linux-kernel - Re: [RFC][PATCH 13/15] videobuf2: do not sync buffers for DMABUF queues

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Message-ID: <20200124022544.GD158382@google.com>
Date:   Fri, 24 Jan 2020 11:25:44 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Tomasz Figa <tfiga@...omium.org>,
        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: [RFC][PATCH 13/15] videobuf2: do not sync buffers for DMABUF
 queues

On (20/01/23 12:35), Hans Verkuil wrote:
> On 1/22/20 6:05 AM, Sergey Senozhatsky wrote:
> > On (20/01/10 11:30), Hans Verkuil wrote:

[..]

> But whatever was written in the buffer is going to be overwritten anyway.
> 
> Unless I am mistaken the current situation is that the cache syncs are done
> in both prepare and finish, regardless of the DMA direction.
> 
> I would keep that behavior to avoid introducing any unexpected regressions.

OK.

> Then, if q->allow_cache_hint is set, then default to a cache sync (cache clean)
> in the prepare for OUTPUT buffers and a cache sync (cache invalidate) in the
> finish for CAPTURE buffers.

We alter default cache sync behaviour based both on queue ->memory
type and queue ->dma_dir. Shall both of those cases depend on
->allow_cache_hints, or q->memory can be independent?

static void set_buffer_cache_hints(struct vb2_queue *q,
				   struct vb2_buffer *vb,
				   struct v4l2_buffer *b)
{
	if (!q->allow_cache_hints)
		return;

	/*
	 * 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;
	}

	/*
	 * ->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;
	else
		vb->need_cache_sync_on_finish = 1;

	/*
	 * ->prepare() cache sync can be avoided when queue direction is
	 * FROM_DEVICE.
	 */
	if (q->dma_dir == DMA_FROM_DEVICE)
		vb->need_cache_sync_on_prepare = 0;
	else
		vb->need_cache_sync_on_prepare = 1;

	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;
}