[libcamera-devel] [PATCH v2 4/4] pipeline: raspberrypi: Add notion of immediate write to StaggeredCtrl

[Naushir Patuck]

If an exposure time change adjusts the vblanking limits, and we write
both VBLANK and EXPOSURE controls as a set, the latter may fail if the
value is outside of the limits calculated by the old VBLANK value. This
is a limitation in V4L2 and cannot be fixed by writing VBLANK before
EXPOSURE.

The workaround here is to have the StaggeredCtrl mark the
VBLANK control as "immediate write", which then write VBLANK separately
from (and ahead of) any other controls. This way, the sensor driver will
update the EXPOSURE control with new limits before the new values is
presented, and will thus be seen as valid.

StaggeredCtrl is due to be deprecated and replaced by DelayedCtrl, so
this change serves more a working proof-of-concept on the workaround,
and not much care has been taken to provide a nice new API for applying
this immediate write flag to the control. A similar workaround must be
available to DelayedCtrl eventually.

Signed-off-by: Naushir Patuck <naush at raspberrypi.com>
---
 src/ipa/raspberrypi/raspberrypi.cpp           |  5 ++-
 .../pipeline/raspberrypi/raspberrypi.cpp      | 11 ++++--
 .../pipeline/raspberrypi/staggered_ctrl.cpp   | 39 ++++++++++++++-----
 .../pipeline/raspberrypi/staggered_ctrl.h     |  3 +-
 4 files changed, 43 insertions(+), 15 deletions(-)

diff --git a/src/ipa/raspberrypi/raspberrypi.cpp b/src/ipa/raspberrypi/raspberrypi.cpp
index 75c9e404dcc1..fefca32ce187 100644
--- a/src/ipa/raspberrypi/raspberrypi.cpp
+++ b/src/ipa/raspberrypi/raspberrypi.cpp
@@ -1040,8 +1040,9 @@ void IPARPi::applyAGC(const struct AgcStatus *agcStatus, ControlList &ctrls)
 
 	/*
 	 * Due to the behavior of V4L2, the current value of VBLANK could clip the
-	 * exposure time without us knowing. The next time though this function should
-	 * clip exposure correctly.
+	 * exposure time without us knowing. We get around this by ensuring the
+	 * staggered write component submits VBLANK separately from, and before the
+	 * EXPOSURE control.
 	 */
 	ctrls.set(V4L2_CID_VBLANK, vblanking);
 	ctrls.set(V4L2_CID_EXPOSURE, exposureLines);
diff --git a/src/libcamera/pipeline/raspberrypi/raspberrypi.cpp b/src/libcamera/pipeline/raspberrypi/raspberrypi.cpp
index 524cc960dd37..1485999ad2a0 100644
--- a/src/libcamera/pipeline/raspberrypi/raspberrypi.cpp
+++ b/src/libcamera/pipeline/raspberrypi/raspberrypi.cpp
@@ -1229,12 +1229,17 @@ int RPiCameraData::configureIPA(const CameraConfiguration *config)
 		/*
 		 * Setup our staggered control writer with the sensor default
 		 * gain and exposure delays.
+		 *
+		 * VBLANK must be flagged as "immediate write" to allow it to
+		 * be set immediately instead of being batched with all other
+		 * controls. This is needed so that any update to the EXPOSURE
+		 * control will be validated based on the new VBLANK control value.
 		 */
 		if (!staggeredCtrl_) {
 			staggeredCtrl_.init(unicam_[Unicam::Image].dev(),
-					    { { V4L2_CID_ANALOGUE_GAIN, result.data[resultIdx++] },
-					      { V4L2_CID_EXPOSURE, result.data[resultIdx++] },
-					      { V4L2_CID_VBLANK, result.data[resultIdx++] } });
+					    { { V4L2_CID_ANALOGUE_GAIN, result.data[resultIdx++], false },
+					      { V4L2_CID_EXPOSURE, result.data[resultIdx++], false },
+					      { V4L2_CID_VBLANK, result.data[resultIdx++], true } });
 			sensorMetadata_ = result.data[resultIdx++];
 		}
 	}
diff --git a/src/libcamera/pipeline/raspberrypi/staggered_ctrl.cpp b/src/libcamera/pipeline/raspberrypi/staggered_ctrl.cpp
index 62605c0fceee..07f8c95d4f2c 100644
--- a/src/libcamera/pipeline/raspberrypi/staggered_ctrl.cpp
+++ b/src/libcamera/pipeline/raspberrypi/staggered_ctrl.cpp
@@ -22,21 +22,29 @@ LOG_DEFINE_CATEGORY(RPI_S_W)
 namespace RPi {
 
