[libcamera-devel,v3,1/8] libcamera: delayed_controls: Add helper for controls that applies with a delay

10481 diff mbox series

Some sensor controls take effect with a delay as the sensor needs time
to adjust, for example exposure. Add a 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 thru 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>
---
* Changes since v2
- Improve error logic in queue() as suggested by Jean-Michel Hautbois.
- s/fistSequence_/firstSequence_/

* Changes since v1
- Correct copyright to reflect work is derived from Raspberry Pi
  pipeline handler. This was always the intention and was wrong in v1.
- Rewrite large parts of the documentation.
- Join two loops to one in DelayedControls::DelayedControls()
---
 include/libcamera/internal/delayed_controls.h |  87 ++++++
 src/libcamera/delayed_controls.cpp            | 265 ++++++++++++++++++
 src/libcamera/meson.build                     |   1 +
 3 files changed, 353 insertions(+)
 create mode 100644 include/libcamera/internal/delayed_controls.h
 create mode 100644 src/libcamera/delayed_controls.cpp

Hi Niklas,

On 23/11/2020 22:12, Niklas Söderlund wrote:
> Some sensor controls take effect with a delay as the sensor needs time
> to adjust, for example exposure. Add a 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 thru 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.

s/behavior/behaviour/

But that could be a valid americanism (behavior)

> 
> Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
> ---
> * Changes since v2
> - Improve error logic in queue() as suggested by Jean-Michel Hautbois.
> - s/fistSequence_/firstSequence_/
> 
> * Changes since v1
> - Correct copyright to reflect work is derived from Raspberry Pi
>   pipeline handler. This was always the intention and was wrong in v1.
> - Rewrite large parts of the documentation.
> - Join two loops to one in DelayedControls::DelayedControls()
> ---
>  include/libcamera/internal/delayed_controls.h |  87 ++++++
>  src/libcamera/delayed_controls.cpp            | 265 ++++++++++++++++++
>  src/libcamera/meson.build                     |   1 +
>  3 files changed, 353 insertions(+)
>  create mode 100644 include/libcamera/internal/delayed_controls.h
>  create mode 100644 src/libcamera/delayed_controls.cpp
> 
> diff --git a/include/libcamera/internal/delayed_controls.h b/include/libcamera/internal/delayed_controls.h
> new file mode 100644
> index 0000000000000000..e9cb0b90aac99847
> --- /dev/null
> +++ b/include/libcamera/internal/delayed_controls.h
> @@ -0,0 +1,87 @@
> +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> +/*
> + * Copyright (C) 2020, Raspberry Pi (Trading) Ltd.
> + *
> + * delayed_controls.h - Helper to deal with controls that are applied with a delay
> + */
> +#ifndef __LIBCAMERA_INTERNAL_DELAYED_CONTROLS_H__
> +#define __LIBCAMERA_INTERNAL_DELAYED_CONTROLS_H__
> +
> +#include <mutex>
> +#include <stdint.h>
> +#include <unordered_map>
> +
> +#include <libcamera/controls.h>
> +
> +namespace libcamera {
> +
> +class V4L2Device;
> +
> +class DelayedControls
> +{
> +public:
> +	DelayedControls(V4L2Device *device,
> +			const std::unordered_map<uint32_t, unsigned int> &delays);
> +
> +	void reset(ControlList *controls = nullptr);
> +
> +	bool push(const ControlList &controls);
> +	ControlList get(uint32_t sequence);
> +
> +	void frameStart(uint32_t sequence);
> +
> +private:
> +	class ControlInfo
> +	{
> +	public:
> +		ControlInfo()
> +			: updated(false)
> +		{
> +		}
> +
> +		ControlInfo(const ControlValue &v)
> +			: value(v), updated(true)
> +		{
> +		}
> +
> +		ControlValue value;
> +		bool updated;
> +	};
> +
> +	static constexpr int listSize = 16;
> +	class ControlArray : public std::array<ControlInfo, listSize>
> +	{
> +	public:
> +		ControlInfo &operator[](unsigned int index)
> +		{
> +			return std::array<ControlInfo, listSize>::operator[](index % listSize);
> +		}
> +
> +		const ControlInfo &operator[](unsigned int index) const
> +		{
> +			return std::array<ControlInfo, listSize>::operator[](index % listSize);
> +		}
> +	};
> +
> +	using ControlsDelays = std::unordered_map<const ControlId *, unsigned int>;
> +	using ControlsValues = std::unordered_map<const ControlId *, ControlArray>;

