media: nxp: imx8-isi: Prioritize pending buffers over discard buffers

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

Signed-off-by: Guoniu Zhou <guoniu.zhou@nxp.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patch.msgid.link/20260320-isi_min_buffers-v3-2-66e0fabccca3@oss.nxp.com
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>

drivers/media/platform/nxp/imx8-isi/imx8-isi-video.c

@@ -792,7 +792,11 @@ 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.
+		 */
+		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);