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

From: Sergey Senozhatsky <senozhatsky@chromium.org>
To: Hans Verkuil <hverkuil@xs4all.nl>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>,
	Hans Verkuil <hans.verkuil@cisco.com>,
	Tomasz Figa <tfiga@chromium.org>,
	Mauro Carvalho Chehab <mchehab@kernel.org>,
	Kyungmin Park <kyungmin.park@samsung.com>,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	Sakari Ailus <sakari.ailus@iki.fi>,
	Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
	Pawel Osciak <posciak@chromium.org>,
	linux-media@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCH 13/15] videobuf2: do not sync buffers for DMABUF queues
Date: Wed, 22 Jan 2020 14:05:15 +0900	[thread overview]
Message-ID: <20200122050515.GB49953@google.com> (raw)
In-Reply-To: <2d0e1a9b-6c5e-ff70-9862-32c8b8aaf65f@xs4all.nl>

On (20/01/10 11:30), Hans Verkuil wrote:
[..]
> > diff --git a/drivers/media/common/videobuf2/videobuf2-v4l2.c b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> > index 1762849288ae..2b9d3318e6fb 100644
> > --- a/drivers/media/common/videobuf2/videobuf2-v4l2.c
> > +++ b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> > @@ -341,8 +341,22 @@ static void set_buffer_cache_hints(struct vb2_queue *q,
> >  				   struct vb2_buffer *vb,
> >  				   struct v4l2_buffer *b)
> >  {
> > -	vb->need_cache_sync_on_prepare = 1;
> > +	/*
> > +	 * DMA exporter should take care of cache syncs, so we can avoid
> > +	 * explicit ->prepare()/->finish() syncs.
> > +	 */
> > +	if (q->memory == VB2_MEMORY_DMABUF) {
> > +		vb->need_cache_sync_on_finish = 0;
> > +		vb->need_cache_sync_on_prepare = 0;
> > +		return;
> > +	}
> >  
> > +	/*
> > +	 * For other ->memory types we always need ->prepare() cache
> > +	 * sync. ->finish() cache sync, however, can be avoided when queue
> > +	 * direction is TO_DEVICE.
> > +	 */
> > +	vb->need_cache_sync_on_prepare = 1;
> 
> I'm trying to remember: what needs to be done in prepare()
> for a capture buffer? I thought that for capture you only
> needed to invalidate the cache in finish(), but nothing needs
> to be done in the prepare().

Hmm. Not sure. A precaution in case if user-space wrote to that buffer?

+	if (q->dma_dir == DMA_FROM_DEVICE)
+		q->need_cache_sync_on_prepare = 0;

?

	-ss