I'd remove the half-way pluralisation here
	ControlDelays
	ControlValues



> +
> +	bool queue(const ControlList &controls);
> +
> +	std::mutex lock_;
> +
> +	V4L2Device *device_;
> +	ControlsDelays delays_;
> +	unsigned int maxDelay_;
> +
> +	bool running_;
> +	uint32_t firstSequence_;
> +
> +	uint32_t queueCount_;
> +	uint32_t writeCount_;
> +	ControlsValues ctrls_;
> +};
> +
> +} /* namespace libcamera */
> +
> +#endif /* __LIBCAMERA_INTERNAL_DELAYED_CONTROLS_H__ */
> diff --git a/src/libcamera/delayed_controls.cpp b/src/libcamera/delayed_controls.cpp
> new file mode 100644
> index 0000000000000000..c810be48a36be515
> --- /dev/null
> +++ b/src/libcamera/delayed_controls.cpp
> @@ -0,0 +1,265 @@
> +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> +/*
> + * Copyright (C) 2020, Raspberry Pi (Trading) Ltd.
> + *
> + * delayed_controls.h - Helper to deal with controls that are applied with a delay
> + */
> +
> +#include "libcamera/internal/delayed_controls.h"
> +#include "libcamera/internal/v4l2_device.h"
> +
> +#include <libcamera/controls.h>
> +
> +#include "libcamera/internal/log.h"
> +
> +/**
> + * \file delayed_controls.h
> + * \brief Helper to deal with controls that are applied 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 focus. This is an optional 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 need 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 grates
> + * delay.
> + */
> +
> +/**
> + * \brief Construct a DelayedControls
> + * \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 itControl = controls.find(delay.first);
> +		if (itControl == 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 = itControl->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 and controls
> + * \param[in] controls Optional list of controls to apply to the device
> + *
> + * Resets the state machine to a starting position based on control values
> + * retrieved from the device. Controls may optionally be set before they are
> + * read back using \a controls.
> + */
> +void DelayedControls::reset(ControlList *controls)
> +{
> +	std::lock_guard<std::mutex> lock(lock_);
> +
> +	running_ = false;
> +	firstSequence_ = 0;
> +	queueCount_ = 0;
> +	writeCount_ = 0;
> +
> +	/* Set the controls on the device if requested. */
> +	if (controls)
> +		device_->setControls(controls);
> +
> +	/* Retrieve current control values reported by the device. */
> +	std::vector<uint32_t> ids;
> +	for (auto const &delay : delays_)
> +		ids.push_back(delay.first->id());
> +
> +	ControlList devCtrls = device_->getControls(ids);
> +
> +	/* Seed the control queue with the controls reported by the device. */
> +	ctrls_.clear();
> +	for (const auto &ctrl : devCtrls) {
> +		const ControlId *id = devCtrls.infoMap()->idmap().at(ctrl.first);
> +		ctrls_[id][queueCount_] = ControlInfo(ctrl.second);
> +	}
> +
> +	queueCount_++;
> +}
> +
> +/**
> + * \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)
> +{
> +	std::lock_guard<std::mutex> lock(lock_);
> +
> +	return queue(controls);
> +}
> +
> +bool DelayedControls::queue(const ControlList &controls)
> +{
> +	/* Copy state from previous frame. */
> +	for (auto &ctrl : ctrls_) {
> +		ControlInfo &info = ctrl.second[queueCount_];
> +		info.value = ctrls_[ctrl.first][queueCount_ - 1].value;
> +		info.updated = false;
> +	}
> +
> +	/* Update with new controls. */
> +	const ControlIdMap &idmap = device_->controls().idmap();
> +	for (const auto &control : controls) {
> +		const auto &itCtrl = idmap.find(control.first);
> +		if (itCtrl == idmap.end())
> +			return false;
> +
> +		const ControlId *id = itCtrl->second;
> +
> +		if (delays_.find(id) == delays_.end())
> +			return false;
> +
> +		ControlInfo &info = ctrls_[id][queueCount_];
> +
> +		info.value = control.second;
> +		info.updated = true;
> +
> +		LOG(DelayedControls, Debug)
> +			<< "Queuing " << id->name()
> +			<< " to " << info.value.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 to 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)
> +{
> +	std::lock_guard<std::mutex> lock(lock_);
> +
> +	uint32_t adjustedSeq = sequence - firstSequence_ + 1;
> +	unsigned int index = std::max<int>(0, adjustedSeq - maxDelay_);
> +
> +	ControlList out(device_->controls());
> +	for (const auto &ctrl : ctrls_) {
> +		const ControlId *id = ctrl.first;
> +		const ControlInfo &info = ctrl.second[index];
> +
> +		out.set(id->id(), info.value);
> +
> +		LOG(DelayedControls, Debug)
> +			<< "Reading " << id->name()
> +			<< " to " << info.value.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::frameStart(uint32_t sequence)
> +{
> +	LOG(DelayedControls, Debug) << "frame " << sequence << " started";
> +
> +	std::lock_guard<std::mutex> lock(lock_);
> +
> +	if (!running_) {
> +		firstSequence_ = sequence;
> +		running_ = true;
> +	}
> +
> +	/*
> +	 * Create control list peaking 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 : ctrls_) {
> +		const ControlId *id = ctrl.first;
> +		unsigned int delayDiff = maxDelay_ - delays_[id];
> +		unsigned int index = std::max<int>(0, writeCount_ - delayDiff);
> +		const ControlInfo &info = ctrl.second[index];
> +
> +		if (info.updated) {
> +			out.set(id->id(), info.value);
> +			LOG(DelayedControls, Debug)
> +				<< "Setting " << id->name()
> +				<< " to " << info.value.toString()
> +				<< " at index " << index;
> +		}
> +	}
> +
> +	writeCount_++;
> +
> +	while (writeCount_ >= queueCount_) {
> +		LOG(DelayedControls, Debug)
> +			<< "Queue is empty, auto queue no-op.";
> +		queue({});
> +	}
> +
> +	device_->setControls(&out);
> +}
> +
> +} /* namespace libcamera */
> diff --git a/src/libcamera/meson.build b/src/libcamera/meson.build
> index 387d5d88ecae11ad..5a4bf0d7ba4fd231 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',
>

