linux-kernel - Re: [PATCH 8/9] media: cedrus: Add start and stop decoder operations

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Date:   Mon, 25 Jun 2018 15:42:52 +0200
From:   Paul Kocialkowski <paul.kocialkowski@...tlin.com>
To:     Maxime Ripard <maxime.ripard@...tlin.com>
Cc:     hans.verkuil@...co.com, acourbot@...omium.org,
        sakari.ailus@...ux.intel.com,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        tfiga@...omium.org, posciak@...omium.org,
        Chen-Yu Tsai <wens@...e.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org,
        nicolas.dufresne@...labora.com, jenskuske@...il.com,
        linux-sunxi@...glegroups.com,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH 8/9] media: cedrus: Add start and stop decoder operations

Hi,

On Mon, 2018-06-25 at 15:32 +0200, Maxime Ripard wrote:
> On Thu, Jun 21, 2018 at 05:38:23PM +0200, Paul Kocialkowski wrote:
> > Hi,
> > 
> > On Wed, 2018-06-13 at 16:07 +0200, Maxime Ripard wrote:
> > > Some codec needs to perform some additional task when a decoding is started
> > > and stopped, and not only at every frame.
> > > 
> > > For example, the H264 decoding support needs to allocate buffers that will
> > > be used in the decoding process, but do not need to change over time, or at
> > > each frame.
> > > 
> > > In order to allow that for codecs, introduce a start and stop hook that
> > > will be called if present at start_streaming and stop_streaming time.
> > > 
> > > Signed-off-by: Maxime Ripard <maxime.ripard@...tlin.com>
> > > ---
> > >  .../platform/sunxi/cedrus/sunxi_cedrus_common.h    |  2 ++
> > >  .../platform/sunxi/cedrus/sunxi_cedrus_video.c     | 14 +++++++++++++-
> > >  2 files changed, 15 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/media/platform/sunxi/cedrus/sunxi_cedrus_common.h b/drivers/media/platform/sunxi/cedrus/sunxi_cedrus_common.h
> > > index a2a507eb9fc9..20c78ec1f037 100644
> > > --- a/drivers/media/platform/sunxi/cedrus/sunxi_cedrus_common.h
> > > +++ b/drivers/media/platform/sunxi/cedrus/sunxi_cedrus_common.h
> > > @@ -120,6 +120,8 @@ struct sunxi_cedrus_dec_ops {
> > >  	enum sunxi_cedrus_irq_status (*irq_status)(struct sunxi_cedrus_ctx *ctx);
> > >  	void (*setup)(struct sunxi_cedrus_ctx *ctx,
> > >  		      struct sunxi_cedrus_run *run);
> > > +	int (*start)(struct sunxi_cedrus_ctx *ctx);
> > > +	void (*stop)(struct sunxi_cedrus_ctx *ctx);
> > >  	void (*trigger)(struct sunxi_cedrus_ctx *ctx);
> > >  };
> > >  
> > > diff --git a/drivers/media/platform/sunxi/cedrus/sunxi_cedrus_video.c b/drivers/media/platform/sunxi/cedrus/sunxi_cedrus_video.c
> > > index fb7b081a5bb7..d93461178857 100644
> > > --- a/drivers/media/platform/sunxi/cedrus/sunxi_cedrus_video.c
> > > +++ b/drivers/media/platform/sunxi/cedrus/sunxi_cedrus_video.c
> > > @@ -416,6 +416,8 @@ static int sunxi_cedrus_buf_prepare(struct vb2_buffer *vb)
> > >  static int sunxi_cedrus_start_streaming(struct vb2_queue *q, unsigned int count)
> > >  {
> > >  	struct sunxi_cedrus_ctx *ctx = vb2_get_drv_priv(q);
> > > +	struct sunxi_cedrus_dev *dev = ctx->dev;
> > > +	int ret = 0;
> > >  
> > >  	switch (ctx->vpu_src_fmt->fourcc) {
> > >  	case V4L2_PIX_FMT_MPEG2_FRAME:
> > > @@ -425,16 +427,26 @@ static int sunxi_cedrus_start_streaming(struct vb2_queue *q, unsigned int count)
> > >  		return -EINVAL;
> > >  	}
> > >  
> > > -	return 0;
> > > +	if (V4L2_TYPE_IS_OUTPUT(q->type) &&
> > 
> > I suppose this check was put in place to ensure that ->start is only
> > called once, but what if start_streaming is called multiple times on
> > output? Am I totally unsure about whether the API guarantees that we
> > only get one start_streaming call per buffer queue, regardless of how
> > many userspace issues.
> > 
> > If we don't have such a guarantee, we probably need an internal
> > mechanism to avoid having ->start called more than once.
> 
> As far as I understand it, start_streaming can only be called once:
> https://elixir.bootlin.com/linux/latest/source/include/media/videobuf2-core.h#L357

And there's definitely a check in vb2_core_streamon:

	if (q->streaming) {
		dprintk(3, "already streaming\n");
		return 0;
	}

so it looks like we're safe :)

Cheers,

Paul

-- 
Paul Kocialkowski, Bootlin (formerly Free Electrons)
Embedded Linux and kernel engineering
https://bootlin.com
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