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

[Guoniu Zhou]

From: Guoniu Zhou <guoniu.zhou@xxxxxxx>

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@xxxxxxx>
---
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@xxxxxxx/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);

-- 
2.34.1