Re: [PATCH v3 32/38] media: ti-vpe: cal: use CSI-2 frame number

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Tomi,

On Mon, Jun 07, 2021 at 03:39:45PM +0300, Tomi Valkeinen wrote:
> On 04/06/2021 17:04, Laurent Pinchart wrote:
> > On Mon, May 24, 2021 at 02:09:03PM +0300, Tomi Valkeinen wrote:
> >> The driver fills buf->vb.sequence with an increasing number which is
> >> incremented by the driver. This feels a bit pointless, as the userspace
> >> could as well track that kind of number itself. Instead, lets use the
> > 
> > s/lets/let's/
> > 
> >> frame number provided in the CSI-2 data from the sensor.
> >>
> >> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
> >> ---
> >>   drivers/media/platform/ti-vpe/cal.c | 7 +++++--
> >>   drivers/media/platform/ti-vpe/cal.h | 1 -
> >>   2 files changed, 5 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/media/platform/ti-vpe/cal.c b/drivers/media/platform/ti-vpe/cal.c
> >> index 888706187fd1..62c45add4efe 100644
> >> --- a/drivers/media/platform/ti-vpe/cal.c
> >> +++ b/drivers/media/platform/ti-vpe/cal.c
> >> @@ -493,7 +493,6 @@ void cal_ctx_unprepare(struct cal_ctx *ctx)
> >>   
> >>   void cal_ctx_start(struct cal_ctx *ctx)
> >>   {
> >> -	ctx->sequence = 0;
> >>   	ctx->dma.state = CAL_DMA_RUNNING;
> >>   
> >>   	/* Configure the CSI-2, pixel processing and write DMA contexts. */
> >> @@ -586,6 +585,10 @@ static inline void cal_irq_wdma_start(struct cal_ctx *ctx)
> >>   static inline void cal_irq_wdma_end(struct cal_ctx *ctx)
> >>   {
> >>   	struct cal_buffer *buf = NULL;
> >> +	u32 frame_num;
> >> +
> >> +	frame_num = cal_read(ctx->cal, CAL_CSI2_STATUS(ctx->phy->instance,
> >> +						       ctx->csi2_ctx)) & 0xffff;
> >>   
> >>   	spin_lock(&ctx->dma.lock);
> >>   
> >> @@ -607,7 +610,7 @@ static inline void cal_irq_wdma_end(struct cal_ctx *ctx)
> >>   	if (buf) {
> >>   		buf->vb.vb2_buf.timestamp = ktime_get_ns();
> >>   		buf->vb.field = ctx->v_fmt.fmt.pix.field;
> >> -		buf->vb.sequence = ctx->sequence++;
> >> +		buf->vb.sequence = frame_num;
> > 
> > We'll need something a bit more complicated. The CSI-2 frame number is
> > not mandatory, and when used, it is a 16-bit number starting at 1 and
> > counting to an unspecified value larger than one, resetting to 1 at the
> > end of the cycle. The V4L2 sequence number, on the other hand, is a
> > monotonic counter starting at 0 and wrapping only at 2^32-1. We should
> > thus keep a software sequence counter and
> > 
> > - increase it by 1 if the frame number is zero
> > - increase it by frame_num - last_frame_num (with wrap-around of
> >    frame_num handled) otherwise
> 
> Ok... I wonder if we need a new field for this, though. The problem I 
> was solving when I changed this to use the CSI-2 frame-number was how to 
> associate a pixel frame and a metadata frame.
> 
> Their CSI-2 frame-numbers match (as they are from the same original 
> CSI-2 frame), so the userspace can use that to figure the matching 
> frames. While the above method you suggest should give us identical 
> sequence numbers for pixel and metadata, I think it's going a bit away 
> from my intended purpose, and possibly risks ending up with different 
> sequences for pixel and metadata.

Why do you think they could get out of sync (assuming the sensor
supports frame numbers of course, if it always returns 0, that's not
usable for the purpose of synchronization).

> So do we need the sequence number, as it is currently, and something new 
> for this buffer matching purpose?

When the sensor doesn't support frame numbers, the only thing that can
be used is the timestamp as far as I can see.

-- 
Regards,

Laurent Pinchart