[PATCH] media: mali-c55: Fix frame sequence numbers on dual-pipe hardware

[David Carlier <devnexen@gmail.com>]

The single frame_sequence counter in struct mali_c55_isp is incremented
in mali_c55_set_plane_done(), which runs once per completed buffer per
capture device rather than once per frame. Hardware fitted with the
downscale pipe always hits this: mali_c55_pipeline_ready() refuses to
start the ISP unless both the full-resolution and the downscale queues
are streaming, so the counter advances twice per frame.

Each video node then reports sequence numbers 0, 2, 4, ..., which
userspace reads as a dropped frame between every pair of frames, and the
two pipes never number the same frame alike, contrary to
Documentation/admin-guide/media/mali-c55.rst. The V4L2_EVENT_FRAME_SYNC
event and the statistics and parameters buffers only read the counter,
so their sequence numbers stop identifying a frame too.

Increment the counter once per frame, when the ISP start interrupt is
handled, and let mali_c55_set_plane_done() only read it. Initialise it
to UINT_MAX so the first increment yields sequence zero. Hardware
without the downscale pipe keeps the numbering it had before.

Fixes: d5f281f3dd29 ("media: mali-c55: Add Mali-C55 ISP driver")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5
Signed-off-by: David Carlier <devnexen@gmail.com>
---
 drivers/media/platform/arm/mali-c55/mali-c55-capture.c | 2 +-
 drivers/media/platform/arm/mali-c55/mali-c55-core.c    | 1 +
 drivers/media/platform/arm/mali-c55/mali-c55-isp.c     | 3 ++-
 3 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/media/platform/arm/mali-c55/mali-c55-capture.c b/drivers/media/platform/arm/mali-c55/mali-c55-capture.c
index ff01553026fb..7e1190d7fffc 100644
--- a/drivers/media/platform/arm/mali-c55/mali-c55-capture.c
+++ b/drivers/media/platform/arm/mali-c55/mali-c55-capture.c
@@ -476,7 +476,7 @@ void mali_c55_set_plane_done(struct mali_c55_cap_dev *cap_dev,
 
 	/* If the other plane is also done... */
 	buf->vb.vb2_buf.timestamp = ktime_get_boottime_ns();
-	buf->vb.sequence = isp->frame_sequence++;
+	buf->vb.sequence = isp->frame_sequence;
 	vb2_buffer_done(&buf->vb.vb2_buf, VB2_BUF_STATE_DONE);
 }
 
diff --git a/drivers/media/platform/arm/mali-c55/mali-c55-core.c b/drivers/media/platform/arm/mali-c55/mali-c55-core.c
index fb81141d1653..be562156b295 100644
--- a/drivers/media/platform/arm/mali-c55/mali-c55-core.c
+++ b/drivers/media/platform/arm/mali-c55/mali-c55-core.c
@@ -573,6 +573,7 @@ static irqreturn_t mali_c55_isr(int irq, void *context)
 	for_each_set_bit(i, &interrupt_status, MALI_C55_NUM_IRQ_BITS) {
 		switch (i) {
 		case MALI_C55_IRQ_ISP_START:
+			mali_c55->isp.frame_sequence++;
 			mali_c55_isp_queue_event_sof(mali_c55);
 
 			mali_c55_set_next_buffer(&mali_c55->cap_devs[MALI_C55_CAP_DEV_FR]);
diff --git a/drivers/media/platform/arm/mali-c55/mali-c55-isp.c b/drivers/media/platform/arm/mali-c55/mali-c55-isp.c
index e128adf6ee37..39178f6a1bd6 100644
--- a/drivers/media/platform/arm/mali-c55/mali-c55-isp.c
+++ b/drivers/media/platform/arm/mali-c55/mali-c55-isp.c
@@ -342,7 +342,8 @@ static int mali_c55_isp_enable_streams(struct v4l2_subdev *sd,
 
 	src_sd = media_entity_to_v4l2_subdev(isp->remote_src->entity);
 
-	isp->frame_sequence = 0;
+	/* first pre-increment at SOF yields sequence 0 */
+	isp->frame_sequence = UINT_MAX;
 	ret = mali_c55_isp_start(mali_c55, state);
 	if (ret) {
 		dev_err(mali_c55->dev, "Failed to start ISP\n");
-- 
2.53.0