Hi Niklas,

On Mon, Nov 23, 2020 at 11:12:27PM +0100, Niklas Söderlund wrote:
> Some sensor controls take effect with a delay as the sensor needs time
> to adjust, for example exposure. Add a 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 thru 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.
>

Now tha the whole mechanism clicked for me (it only took three days
after all) I mostly have comments on the 'cosmetic' part

> Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
> ---
> * Changes since v2
> - Improve error logic in queue() as suggested by Jean-Michel Hautbois.
> - s/fistSequence_/firstSequence_/
>
> * Changes since v1
> - Correct copyright to reflect work is derived from Raspberry Pi
>   pipeline handler. This was always the intention and was wrong in v1.
> - Rewrite large parts of the documentation.
> - Join two loops to one in DelayedControls::DelayedControls()
> ---
>  include/libcamera/internal/delayed_controls.h |  87 ++++++
>  src/libcamera/delayed_controls.cpp            | 265 ++++++++++++++++++
>  src/libcamera/meson.build                     |   1 +
>  3 files changed, 353 insertions(+)
>  create mode 100644 include/libcamera/internal/delayed_controls.h
>  create mode 100644 src/libcamera/delayed_controls.cpp
>
> diff --git a/include/libcamera/internal/delayed_controls.h b/include/libcamera/internal/delayed_controls.h
> new file mode 100644
> index 0000000000000000..e9cb0b90aac99847
> --- /dev/null
> +++ b/include/libcamera/internal/delayed_controls.h
> @@ -0,0 +1,87 @@
> +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> +/*
> + * Copyright (C) 2020, Raspberry Pi (Trading) Ltd.
> + *
> + * delayed_controls.h - Helper to deal with controls that are applied with a delay
> + */
> +#ifndef __LIBCAMERA_INTERNAL_DELAYED_CONTROLS_H__
> +#define __LIBCAMERA_INTERNAL_DELAYED_CONTROLS_H__
> +
> +#include <mutex>
> +#include <stdint.h>
> +#include <unordered_map>
> +
> +#include <libcamera/controls.h>
> +
> +namespace libcamera {
> +
> +class V4L2Device;
> +
> +class DelayedControls
> +{
> +public:
> +	DelayedControls(V4L2Device *device,
> +			const std::unordered_map<uint32_t, unsigned int> &delays);
> +
> +	void reset(ControlList *controls = nullptr);
> +
> +	bool push(const ControlList &controls);
> +	ControlList get(uint32_t sequence);

I liked the set/get pair that we had in StaggeredControls more

> +
> +	void frameStart(uint32_t sequence);

I also think 'write' was probably nicer, but that's minor.
I would have called this 'notifySOF()' but that's just my preference,
so keep whatever you have here

> +
> +private:
> +	class ControlInfo

As I've commented on v2, this is already the name of a library wide
type.

