[PATCH] media: rkisp1: Reduce min_queued_buffers to 1

[Jacopo Mondi]

Hi Laurent

On Mon, Oct 07, 2024 at 04:55:32PM GMT, Laurent Pinchart wrote:
> Hi Jacopo,
>
> Thank you for the patch.
>
> On Mon, Oct 07, 2024 at 02:42:24PM +0200, Jacopo Mondi wrote:
> > There apparently is no reason to require 3 queued buffers to call
> > streamon() for the RkISP1 as the driver operates with a scratch buffer
> > where frames can be directed to if there's no available buffer provided
> > by userspace.
> >
> > Reduce the number of required buffers to 1 to allow applications to
> > operate with a single queued buffer.
> >
> > Tested with libcamera, by operating with a single capture a request. The
>
> s/capture a/capture/
>
> > same request (and associated capture buffer) gets recycled once
> > completed. This of course causes a frame rate drop but doesn't hinder
> > operations.
> >
> > Signed-off-by: Jacopo Mondi <jacopo.mondi at ideasonboard.com>
> > ---
> >
> > Adam,
> >    a few months ago you were exercizing your pinhole app with a single capture
> > request for StillCapture operations and you got the video device to hang because
> > no enough buffers where provided.
> >
> > This small change should be enough to unblock you. Could you maybe give it a
> > spin if you're still working on this ?
> >
> > Thanks
> >    j
> > ---
> >
> >  drivers/media/platform/rockchip/rkisp1/rkisp1-capture.c | 4 +---
> >  1 file changed, 1 insertion(+), 3 deletions(-)
> >
> > diff --git a/drivers/media/platform/rockchip/rkisp1/rkisp1-capture.c b/drivers/media/platform/rockchip/rkisp1/rkisp1-capture.c
> > index 2bddb4fa8a5c..34adaecdee54 100644
> > --- a/drivers/media/platform/rockchip/rkisp1/rkisp1-capture.c
> > +++ b/drivers/media/platform/rockchip/rkisp1/rkisp1-capture.c
> > @@ -35,8 +35,6 @@
> >  #define RKISP1_SP_DEV_NAME	RKISP1_DRIVER_NAME "_selfpath"
> >  #define RKISP1_MP_DEV_NAME	RKISP1_DRIVER_NAME "_mainpath"
> >
> > -#define RKISP1_MIN_BUFFERS_NEEDED 3
> > -
> >  enum rkisp1_plane {
> >  	RKISP1_PLANE_Y	= 0,
> >  	RKISP1_PLANE_CB	= 1,
> > @@ -1563,7 +1561,7 @@ static int rkisp1_register_capture(struct rkisp1_capture *cap)
> >  	q->ops = &rkisp1_vb2_ops;
> >  	q->mem_ops = &vb2_dma_contig_memops;
> >  	q->buf_struct_size = sizeof(struct rkisp1_buffer);
> > -	q->min_queued_buffers = RKISP1_MIN_BUFFERS_NEEDED;
> > +	q->min_queued_buffers = 1;
>
> min_queued_buffers controls two things in vb2:
>
> - It controls the minimum number of buffers that can be allocated, by
>   setting
>
>         if (q->min_reqbufs_allocation < q->min_queued_buffers + 1)
>                 q->min_reqbufs_allocation = q->min_queued_buffers + 1;
>
>   in vb2_core_queue_init(). Note that this ony impacts the
>   VIDIOC_REQBUFS ioctl, VIDIOC_CREATE_BUFS can still allocate a lower
>   number of buffers.
>
> - It delays the .start_streaming() call until min_queued_buffers buffers
>   have been queued.
>
> This patch brings a clear improvement as it allows operating with a
> single buffer. Ideally though, it would be nice to set
> min_queued_buffers to 0, so that .start_streaming() gets called
> synchronously with VIDIOC_STREAMON. Otherwise stream start errors can be
> reported later, at VIDIOC_QBUF time.
>
> I expect going for 0 will require more changes in the driver, so I'm
> fine merging this patch as-is as a first step.

Well well well, I tried setting it to 0 and not provide any buffer to
the video device.

I see the number of interrupts received from the rkisp1 driver
increase with a frequency compatible with the frame rate, so it might
be possible that things work without modifications.

I'll keep digging

>
> Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
>
> >  	q->timestamp_flags = V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC;
> >  	q->lock = &node->vlock;
> >  	q->dev = cap->rkisp1->dev;
>
> --
> Regards,
>
> Laurent Pinchart