
> +
> +	if (b->flags & V4L2_BUF_FLAG_NO_CACHE_INVALIDATE)
> +		vb->need_cache_sync_on_finish = 0;
> +
> +	if (b->flags & V4L2_BUF_FLAG_NO_CACHE_CLEAN)
> +		vb->need_cache_sync_on_prepare = 0;
> +}
> +
>  static int vb2_queue_or_prepare_buf(struct vb2_queue *q, struct media_device *mdev,
>  				    struct v4l2_buffer *b, bool is_prepare,
>  				    struct media_request **p_req)
> @@ -381,6 +429,7 @@ static int vb2_queue_or_prepare_buf(struct vb2_queue *q, struct media_device *md
>  	}
>  
>  	if (!vb->prepared) {
> +		set_buffer_cache_hints(q, vb, b);
>  		/* Copy relevant information provided by the userspace */
>  		memset(vbuf->planes, 0,
>  		       sizeof(vbuf->planes[0]) * vb->num_planes);
> diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
> index 4a19170672ac..d307869bfb15 100644
> --- a/include/media/videobuf2-core.h
> +++ b/include/media/videobuf2-core.h
> @@ -632,6 +632,17 @@ struct vb2_queue {
>  #endif
>  };
>  
> +/**
> + * vb2_queue_allows_cache_hints() - Return true if the queue allows cache
> + * and memory consistency hints.
> + *
> + * @q:		pointer to &struct vb2_queue with videobuf2 queue
> + */
> +static inline bool vb2_queue_allows_cache_hints(struct vb2_queue *q)
> +{
> +	return (q->allow_cache_hints != 0) && (q->memory == VB2_MEMORY_MMAP);

Simply to:

	return q->allow_cache_hints && q->memory == VB2_MEMORY_MMAP;

Regards,

	Hans

> +}
> +
>  /**
>   * vb2_plane_vaddr() - Return a kernel virtual address of a given plane.
>   * @vb:		pointer to &struct vb2_buffer to which the plane in
>