To be honest I would drop the 'Control' suffix for all the internal
types

        ControlInfo -> Info
        ControlsDelays -> Delays
        ControlsValues -> Values

They're internal after all (I know this goes both ways, but for sure
they're shorter)

> +	{
> +	public:
> +		ControlInfo()
> +			: updated(false)
> +		{
> +		}
> +
> +		ControlInfo(const ControlValue &v)
> +			: value(v), updated(true)
> +		{
> +		}
> +
> +		ControlValue value;
> +		bool updated;
> +	};
> +
> +	static constexpr int listSize = 16;
> +	class ControlArray : public std::array<ControlInfo, listSize>
> +	{
> +	public:
> +		ControlInfo &operator[](unsigned int index)
> +		{
> +			return std::array<ControlInfo, listSize>::operator[](index % listSize);
> +		}
> +
> +		const ControlInfo &operator[](unsigned int index) const
> +		{
> +			return std::array<ControlInfo, listSize>::operator[](index % listSize);
> +		}
> +	};
> +
> +	using ControlsDelays = std::unordered_map<const ControlId *, unsigned int>;
> +	using ControlsValues = std::unordered_map<const ControlId *, ControlArray>;
> +
> +	bool queue(const ControlList &controls);
> +
> +	std::mutex lock_;
> +
> +	V4L2Device *device_;
> +	ControlsDelays delays_;
> +	unsigned int maxDelay_;
> +
> +	bool running_;
> +	uint32_t firstSequence_;
> +
> +	uint32_t queueCount_;
> +	uint32_t writeCount_;
> +	ControlsValues ctrls_;

I would then call this 'values_'

> +};
> +
> +} /* namespace libcamera */
> +
> +#endif /* __LIBCAMERA_INTERNAL_DELAYED_CONTROLS_H__ */
> diff --git a/src/libcamera/delayed_controls.cpp b/src/libcamera/delayed_controls.cpp
> new file mode 100644
> index 0000000000000000..c810be48a36be515
> --- /dev/null
> +++ b/src/libcamera/delayed_controls.cpp
> @@ -0,0 +1,265 @@
> +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> +/*
> + * Copyright (C) 2020, Raspberry Pi (Trading) Ltd.
> + *
> + * delayed_controls.h - Helper to deal with controls that are applied with a delay
> + */
> +
> +#include "libcamera/internal/delayed_controls.h"
> +#include "libcamera/internal/v4l2_device.h"
> +
> +#include <libcamera/controls.h>
> +
> +#include "libcamera/internal/log.h"
> +
> +/**
> + * \file delayed_controls.h
> + * \brief Helper to deal with controls that are applied 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 focus. This is an optional 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 need 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 grates

'greatest'

I find the reference to the buffer depth more confusing than helping.

> + * delay.
> + */
> +
> +/**
> + * \brief Construct a DelayedControls
 \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 itControl = controls.find(delay.first);
> +		if (itControl == 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 = itControl->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 and controls
> + * \param[in] controls Optional list of controls to apply to the device
> + *
> + * Resets the state machine to a starting position based on control values
> + * retrieved from the device. Controls may optionally be set before they are
> + * read back using \a controls.
> + */
> +void DelayedControls::reset(ControlList *controls)
> +{
> +	std::lock_guard<std::mutex> lock(lock_);
> +
> +	running_ = false;
> +	firstSequence_ = 0;
> +	queueCount_ = 0;
> +	writeCount_ = 0;
> +
> +	/* Set the controls on the device if requested. */
> +	if (controls)
> +		device_->setControls(controls);

Checking for the return value would guarantee Controls not supported
by the device are not supplied and not erronously added to the ctrls_
map below ?

> +
> +	/* Retrieve current control values reported by the device. */
> +	std::vector<uint32_t> ids;
> +	for (auto const &delay : delays_)
> +		ids.push_back(delay.first->id());
> +
> +	ControlList devCtrls = device_->getControls(ids);
> +
> +	/* Seed the control queue with the controls reported by the device. */
> +	ctrls_.clear();
> +	for (const auto &ctrl : devCtrls) {
> +		const ControlId *id = devCtrls.infoMap()->idmap().at(ctrl.first);

