libcamera: delayed_controls: Add helper for controls that apply with a delay

Some sensor controls take effect with a delay as the sensor needs time
to adjust, for example exposure. Add an optional helper DelayedControls
to help pipelines deal with such controls.

The idea is to provide a queue of controls towards the V4L2 device and
apply individual controls with the specified delay with the aim to get
predictable and retrievable control values for any given frame. To do
this the queue of controls needs to be at least as deep as the control
with the largest delay.

The DelayedControls needs to be informed of every start of exposure.
This can be emulated but the helper is designed to be used with this
event being provide by the kernel through V4L2 events.

This helper is based on StaggeredCtrl from the Raspberry Pi pipeline
handler but expands on its API. This helpers aims to replace the
Raspberry Pi implementations and mimics it behavior perfectly.

Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>

2 files changed, 248 insertions, 0 deletions

diff --git a/src/libcamera/delayed_controls.cpp b/src/libcamera/delayed_controls.cpp
new file mode 100644
index 00000000..ab1d4005
--- /dev/null
+++ b/src/libcamera/delayed_controls.cpp
@@ -0,0 +1,247 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2020, Raspberry Pi (Trading) Ltd.
+ *
+ * delayed_controls.h - Helper to deal with controls that take effect with a delay
+ */
+
+#include "libcamera/internal/delayed_controls.h"
+
+#include <libcamera/controls.h>
+
+#include "libcamera/internal/log.h"
+#include "libcamera/internal/v4l2_device.h"
+
+/**
+ * \file delayed_controls.h
+ * \brief Helper to deal with controls that take effect with a delay
+ */
+
+namespace libcamera {
+
+LOG_DEFINE_CATEGORY(DelayedControls)
+
+/**
+ * \class DelayedControls
+ * \brief Helper to deal with controls that take effect with a delay
+ *
+ * Some sensor controls take effect with a delay as the sensor needs time to
+ * adjust, for example exposure and analog gain. This is a helper class to deal
+ * with such controls and the intended users are pipeline handlers.
+ *
+ * The idea is to extend the concept of the buffer depth of a pipeline the
+ * application needs to maintain to also cover controls. Just as with buffer
+ * depth if the application keeps the number of requests queued above the
+ * control depth the controls are guaranteed to take effect for the correct
+ * request. The control depth is determined by the control with the greatest
+ * delay.
+ */
+
+/**
+ * \brief Construct a DelayedControls instance
+ * \param[in] device The V4L2 device the controls have to be applied to
+ * \param[in] delays Map of the numerical V4L2 control ids to their associated
+ * delays (in frames)
+ *
+ * Only controls specified in \a delays are handled. If it's desired to mix
+ * delayed controls and controls that take effect immediately the immediate
+ * controls must be listed in the \a delays map with a delay value of 0.
+ */
+DelayedControls::DelayedControls(V4L2Device *device,
+				 const std::unordered_map<uint32_t, unsigned int> &delays)
+	: device_(device), maxDelay_(0)
+{
+	const ControlInfoMap &controls = device_->controls();
+
+	/*
+	 * Create a map of control ids to delays for controls exposed by the
+	 * device.
+	 */
+	for (auto const &delay : delays) {
+		auto it = controls.find(delay.first);
+		if (it == controls.end()) {
+			LOG(DelayedControls, Error)
+				<< "Delay request for control id "
+				<< utils::hex(delay.first)
+				<< " but control is not exposed by device "
+				<< device_->deviceNode();
+			continue;
+		}
+
+		const ControlId *id = it->first;
+
+		delays_[id] = delay.second;
+
+		LOG(DelayedControls, Debug)
+			<< "Set a delay of " << delays_[id]
+			<< " for " << id->name();
+
+		maxDelay_ = std::max(maxDelay_, delays_[id]);
+	}
+
+	reset();
+}
+
+/**
+ * \brief Reset state machine
+ *
+ * Resets the state machine to a starting position based on control values
+ * retrieved from the device.
+ */
+void DelayedControls::reset()
+{
+	running_ = false;
+	firstSequence_ = 0;
+	queueCount_ = 1;
+	writeCount_ = 0;
+
+	/* Retrieve control as reported by the device. */
+	std::vector<uint32_t> ids;
+	for (auto const &delay : delays_)
+		ids.push_back(delay.first->id());
+
+	ControlList controls = device_->getControls(ids);
+
+	/* Seed the control queue with the controls reported by the device. */
+	values_.clear();
