[PATCH v2] 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 stamp each capture buffer when mali_c55_set_next_buffer()
programs it, as it is written out during the following frame. Stamping
at completion would be off by one whenever DONE(n) and START(n+1) are
handled in the same interrupt, since ISP_START is processed first. Hold
the counter at UINT_MAX while the ISP is stopped, as the first buffers
are programmed before it starts, so the first frame gets sequence zero.

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>
---
v2:
 - Stamp the buffer sequence in mali_c55_set_next_buffer() instead of
   at completion, which broke when START(n+1) and DONE(n) are handled
   together (Jacopo)
 - Reset the counter when the ISP stops instead of when it starts

v1: https://lore.kernel.org/r/20260801205311.386692-1-devnexen@gmail.com
---
 drivers/media/platform/arm/mali-c55/mali-c55-capture.c | 5 +++--
 drivers/media/platform/arm/mali-c55/mali-c55-common.h  | 4 ++++
 drivers/media/platform/arm/mali-c55/mali-c55-core.c    | 1 +
 drivers/media/platform/arm/mali-c55/mali-c55-isp.c     | 3 ++-
 4 files changed, 10 insertions(+), 3 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..fa073ce4b2bd 100644
--- a/drivers/media/platform/arm/mali-c55/mali-c55-capture.c
+++ b/drivers/media/platform/arm/mali-c55/mali-c55-capture.c
@@ -413,6 +413,9 @@ void mali_c55_set_next_buffer(struct mali_c55_cap_dev *cap_dev)
 		return;
 	}
 
+	/* The buffer is written out during the frame following this one. */
+	buf->vb.sequence = cap_dev->mali_c55->isp.frame_sequence + 1;
+
 	pix_mp = &cap_dev->format.format;
 
 	mali_c55_cap_dev_update_bits(cap_dev, MALI_C55_REG_Y_WRITER_MODE,
@@ -457,7 +460,6 @@ void mali_c55_set_next_buffer(struct mali_c55_cap_dev *cap_dev)
 void mali_c55_set_plane_done(struct mali_c55_cap_dev *cap_dev,
 			     enum mali_c55_planes plane)
 {
-	struct mali_c55_isp *isp = &cap_dev->mali_c55->isp;
 	struct mali_c55_buffer *buf;
 
 	scoped_guard(spinlock, &cap_dev->buffers.processing_lock) {
@@ -476,7 +478,6 @@ 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++;
 	vb2_buffer_done(&buf->vb.vb2_buf, VB2_BUF_STATE_DONE);
 }
 
diff --git a/drivers/media/platform/arm/mali-c55/mali-c55-common.h b/drivers/media/platform/arm/mali-c55/mali-c55-common.h
index 13a3e9dc4243..50f2b7a85f62 100644
--- a/drivers/media/platform/arm/mali-c55/mali-c55-common.h
+++ b/drivers/media/platform/arm/mali-c55/mali-c55-common.h
@@ -76,6 +76,10 @@ struct mali_c55_isp {
 	struct media_pad *remote_src;
 	/* Mutex to guard vb2 start/stop streaming */
 	struct mutex capture_lock;
+	/*
+	 * Sequence of the frame being processed, incremented at SOF. Held at
+	 * UINT_MAX while stopped so the first frame gets sequence 0.
+	 */
 	unsigned int frame_sequence;
 };
 
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..dd3bce287a3d 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 +342,6 @@ 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;
 	ret = mali_c55_isp_start(mali_c55, state);
 	if (ret) {
 		dev_err(mali_c55->dev, "Failed to start ISP\n");
@@ -380,6 +379,7 @@ static int mali_c55_isp_disable_streams(struct v4l2_subdev *sd,
 	isp->remote_src = NULL;
 
 	mali_c55_isp_stop(mali_c55);
+	isp->frame_sequence = UINT_MAX;
 
 	return 0;
 }
@@ -584,6 +584,7 @@ int mali_c55_register_isp(struct mali_c55 *mali_c55)
 	int ret;
 
 	isp->mali_c55 = mali_c55;
+	isp->frame_sequence = UINT_MAX;
 
 	v4l2_subdev_init(sd, &mali_c55_isp_ops);
 	sd->flags |= V4L2_SUBDEV_FL_HAS_DEVNODE | V4L2_SUBDEV_FL_HAS_EVENTS;

base-commit: 95f76f51937fdfb0fc1e14cae606b1ef574a56f3
-- 
2.55.0