As .at() with an unsupported id would fail afaict

> +		ctrls_[id][queueCount_] = ControlInfo(ctrl.second);
> +	}
> +
> +	queueCount_++;
> +}
> +
> +/**
> + * \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)
> +{
> +	std::lock_guard<std::mutex> lock(lock_);
> +
> +	return queue(controls);
> +}
> +
> +bool DelayedControls::queue(const ControlList &controls)
> +{
> +	/* Copy state from previous frame. */
> +	for (auto &ctrl : ctrls_) {
> +		ControlInfo &info = ctrl.second[queueCount_];
> +		info.value = ctrls_[ctrl.first][queueCount_ - 1].value;
> +		info.updated = false;
> +	}
> +
> +	/* Update with new controls. */
> +	const ControlIdMap &idmap = device_->controls().idmap();
> +	for (const auto &control : controls) {
> +		const auto &itCtrl = idmap.find(control.first);
> +		if (itCtrl == idmap.end())
> +			return false;
> +
> +		const ControlId *id = itCtrl->second;
> +
> +		if (delays_.find(id) == delays_.end())
> +			return false;
> +
> +		ControlInfo &info = ctrls_[id][queueCount_];
> +
> +		info.value = control.second;
> +		info.updated = true;
> +
> +		LOG(DelayedControls, Debug)
> +			<< "Queuing " << id->name()
> +			<< " to " << info.value.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 to 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)
> +{
> +	std::lock_guard<std::mutex> lock(lock_);
> +
> +	uint32_t adjustedSeq = sequence - firstSequence_ + 1;
> +	unsigned int index = std::max<int>(0, adjustedSeq - maxDelay_);
> +
> +	ControlList out(device_->controls());
> +	for (const auto &ctrl : ctrls_) {
> +		const ControlId *id = ctrl.first;
> +		const ControlInfo &info = ctrl.second[index];
> +
> +		out.set(id->id(), info.value);
> +
> +		LOG(DelayedControls, Debug)
> +			<< "Reading " << id->name()
> +			<< " to " << info.value.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.

This is all true, this function informs about an SOE events, but
what's relevant is that in response to this event a new set of
controls are written to the device. It is not mentioned anywhere in
documentation.

> + */
> +void DelayedControls::frameStart(uint32_t sequence)
> +{
> +	LOG(DelayedControls, Debug) << "frame " << sequence << " started";
> +
> +	std::lock_guard<std::mutex> lock(lock_);
> +
> +	if (!running_) {
> +		firstSequence_ = sequence;
> +		running_ = true;
> +	}
> +
> +	/*
> +	 * Create control list peaking 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 : ctrls_) {
> +		const ControlId *id = ctrl.first;
> +		unsigned int delayDiff = maxDelay_ - delays_[id];
> +		unsigned int index = std::max<int>(0, writeCount_ - delayDiff);
> +		const ControlInfo &info = ctrl.second[index];
> +
> +		if (info.updated) {
> +			out.set(id->id(), info.value);
> +			LOG(DelayedControls, Debug)
> +				<< "Setting " << id->name()
> +				<< " to " << info.value.toString()
> +				<< " at index " << index;
> +		}
> +	}
> +
> +	writeCount_++;
> +
> +	while (writeCount_ >= queueCount_) {
> +		LOG(DelayedControls, Debug)
> +			<< "Queue is empty, auto queue no-op.";
> +		queue({});

I would check at the beginning of the function if writeCount_ >
queueCount_ and in the case exit immediately to avoid pushing a set of
empty controls just to create a new slot. If I'm not mistaken it has
the same effect ?

This would allow you merge queue() and push() in a single function (I
still don't like queue({}) and still seems a bit of an hack :)

Thanks
  j

> +	}
> +
> +	device_->setControls(&out);
> +}
> +
> +} /* namespace libcamera */
> diff --git a/src/libcamera/meson.build b/src/libcamera/meson.build
> index 387d5d88ecae11ad..5a4bf0d7ba4fd231 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',
> --
> 2.29.2
>
> _______________________________________________
> libcamera-devel mailing list
> libcamera-devel@lists.libcamera.org
> https://lists.libcamera.org/listinfo/libcamera-devel

Hi Niklas,

Thank you for the patch.

On Thu, Nov 26, 2020 at 06:38:59PM +0100, Jacopo Mondi wrote:
> On Mon, Nov 23, 2020 at 11:12:27PM +0100, Niklas Söderlund wrote:
> > Some sensor controls take effect with a delay as the sensor needs time
> > to adjust, for example exposure. Add a optional helper DelayedControls

s/a optional/an optional/

> > 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 thru V4L2 events.

