Re: [PATCH 2/2] media: v4l2-mem2mem: add a list for buf used by hw

[Nicolas Dufresne]

Le mercredi 12 juillet 2023 à 09:33 +0000, Tomasz Figa a écrit :
> On Tue, Jul 04, 2023 at 12:00:38PM +0800, Hsia-Jun Li wrote:
> > From: "Hsia-Jun(Randy) Li" <randy.li@xxxxxxxxxxxxx>
> > 
> > Many drivers have to create its own buf_struct for a
> > vb2_queue to track such a state. Also driver has to
> > iterate over rdy_queue every times to find out a buffer
> > which is not sent to hardware(or firmware), this new
> > list just offers the driver a place to store the buffer
> > that hardware(firmware) has acknowledged.
> > 
> > One important advance about this list, it doesn't like
> > rdy_queue which both bottom half of the user calling
> > could operate it, while the v4l2 worker would as well.
> > The v4l2 core could only operate this queue when its
> > v4l2_context is not running, the driver would only
> > access this new hw_queue in its own worker.
> 
> Could you describe in what case such a list would be useful for a
> mem2mem driver?

Today all driver must track buffers that are "owned by the hardware". This is a
concept dictated by the m2m framework and enforced through the ACTIVE flag. All
buffers from this list must be mark as done/error/queued after streamoff of the
respective queue in order to acknowledge that they are no longer in use by the
HW. Not doing so will warn:

  videobuf2_common: driver bug: stop_streaming operation is leaving buf ...

Though, there is no queue to easily iterate them. All driver endup having their
own queue, or just leaving the buffers in the rdy_queue (which isn't better).

Nicolas
> 
> > 
> > Signed-off-by: Hsia-Jun(Randy) Li <randy.li@xxxxxxxxxxxxx>
> > ---
> >  drivers/media/v4l2-core/v4l2-mem2mem.c | 25 +++++++++++++++++--------
> >  include/media/v4l2-mem2mem.h           | 10 +++++++++-
> >  2 files changed, 26 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/media/v4l2-core/v4l2-mem2mem.c b/drivers/media/v4l2-core/v4l2-mem2mem.c
> > index c771aba42015..b4151147d5bd 100644
> > --- a/drivers/media/v4l2-core/v4l2-mem2mem.c
> > +++ b/drivers/media/v4l2-core/v4l2-mem2mem.c
> > @@ -321,15 +321,21 @@ static void __v4l2_m2m_try_queue(struct v4l2_m2m_dev *m2m_dev,
> >  		goto job_unlock;
> >  	}
> >  
> > -	src = v4l2_m2m_next_src_buf(m2m_ctx);
> > -	dst = v4l2_m2m_next_dst_buf(m2m_ctx);
> > -	if (!src && !m2m_ctx->out_q_ctx.buffered) {
> > -		dprintk("No input buffers available\n");
> > -		goto job_unlock;
> > +	if (list_empty(&m2m_ctx->out_q_ctx.hw_queue)) {
> > +		src = v4l2_m2m_next_src_buf(m2m_ctx);
> > +
> > +		if (!src && !m2m_ctx->out_q_ctx.buffered) {
> > +			dprintk("No input buffers available\n");
> > +			goto job_unlock;
> > +		}
> >  	}
> > -	if (!dst && !m2m_ctx->cap_q_ctx.buffered) {
> > -		dprintk("No output buffers available\n");
> > -		goto job_unlock;
> > +
> > +	if (list_empty(&m2m_ctx->cap_q_ctx.hw_queue)) {
> > +		dst = v4l2_m2m_next_dst_buf(m2m_ctx);
> > +		if (!dst && !m2m_ctx->cap_q_ctx.buffered) {
> > +			dprintk("No output buffers available\n");
> > +			goto job_unlock;
> > +		}
> >  	}
> 
> src and dst would be referenced unitialized below if neither of the
> above ifs hits...
> 
> Best regards,
> Tomasz
> 
> >  
> >  	m2m_ctx->new_frame = true;
> > @@ -896,6 +902,7 @@ int v4l2_m2m_streamoff(struct file *file, struct v4l2_m2m_ctx *m2m_ctx,
> >  	INIT_LIST_HEAD(&q_ctx->rdy_queue);
> >  	q_ctx->num_rdy = 0;
> >  	spin_unlock_irqrestore(&q_ctx->rdy_spinlock, flags);
> > +	INIT_LIST_HEAD(&q_ctx->hw_queue);
> >  
> >  	if (m2m_dev->curr_ctx == m2m_ctx) {
> >  		m2m_dev->curr_ctx = NULL;
> > @@ -1234,6 +1241,8 @@ struct v4l2_m2m_ctx *v4l2_m2m_ctx_init(struct v4l2_m2m_dev *m2m_dev,
> >  
> >  	INIT_LIST_HEAD(&out_q_ctx->rdy_queue);
> >  	INIT_LIST_HEAD(&cap_q_ctx->rdy_queue);
> > +	INIT_LIST_HEAD(&out_q_ctx->hw_queue);
> > +	INIT_LIST_HEAD(&cap_q_ctx->hw_queue);
> >  	spin_lock_init(&out_q_ctx->rdy_spinlock);
> >  	spin_lock_init(&cap_q_ctx->rdy_spinlock);
> >  
> > diff --git a/include/media/v4l2-mem2mem.h b/include/media/v4l2-mem2mem.h
> > index d6c8eb2b5201..2342656e582d 100644
> > --- a/include/media/v4l2-mem2mem.h
> > +++ b/include/media/v4l2-mem2mem.h
> > @@ -53,9 +53,16 @@ struct v4l2_m2m_dev;
> >   *	processed
> >   *
> >   * @q:		pointer to struct &vb2_queue
> > - * @rdy_queue:	List of V4L2 mem-to-mem queues
> > + * @rdy_queue:	List of V4L2 mem-to-mem queues. If v4l2_m2m_buf_queue() is
> > + *		called in struct vb2_ops->buf_queue(), the buffer enqueued
> > + *		by user would be added to this list.
> >   * @rdy_spinlock: spin lock to protect the struct usage
> >   * @num_rdy:	number of buffers ready to be processed
> > + * @hw_queue:	A list for tracking the buffer is occupied by the hardware
> > + * 		(or device's firmware). A buffer could only be in either
> > + * 		this list or @rdy_queue.
> > + * 		Driver may choose not to use this list while uses its own
> > + * 		private data to do this work.
> >   * @buffered:	is the queue buffered?
> >   *
> >   * Queue for buffers ready to be processed as soon as this
> > @@ -68,6 +75,7 @@ struct v4l2_m2m_queue_ctx {
> >  	struct list_head	rdy_queue;
> >  	spinlock_t		rdy_spinlock;
> >  	u8			num_rdy;
> > +	struct list_head	hw_queue;
> >  	bool			buffered;
> >  };
> >  
> > -- 
> > 2.17.1
> >