[PATCH v3 2/2] media: nxp: imx8-isi: Prioritize pending buffers over discard buffers

[Laurent Pinchart]

Hello Guoniu,

Thank you for the patch.

On Fri, Mar 20, 2026 at 02:42:02PM +0800, Guoniu Zhou wrote:
> From: Guoniu Zhou <guoniu.zhou at nxp.com>
> 
> The number of times to use the discard buffer is determined by the
> out_pending list size:
> 
>   discard = list_empty(&video->out_pending) ? 2
>             : list_is_singular(&video->out_pending) ? 1
> 	    : 0;
> 
> In the current buffer selection logic, when both discard and pending
> buffers are available, the driver fills hardware slots with discard
> buffers first which results in an unnecessary frame drop even though
> a user buffer was queued and ready.
> 
> Change the buffer selection logic to use pending buffers first (up to
> the number available), and only use discard buffers to fill remaining
> slots when insufficient pending buffers are queued.
> 
> This improves behavior by:
> - Reducing discarded frames at stream start when user buffers are ready
> - Decreasing latency in delivering captured frames to user-space
> - Ensuring user buffers are utilized as soon as they are queued
> - Improving overall buffer utilization efficiency

There's a bit of repeat here, but that's OK.

Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>

> Signed-off-by: Guoniu Zhou <guoniu.zhou at nxp.com>
> ---
> Changes in v3:
> - Expanded commit message to explain the problem in current driver and the
>   benefits gained from this change
> - No code changes
> 
> Changes in v2:
> - Replace "This ensures" with "ensure"
> - Put example from commit message to comment in driver suggested by Frank
>   https://lore.kernel.org/linux-media/20260311-isi_min_buffers-v1-0-c9299d6e8ae6@nxp.com/T/#m2774912ed31553ef1fdcc840bd6eae53a03ecccd
> ---
>  drivers/media/platform/nxp/imx8-isi/imx8-isi-video.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/media/platform/nxp/imx8-isi/imx8-isi-video.c b/drivers/media/platform/nxp/imx8-isi/imx8-isi-video.c
> index 1be3a728f32f..77ebff03323a 100644
> --- a/drivers/media/platform/nxp/imx8-isi/imx8-isi-video.c
> +++ b/drivers/media/platform/nxp/imx8-isi/imx8-isi-video.c
> @@ -792,7 +792,14 @@ static void mxc_isi_video_queue_first_buffers(struct mxc_isi_video *video)
>  		struct mxc_isi_buffer *buf;
>  		struct list_head *list;
>  
> -		list = i < discard ? &video->out_discard : &video->out_pending;
> +		/*
> +		 * Queue buffers: prioritize pending buffers, then discard buffers
> +		 * For example:
> +		 * - 2 pending buffers: both slots use pending buffers
> +		 * - 1 pending buffer: first slot uses pending, second uses discard
> +		 * - 0 pending buffers: both slots use discard buffers
> +		 */
> +		list = (i < 2 - discard) ? &video->out_pending : &video->out_discard;
>  		buf = list_first_entry(list, struct mxc_isi_buffer, list);
>  
>  		mxc_isi_channel_set_outbuf(video->pipe, buf->dma_addrs, buf_id);

-- 
Regards,

Laurent Pinchart