I think that's a good design. If we ever have to emulate this using
timers, that can always be implement this as an extension (possibly in a
separate class whose purpose would be to generate the emulate 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.
> 
> Now tha the whole mechanism clicked for me (it only took three days
> after all) I mostly have comments on the 'cosmetic' part
> 
> > Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
> > ---
> > * Changes since v2
> > - Improve error logic in queue() as suggested by Jean-Michel Hautbois.
> > - s/fistSequence_/firstSequence_/
> >
> > * Changes since v1
> > - Correct copyright to reflect work is derived from Raspberry Pi
> >   pipeline handler. This was always the intention and was wrong in v1.
> > - Rewrite large parts of the documentation.
> > - Join two loops to one in DelayedControls::DelayedControls()
> > ---
> >  include/libcamera/internal/delayed_controls.h |  87 ++++++
> >  src/libcamera/delayed_controls.cpp            | 265 ++++++++++++++++++
> >  src/libcamera/meson.build                     |   1 +
> >  3 files changed, 353 insertions(+)
> >  create mode 100644 include/libcamera/internal/delayed_controls.h
> >  create mode 100644 src/libcamera/delayed_controls.cpp
> >
> > diff --git a/include/libcamera/internal/delayed_controls.h b/include/libcamera/internal/delayed_controls.h
> > new file mode 100644
> > index 0000000000000000..e9cb0b90aac99847
> > --- /dev/null
> > +++ b/include/libcamera/internal/delayed_controls.h
> > @@ -0,0 +1,87 @@
> > +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> > +/*
> > + * Copyright (C) 2020, Raspberry Pi (Trading) Ltd.
> > + *
> > + * delayed_controls.h - Helper to deal with controls that are applied with a delay
> > + */
> > +#ifndef __LIBCAMERA_INTERNAL_DELAYED_CONTROLS_H__
> > +#define __LIBCAMERA_INTERNAL_DELAYED_CONTROLS_H__
> > +
> > +#include <mutex>
> > +#include <stdint.h>
> > +#include <unordered_map>
> > +
> > +#include <libcamera/controls.h>
> > +
> > +namespace libcamera {
> > +
> > +class V4L2Device;
> > +
> > +class DelayedControls
> > +{
> > +public:
> > +	DelayedControls(V4L2Device *device,
> > +			const std::unordered_map<uint32_t, unsigned int> &delays);

I could imagine this being useful for non-V4L2 devices too. We could
instead pass a ControlInfoMap to the constructor, and move the
setControls() call to the user of this class. There's no need to do so
now (but of course if you think it's a good idea, I'm not asking to
delay such rework :-)), but could you capture this in a \todo comment
(if you agree with the idea) ?

> > +
> > +	void reset(ControlList *controls = nullptr);
> > +
> > +	bool push(const ControlList &controls);
> > +	ControlList get(uint32_t sequence);
> 
> I liked the set/get pair that we had in StaggeredControls more
> 
> > +
> > +	void frameStart(uint32_t sequence);
> 
> I also think 'write' was probably nicer, but that's minor.
> I would have called this 'notifySOF()' but that's just my preference,
> so keep whatever you have here

applyControls() could also be an option. Or, if we decide to move
setControls() out of this class, we would need to make this function
return a ControlList.

> > +
> > +private:
> > +	class ControlInfo
> 
> As I've commented on v2, this is already the name of a library wide
> type.
> 
> To be honest I would drop the 'Control' suffix for all the internal
> types
> 
>         ControlInfo -> Info
>         ControlsDelays -> Delays
>         ControlsValues -> Values
> 
> They're internal after all (I know this goes both ways, but for sure
> they're shorter)

That could indeed avoid confusion.

> > +	{
> > +	public:
> > +		ControlInfo()
> > +			: updated(false)
> > +		{
> > +		}
> > +
> > +		ControlInfo(const ControlValue &v)
> > +			: value(v), updated(true)
> > +		{
> > +		}
> > +
> > +		ControlValue value;
> > +		bool updated;
> > +	};

You could also make this class inherit from ControlValue, to avoid the
explicit .value below.

> > +
> > +	static constexpr int listSize = 16;
> > +	class ControlArray : public std::array<ControlInfo, listSize>

How about a name that conveys this is a ring buffer, such as
ControlCircularArray, ControlRingBuffer, CircularArray, ... ? 

> > +	{
> > +	public:
> > +		ControlInfo &operator[](unsigned int index)
> > +		{
> > +			return std::array<ControlInfo, listSize>::operator[](index % listSize);
> > +		}
> > +
> > +		const ControlInfo &operator[](unsigned int index) const
> > +		{
> > +			return std::array<ControlInfo, listSize>::operator[](index % listSize);
> > +		}
> > +	};
> > +
> > +	using ControlsDelays = std::unordered_map<const ControlId *, unsigned int>;
> > +	using ControlsValues = std::unordered_map<const ControlId *, ControlArray>;

As these two types are only used once, below in the class declaration,
how about dropping the aliases ? Aliases are useful to avoid typing
types out explicitly in every location, but they have the drawback of
possibly obfuscating the code. I would then rename the above ControlInfo
to Value, as it stores a value.

I wonder if we should bundle the delay and array in a single class, to
have a single map (now or on top).

I also wonder if we shouldn't use the integer control ID as the key,
this would simplify DelayedControls::queue() that wouldn't have to look
up the ControlId pointer from the idmap.

> > +
> > +	bool queue(const ControlList &controls);
> > +
> > +	std::mutex lock_;

Do we need a lock, isn't this class used in a single thread ? If we want
to make it thread-safe, we should document this explicitly for each
function.