 void StaggeredCtrl::init(V4L2VideoDevice *dev,
-	  std::initializer_list<std::pair<const uint32_t, uint8_t>> delayList)
+			 std::initializer_list<std::tuple<uint32_t, uint8_t, bool>> delayList)
 {
 	std::lock_guard<std::mutex> lock(lock_);
 
 	dev_ = dev;
-	delay_ = delayList;
+	delay_.clear();
 	ctrl_.clear();
 
-	/* Find the largest delay across all controls. */
 	maxDelay_ = 0;
-	for (auto const &p : delay_) {
+	for (auto const &c : delayList) {
+		uint32_t id = std::get<0>(c);
+		uint8_t delay = std::get<1>(c);
+		bool immediateWrite = std::get<2>(c);
+
+		delay_[id] = delay;
+		immediateWrite_[id] = immediateWrite;
+
 		LOG(RPI_S_W, Info) << "Init ctrl "
-				   << utils::hex(p.first) << " with delay "
-				   << static_cast<int>(p.second);
-		maxDelay_ = std::max(maxDelay_, p.second);
+				   << utils::hex(id) << " with delay "
+				   << static_cast<int>(delay);
+
+		/* Find the largest delay across all controls. */
+		maxDelay_ = std::max(maxDelay_, delay);
 	}
 
 	init_ = true;
@@ -121,8 +129,21 @@ int StaggeredCtrl::write()
 		int index = std::max<int>(0, setCount_ - delayDiff);
 
 		if (p.second[index].updated) {
-			/* We need to write this value out. */
-			controls.set(p.first, p.second[index].value);
+			if (immediateWrite_[p.first]) {
+				/*
+				 * This control must be written now, it could
+				 * affect validity of the other controls.
+				 */
+				ControlList immediate(dev_->controls());
+				immediate.set(p.first, p.second[index].value);
+				dev_->setControls(&immediate);
+			} else {
+				/*
+				 * Batch up the list of controls and write them
+				 * at the end of the function.
+				 */
+				controls.set(p.first, p.second[index].value);
+			}
 			p.second[index].updated = false;
 			LOG(RPI_S_W, Debug) << "Writing ctrl "
 					    << utils::hex(p.first) << " to "
diff --git a/src/libcamera/pipeline/raspberrypi/staggered_ctrl.h b/src/libcamera/pipeline/raspberrypi/staggered_ctrl.h
index 382fa31a6853..7c920c3a13c7 100644
--- a/src/libcamera/pipeline/raspberrypi/staggered_ctrl.h
+++ b/src/libcamera/pipeline/raspberrypi/staggered_ctrl.h
@@ -34,7 +34,7 @@ public:
 	}
 
 	void init(V4L2VideoDevice *dev,
-		  std::initializer_list<std::pair<const uint32_t, uint8_t>> delayList);
+		  std::initializer_list<std::tuple<uint32_t, uint8_t, bool>> delayList);
 	void reset();
 
 	void get(std::unordered_map<uint32_t, int32_t> &ctrl, uint8_t offset = 0);
@@ -85,6 +85,7 @@ private:
 	uint8_t maxDelay_;
 	V4L2VideoDevice *dev_;
 	std::unordered_map<uint32_t, uint8_t> delay_;
+	std::unordered_map<uint32_t, bool> immediateWrite_;
 	std::unordered_map<uint32_t, CircularArray> ctrl_;
 	std::mutex lock_;
 };
-- 
2.25.1