[PATCH v2] media: rkisp1: Reduce min_queued_buffers to 0

[Hans Verkuil]

On 28/10/2024 15:35, 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 0 to allow applications to
> operate by queueing capture buffers on-demand.
> 
> Tested with libcamera, by operating with a single capture request. The
> 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>
> ---
> The first version of this patch set min_queued_buffers to 1, but setting it
> to 0 doesn't compromise operations and it's even better as it allows application
> to queue buffers to the capture devices on-demand. If a buffer is not provided
> to the DMA engines, image data gets directed to the driver's internal scratch
> buffer.
> ---
>  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..5fcf9731f41b 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 = 0;

You can probably just drop this since the vb2_queue struct is zeroed when it
is allocated. So no need to set it to 0.

And is the RKISP1_MIN_BUFFERS_NEEDED define still needed after this change?

Also, see my RFC I posted today:

https://lore.kernel.org/linux-media/126cd76a-6224-483b-a18d-a3cc89e5ff2d@xs4all.nl/T/#u

My main concern is that applications that just call VIDIOC_REQBUFS with count = 1
and expect the driver to change that to a workable value, will, in fact, now just get
one buffer. And streaming that will cause lots of frame drops.

It makes sense to leave min_queued_buffers at 0 if a scratch buffer is available,
but I'm unhappy with the fact that you get a poor experience when REQBUFS(1) is called.

My RFC suggests improvements in the uAPI. With that in place you can use CREATE_BUFS in
libcamera to get much better control over how many buffers should be allocated.

Regards,

	Hans

>  	q->timestamp_flags = V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC;
>  	q->lock = &node->vlock;
>  	q->dev = cap->rkisp1->dev;
> --
> 2.47.0
> 
>