> > +
> > +	V4L2Device *device_;
> > +	ControlsDelays delays_;
> > +	unsigned int maxDelay_;
> > +
> > +	bool running_;
> > +	uint32_t firstSequence_;
> > +
> > +	uint32_t queueCount_;
> > +	uint32_t writeCount_;
> > +	ControlsValues ctrls_;
> 
> I would then call this 'values_'
> 
> > +};
> > +
> > +} /* namespace libcamera */
> > +
> > +#endif /* __LIBCAMERA_INTERNAL_DELAYED_CONTROLS_H__ */
> > diff --git a/src/libcamera/delayed_controls.cpp b/src/libcamera/delayed_controls.cpp
> > new file mode 100644
> > index 0000000000000000..c810be48a36be515
> > --- /dev/null
> > +++ b/src/libcamera/delayed_controls.cpp
> > @@ -0,0 +1,265 @@
> > +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> > +/*
> > + * Copyright (C) 2020, Raspberry Pi (Trading) Ltd.
> > + *
> > + * delayed_controls.h - Helper to deal with controls that are applied with a delay
> > + */
> > +
> > +#include "libcamera/internal/delayed_controls.h"
> > +#include "libcamera/internal/v4l2_device.h"

This header...

> > +
> > +#include <libcamera/controls.h>
> > +
> > +#include "libcamera/internal/log.h"

... should go here.

> > +
> > +/**
> > + * \file delayed_controls.h
> > + * \brief Helper to deal with controls that are applied 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 focus. This is an optional helper class to

s/an optional helper/a helper/ ?

> > + * 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 need to maintain to also cover controls. Just as with buffer

s/need/needs/

> > + * 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 grates
> 
> 'greatest'
> 
> I find the reference to the buffer depth more confusing than helping.
> 
> > + * delay.
> > + */
> > +
> > +/**
> > + * \brief Construct a DelayedControls
>
>  \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 itControl = controls.find(delay.first);

As there's a single iterator in this function we usually use "it" or
"iter". Up to you.

> > +		if (itControl == 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 = itControl->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 and controls
> > + * \param[in] controls Optional list of controls to apply to the device
> > + *
> > + * Resets the state machine to a starting position based on control values
> > + * retrieved from the device. Controls may optionally be set before they are
> > + * read back using \a controls.
> > + */
> > +void DelayedControls::reset(ControlList *controls)
> > +{
> > +	std::lock_guard<std::mutex> lock(lock_);
> > +
> > +	running_ = false;
> > +	firstSequence_ = 0;
> > +	queueCount_ = 0;
> > +	writeCount_ = 0;
> > +
> > +	/* Set the controls on the device if requested. */
> > +	if (controls)
> > +		device_->setControls(controls);
> 
> Checking for the return value would guarantee Controls not supported
> by the device are not supplied and not erronously added to the ctrls_
> map below ?

Should this be moved to the caller ? Looking at how this is used, only
the Raspberry Pi pipeline handler calls this function with a non-null
controls parameter, to set initial control values at configure time. It
doesn't seem to really fit in the DelayedControls helper.

> > +
> > +	/* Retrieve current control values reported by the device. */
> > +	std::vector<uint32_t> ids;
> > +	for (auto const &delay : delays_)
> > +		ids.push_back(delay.first->id());
> > +
> > +	ControlList devCtrls = device_->getControls(ids);

(This variable could then be called controls)

> > +
> > +	/* Seed the control queue with the controls reported by the device. */
> > +	ctrls_.clear();
> > +	for (const auto &ctrl : devCtrls) {
> > +		const ControlId *id = devCtrls.infoMap()->idmap().at(ctrl.first);

You use devCtrls.infoMap() here, and device_->controls() below. They
should be identical (or there's something I don't get :-)), so I think
the code would be easier to understand if we used one of the two
consistently.

> As .at() with an unsupported id would fail afaict
> 
> > +		ctrls_[id][queueCount_] = ControlInfo(ctrl.second);

Maybe s/queueCount/0/ to make this more explicit ?

> > +	}
> > +
> > +	queueCount_++;

You could then fold this above, making it queueCount_ = 1.

> > +}
> > +
> > +/**
> > + * \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)
> > +{
> > +	std::lock_guard<std::mutex> lock(lock_);
> > +
> > +	return queue(controls);
> > +}
> > +
> > +bool DelayedControls::queue(const ControlList &controls)
> > +{
> > +	/* Copy state from previous frame. */
> > +	for (auto &ctrl : ctrls_) {
> > +		ControlInfo &info = ctrl.second[queueCount_];
> > +		info.value = ctrls_[ctrl.first][queueCount_ - 1].value;
> > +		info.updated = false;
> > +	}
> > +
> > +	/* Update with new controls. */
> > +	const ControlIdMap &idmap = device_->controls().idmap();
> > +	for (const auto &control : controls) {
> > +		const auto &itCtrl = idmap.find(control.first);

Same comment about the iterator name.

