linux-kernel - Re: [RFC v5 10/11] [media] vb2: add out-fence support to QBUF

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Message-ID: <20171117114835.GD19033@jade>
Date:   Fri, 17 Nov 2017 09:48:35 -0200
From:   Gustavo Padovan <gustavo@...ovan.org>
To:     Alexandre Courbot <acourbot@...omium.org>
Cc:     linux-media@...r.kernel.org, Hans Verkuil <hverkuil@...all.nl>,
        Mauro Carvalho Chehab <mchehab@....samsung.com>,
        Shuah Khan <shuahkh@....samsung.com>,
        Pawel Osciak <pawel@...iak.com>,
        Sakari Ailus <sakari.ailus@....fi>,
        Brian Starkey <brian.starkey@....com>,
        Thierry Escande <thierry.escande@...labora.com>,
        linux-kernel@...r.kernel.org,
        Gustavo Padovan <gustavo.padovan@...labora.com>
Subject: Re: [RFC v5 10/11] [media] vb2: add out-fence support to QBUF

2017-11-17 Alexandre Courbot <acourbot@...omium.org>:

> On Thursday, November 16, 2017 2:10:56 AM JST, Gustavo Padovan wrote:
> > From: Gustavo Padovan <gustavo.padovan@...labora.com>
> > 
> > If V4L2_BUF_FLAG_OUT_FENCE flag is present on the QBUF call we create
> > an out_fence and send its fd to userspace on the fence_fd field as a
> > return arg for the QBUF call.
> > 
> > The fence is signaled on buffer_done(), when the job on the buffer is
> > finished.
> > 
> > With out-fences we do not allow drivers to requeue buffers through vb2,
> > instead we flag an error on the buffer, signals it fence with error and
> 
> s/it/its
> 
> > return it to userspace.
> > 
> > v6
> > 	- get rid of the V4L2_EVENT_OUT_FENCE event. We always keep the
> > 	ordering in vb2 for queueing in the driver, so the event is not
> > 	necessary anymore and the out_fence_fd is sent back to userspace
> > 	on QBUF call return arg
> > 	- do not allow requeueing with out-fences, instead mark the buffer
> > 	with an error and wake up to userspace.
> > 	- send the out_fence_fd back to userspace on the fence_fd field
> > 
> > v5:
> > 	- delay fd_install to DQ_EVENT (Hans)
> > 	- if queue is fully ordered send OUT_FENCE event right away
> > 	(Brian)
> > 	- rename 'q->ordered' to 'q->ordered_in_driver'
> > 	- merge change to implement OUT_FENCE event here
> > 
> > v4:
> > 	- return the out_fence_fd in the BUF_QUEUED event(Hans)
> > 
> > v3:	- add WARN_ON_ONCE(q->ordered) on requeueing (Hans)
> > 	- set the OUT_FENCE flag if there is a fence pending (Hans)
> > 	- call fd_install() after vb2_core_qbuf() (Hans)
> > 	- clean up fence if vb2_core_qbuf() fails (Hans)
> > 	- add list to store sync_file and fence for the next queued buffer
> > 
> > v2: check if the queue is ordered.
> > 
> > Signed-off-by: Gustavo Padovan <gustavo.padovan@...labora.com>
> > ---
> >  drivers/media/v4l2-core/videobuf2-core.c | 42
> > +++++++++++++++++++++++++++++---
> >  drivers/media/v4l2-core/videobuf2-v4l2.c | 21 +++++++++++++++-
> >  2 files changed, 58 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/media/v4l2-core/videobuf2-core.c
> > b/drivers/media/v4l2-core/videobuf2-core.c
> > index 8b4f0e9bcb36..2eb5ffa8e028 100644
> > --- a/drivers/media/v4l2-core/videobuf2-core.c
> > +++ b/drivers/media/v4l2-core/videobuf2-core.c
> > @@ -354,6 +354,7 @@ static int __vb2_queue_alloc(struct vb2_queue *q,
> > enum vb2_memory memory,
> >  			vb->planes[plane].length = plane_sizes[plane];
> >  			vb->planes[plane].min_length = plane_sizes[plane];
> >  		}
> > +		vb->out_fence_fd = -1;
> >  		q->bufs[vb->index] = vb;
> >  		/* Allocate video buffer memory for the MMAP type */
> > @@ -934,10 +935,26 @@ void vb2_buffer_done(struct vb2_buffer *vb, enum
> > vb2_buffer_state state)
> >  	case VB2_BUF_STATE_QUEUED:
> >  		return;
> >  	case VB2_BUF_STATE_REQUEUEING:
> > -		if (q->start_streaming_called)
> > -			__enqueue_in_driver(vb);
> > -		return;
> > +
> > +		if (!vb->out_fence) {
> > +			if (q->start_streaming_called)
> > +				__enqueue_in_driver(vb);
> > +			return;
> > +		}
> > +
> > +		/* Do not allow requeuing with explicit synchronization,
> > +		 * report it as an error to userspace */
> > +		state = VB2_BUF_STATE_ERROR;
> > +
> > +		/* fall through */
> >  	default:
> > +		if (state == VB2_BUF_STATE_ERROR)
> > +			dma_fence_set_error(vb->out_fence, -EFAULT);
> > +		dma_fence_signal(vb->out_fence);
> > +		dma_fence_put(vb->out_fence);
> > +		vb->out_fence = NULL;
> > +		vb->out_fence_fd = -1;
> > +
> >  		/* Inform any processes that may be waiting for buffers */
> >  		wake_up(&q->done_wq);
> >  		break;
> > @@ -1325,12 +1342,18 @@ EXPORT_SYMBOL_GPL(vb2_core_prepare_buf);
> >  int vb2_setup_out_fence(struct vb2_queue *q, unsigned int index)
> >  {
> >  	struct vb2_buffer *vb;
> > +	u64 context;
> >  	vb = q->bufs[index];
> >  	vb->out_fence_fd = get_unused_fd_flags(O_CLOEXEC);
> > -	vb->out_fence = vb2_fence_alloc(q->out_fence_context);
> > +	if (q->ordered_in_driver)
> > +		context = q->out_fence_context;
> > +	else
> > +		context = dma_fence_context_alloc(1);
> > +
> > +	vb->out_fence = vb2_fence_alloc(context);
> >  	if (!vb->out_fence) {
> >  		put_unused_fd(vb->out_fence_fd);
> >  		return -ENOMEM;
> > @@ -1594,6 +1617,9 @@ int vb2_core_qbuf(struct vb2_queue *q, unsigned
> > int index, void *pb,
> >  	if (pb)
> >  		call_void_bufop(q, fill_user_buffer, vb, pb);
> > +	fd_install(vb->out_fence_fd, vb->sync_file->file);
> 
> What happens if the buffer does not have an output fence? Shouldn't this be
> protected by a check on whether the buffer has been queued with
> V4L2_BUF_FLAG_OUT_FENCE?

Ouch, I think we would have a nice crash. We should protect it, yes.


Gustavo