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

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Message-ID: <20200122050515.GB49953@google.com>
Date:   Wed, 22 Jan 2020 14:05:15 +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 (20/01/10 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