> > +		if (itCtrl == idmap.end())

Should a warning message be printed here ?

> > +			return false;
> > +
> > +		const ControlId *id = itCtrl->second;
> > +
> > +		if (delays_.find(id) == delays_.end())
> > +			return false;
> > +
> > +		ControlInfo &info = ctrls_[id][queueCount_];
> > +
> > +		info.value = control.second;
> > +		info.updated = true;
> > +
> > +		LOG(DelayedControls, Debug)
> > +			<< "Queuing " << id->name()
> > +			<< " to " << info.value.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 to old sequence numbers that have been

s/to/too/

> > + * pushed out of the history.

What history size do we need in practice, do we need to go higher than
the maximum delay ? I wonder if we should make this dynamic (or just
lower, 16 seems fairly large). Making it dynamic could be done on top (a
\todo comment would then be nice).

> > + *
> > + * 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)
> > +{
> > +	std::lock_guard<std::mutex> lock(lock_);
> > +
> > +	uint32_t adjustedSeq = sequence - firstSequence_ + 1;
> > +	unsigned int index = std::max<int>(0, adjustedSeq - maxDelay_);
> > +
> > +	ControlList out(device_->controls());
> > +	for (const auto &ctrl : ctrls_) {
> > +		const ControlId *id = ctrl.first;
> > +		const ControlInfo &info = ctrl.second[index];
> > +
> > +		out.set(id->id(), info.value);
> > +
> > +		LOG(DelayedControls, Debug)
> > +			<< "Reading " << id->name()
> > +			<< " to " << info.value.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.
> 
> This is all true, this function informs about an SOE events, but
> what's relevant is that in response to this event a new set of
> controls are written to the device. It is not mentioned anywhere in
> documentation.

Once we agree on the review comments (and a new version is posted to
address them if needed), I can help with the documentation if that would
be useful.

> > + */
> > +void DelayedControls::frameStart(uint32_t sequence)
> > +{
> > +	LOG(DelayedControls, Debug) << "frame " << sequence << " started";
> > +
> > +	std::lock_guard<std::mutex> lock(lock_);
> > +
> > +	if (!running_) {
> > +		firstSequence_ = sequence;
> > +		running_ = true;
> > +	}
> > +
> > +	/*
> > +	 * Create control list peaking 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 : ctrls_) {
> > +		const ControlId *id = ctrl.first;
> > +		unsigned int delayDiff = maxDelay_ - delays_[id];
> > +		unsigned int index = std::max<int>(0, writeCount_ - delayDiff);

Let's assume a sensor that only a single control, with a delay of 1
frame. It has been pre-programmed before stream start with an initial
value X with reset(). Then, two requests are queued before the first
SOF, with values A and B. push() is called twice, with A and B.
queueCount is thus 3 (initial value set to 1 in reset(), and increased
twice by push), and the queue contains X, A and B in positions 0, 1 and
2.

The first frame start arrives, frameStart(0) is called. writeCount_ is
0, maxDelay_ is 1, delays_[id] is 1. index will thus be 0, which causes
X to be written to the sensor.

Isn't that an incorrect behaviour ? Shouldn't B be written instead,
given that the value will take effect in frame 1, which will be captured
in request B ?

> > +		const ControlInfo &info = ctrl.second[index];
> > +
> > +		if (info.updated) {
> > +			out.set(id->id(), info.value);
> > +			LOG(DelayedControls, Debug)
> > +				<< "Setting " << id->name()
> > +				<< " to " << info.value.toString()
> > +				<< " at index " << index;
> > +		}
> > +	}
> > +
> > +	writeCount_++;
> > +
> > +	while (writeCount_ >= queueCount_) {
> > +		LOG(DelayedControls, Debug)
> > +			<< "Queue is empty, auto queue no-op.";
> > +		queue({});
> 
> I would check at the beginning of the function if writeCount_ >
> queueCount_ and in the case exit immediately to avoid pushing a set of
> empty controls just to create a new slot. If I'm not mistaken it has
> the same effect ?
> 
> This would allow you merge queue() and push() in a single function (I
> still don't like queue({}) and still seems a bit of an hack :)
> 
> > +	}
> > +
> > +	device_->setControls(&out);
> > +}
> > +
> > +} /* namespace libcamera */
> > diff --git a/src/libcamera/meson.build b/src/libcamera/meson.build
> > index 387d5d88ecae11ad..5a4bf0d7ba4fd231 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',