+	for (const auto &ctrl : controls) {
+		const ControlId *id = device_->controls().idmap().at(ctrl.first);
+		values_[id][0] = Info(ctrl.second);
+	}
+}
+
+/**
+ * \brief Push a set of controls on the queue
+ * \param[in] controls List of controls to add to the device queue
+ *
+ * Push a set of controls to the control queue. This increases the control queue
+ * depth by one.
+ *
+ * \returns true if \a controls are accepted, or false otherwise
+ */
+bool DelayedControls::push(const ControlList &controls)
+{
+	/* Copy state from previous frame. */
+	for (auto &ctrl : values_) {
+		Info &info = ctrl.second[queueCount_];
+		info = values_[ctrl.first][queueCount_ - 1];
+		info.updated = false;
+	}
+
+	/* Update with new controls. */
+	const ControlIdMap &idmap = device_->controls().idmap();
+	for (const auto &control : controls) {
+		const auto &it = idmap.find(control.first);
+		if (it == idmap.end()) {
+			LOG(DelayedControls, Warning)
+				<< "Unknown control " << control.first;
+			return false;
+		}
+
+		const ControlId *id = it->second;
+
+		if (delays_.find(id) == delays_.end())
+			return false;
+
+		Info &info = values_[id][queueCount_];
+
+		info = control.second;
+		info.updated = true;
+
+		LOG(DelayedControls, Debug)
+			<< "Queuing " << id->name()
+			<< " to " << info.toString()
+			<< " at index " << queueCount_;
+	}
+
+	queueCount_++;
+
+	return true;
+}
+
+/**
+ * \brief Read back controls in effect at a sequence number
+ * \param[in] sequence The sequence number to get controls for
+ *
+ * Read back what controls where in effect at a specific sequence number. The
+ * history is a ring buffer of 16 entries where new and old values coexist. It's
+ * the callers responsibility to not read too old sequence numbers that have been
+ * pushed out of the history.
+ *
+ * Historic values are evicted by pushing new values onto the queue using
+ * push(). The max history from the current sequence number that yields valid
+ * values are thus 16 minus number of controls pushed.
+ *
+ * \return The controls at \a sequence number
+ */
+ControlList DelayedControls::get(uint32_t sequence)
+{
+	uint32_t adjustedSeq = sequence - firstSequence_ + 1;
+	unsigned int index = std::max<int>(0, adjustedSeq - maxDelay_);
+
+	ControlList out(device_->controls());
+	for (const auto &ctrl : values_) {
+		const ControlId *id = ctrl.first;
+		const Info &info = ctrl.second[index];
+
+		out.set(id->id(), info);
+
+		LOG(DelayedControls, Debug)
+			<< "Reading " << id->name()
+			<< " to " << info.toString()
+			<< " at index " << index;
+	}
+
+	return out;
+}
+
+/**
+ * \brief Inform DelayedControls of the start of a new frame
+ * \param[in] sequence Sequence number of the frame that started
+ *
+ * Inform the state machine that a new frame has started and of its sequence
+ * number. Any user of these helpers is responsible to inform the helper about
+ * the start of any frame. This can be connected with ease to the start of a
+ * exposure (SOE) V4L2 event.
+ */
+void DelayedControls::applyControls(uint32_t sequence)
+{
+	LOG(DelayedControls, Debug) << "frame " << sequence << " started";
+
+	if (!running_) {
+		firstSequence_ = sequence;
+		running_ = true;
+	}
+
+	/*
+	 * Create control list peeking ahead in the value queue to ensure
+	 * values are set in time to satisfy the sensor delay.
+	 */
+	ControlList out(device_->controls());
+	for (const auto &ctrl : values_) {
+		const ControlId *id = ctrl.first;
+		unsigned int delayDiff = maxDelay_ - delays_[id];
+		unsigned int index = std::max<int>(0, writeCount_ - delayDiff);
+		const Info &info = ctrl.second[index];
+
+		if (info.updated) {
+			out.set(id->id(), info);
+			LOG(DelayedControls, Debug)
+				<< "Setting " << id->name()
+				<< " to " << info.toString()
+				<< " at index " << index;
+		}
+	}
+
+	writeCount_++;
+
+	while (writeCount_ >= queueCount_) {
+		LOG(DelayedControls, Debug)
+			<< "Queue is empty, auto queue no-op.";
+		push({});
+	}
+
+	device_->setControls(&out);
+}
+
+} /* namespace libcamera */
diff --git a/src/libcamera/meson.build b/src/libcamera/meson.build
index 387d5d88..5a4bf0d7 100644
--- a/src/libcamera/meson.build
+++ b/src/libcamera/meson.build
@@ -12,6 +12,7 @@ libcamera_sources = files([
     'controls.cpp',
     'control_serializer.cpp',
     'control_validator.cpp',
+    'delayed_controls.cpp',
     'device_enumerator.cpp',
     'device_enumerator_sysfs.cpp',
     'event_dispatcher.cpp',