README.rst


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.. SPDX-License-Identifier: CC-BY-SA-4.0

.. section-begin-libcamera

===========
 libcamera
===========

**A complex camera support library for Linux, Android, and ChromeOS**

Cameras are complex devices that need heavy hardware image processing
operations. Control of the processing is based on advanced algorithms that must
run on a programmable processor. This has traditionally been implemented in a
dedicated MCU in the camera, but in embedded devices algorithms have been moved
to the main CPU to save cost. Blurring the boundary between camera devices and
Linux often left the user with no other option than a vendor-specific
closed-source solution.

To address this problem the Linux media community has very recently started
collaboration with the industry to develop a camera stack that will be
open-source-friendly while still protecting vendor core IP. libcamera was born
out of that collaboration and will offer modern camera support to Linux-based
systems, including traditional Linux distributions, ChromeOS and Android.

.. section-end-libcamera
.. section-begin-getting-started

Getting Started
---------------

To fetch the sources, build and install:

::

  git clone git://linuxtv.org/libcamera.git
  cd libcamera
  meson build
  ninja -C build install

Dependencies
~~~~~~~~~~~~

The following Debian/Ubuntu packages are required for building libcamera.
Other distributions may have differing package names:

A C++ toolchain: [required]
	Either {g++, clang}

for libcamera: [required]
        meson (>= 0.47) ninja-build python3-yaml

        If your distribution doesn't provide a recent enough version of meson,
        you can install or upgrade it using pip3.

        .. code::

            pip3 install --user meson
            pip3 install --user --upgrade meson

for device hotplug enumeration: [optional]
	pkg-config libudev-dev

for documentation: [optional]
	python3-sphinx doxygen

for gstreamer: [optional]
	libgstreamer1.0-dev libgstreamer-plugins-base1.0-dev

for IPA module signing: [required]
        libgnutls28-dev openssl

for qcam: [optional]
	qtbase5-dev libqt5core5a libqt5gui5 libqt5widgets5

Using GStreamer plugin
~~~~~~~~~~~~~~~~~~~~~~

To use GStreamer plugin from source tree, set the following environment so that
GStreamer can find it.

  export GST_PLUGIN_PATH=$(pwd)/build/src/gstreamer

The debugging tool `gst-launch-1.0` can be used to construct and pipeline and test
it. The following pipeline will stream from the camera named "Camera 1" onto the
default video display element on your system.

.. code::

  gst-launch-1.0 libcamerasrc camera-name="Camera 1" ! videoconvert ! autovideosink

.. section-end-getting-started
/* SPDX-License-Identifier: LGPL-2.1-or-later */
/*
 * Copyright (C) 2019, Google Inc.
 *
 * thread.cpp - Thread support
 */

#include "libcamera/internal/thread.h"

#include <atomic>
#include <condition_variable>
#include <list>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>

#include <libcamera/event_dispatcher.h>

#include "libcamera/internal/event_dispatcher_poll.h"
#include "libcamera/internal/log.h"
#include "libcamera/internal/message.h"

/**
 * \page thread Thread Support
 *
 * libcamera supports multi-threaded applications through a threading model that
 * sets precise rules to guarantee thread-safe usage of the API. Additionally,
 * libcamera makes internal use of threads, and offers APIs that simplify
 * interactions with application threads. Careful compliance with the threading
 * model will ensure avoidance of race conditions.
 *
 * \section thread-objects Threads and Objects
 *
 * Instances of the Object class and all its derived classes are thread-aware
 * and are bound to the thread they are created in. They are said to *live* in
 * a thread, and they interact with the event loop of their thread for the
 * purpose of message passing and signal delivery. Messages posted to the
 * object with Object::postMessage() will be delivered from the event loop of
 * the thread that the object lives in. Signals delivered to the object, unless
 * explicitly connected with ConnectionTypeDirect, will also be delivered from
 * the object thread's event loop.
 *
 * All Object instances created by libcamera are bound to an internal thread,
 * and applications don't need to provide an event loop to support them. Object
 * instances created by applications require an event loop. It is the
 * responsibility of applications to provide that event loop, either explicitly
 * through CameraManager::setEventDispatcher(), or by running the default event
 * loop provided by CameraManager::eventDispatcher() in their main thread. The
 * main thread of an application is the one that calls CameraManager::start().
 *
 * \section thread-signals Threads and Signals
 *
 * When sent to a receiver that does not inherit from the Object class, signals
 * are delivered synchronously in the thread of the sender. When the receiver
 * inherits from the Object class, delivery is by default asynchronous if the
 * sender and receiver live in different threads. In that case, the signal is
 * posted to the receiver's message queue and will be delivered from the
 * receiver's event loop, running in the receiver's thread. This mechanism can
 * be overridden by selecting a different connection type when calling
 * Signal::connect().
 *
 * Asynchronous signal delivery is used internally in libcamera, but is also
 * available to applications if desired. To use this feature, applications
 * shall create receiver classes that inherit from the Object class, and
 * provide an event loop to the CameraManager as explained above. Note that
 * Object instances created by the application are limited to living in the
 * application's main thread. Creating Object instances from another thread of
 * an application causes undefined behaviour.
 *
 * \section thread-reentrancy Reentrancy and Thread-Safety
 *
 * Through the documentation, several terms are used to define how classes and
 * their member functions can be used from multiple threads.
 *
 * - A **reentrant** function may be called simultaneously from multiple
 *   threads if and only if each invocation uses a different instance of the
 *   class. This is the default for all member functions not explictly marked
 *   otherwise.
 *
 * - \anchor thread-safe A **thread-safe** function may be called
 *   simultaneously from multiple threads on the same instance of a class. A
 *   thread-safe function is thus reentrant. Thread-safe functions may also be
 *   called simultaneously with any other reentrant function of the same class
 *   on the same instance.
 *
 * - \anchor thread-bound A **thread-bound** function may be called only from
 *   the thread that the class instances lives in (see section \ref
 *   thread-objects). For instances of classes that do not derive from the
 *   Object class, this is the thread in which the instance was created. A
 *   thread-bound function is not thread-safe, and may or may not be reentrant.
 *
 * Neither reentrancy nor thread-safety, in this context, mean that a function
 * may be called simultaneously from the same thread, for instance from a
 * callback invoked by the function. This may deadlock and isn't allowed unless
 * separately documented.
 *
 * A class is defined as reentrant, thread-safe or thread-bound if all its
 * member functions are reentrant, thread-safe or thread-bound respectively.
 * Some member functions may additionally be documented as having additional
 * thread-related attributes.
 *
 * Most classes are reentrant but not thread-safe, as making them fully
 * thread-safe would incur locking costs considered prohibitive for the
 * expected use cases.
 */

/**
 * \file thread.h
 * \brief Thread support
 */

namespace libcamera {

LOG_DEFINE_CATEGORY(Thread)

class ThreadMain;

/**
 * \brief A queue of posted messages
 */
class MessageQueue
{
public:
	/**
	 * \brief List of queued Message instances
	 */
	std::list<std::unique_ptr<Message>> list_;
	/**
	 * \brief Protects the \ref list_
	 */
	Mutex mutex_;
};

/**
 * \brief Thread-local internal data
 */
class ThreadData
{
public:
	ThreadData()
		: thread_(nullptr), running_(false), dispatcher_(nullptr)
	{
	}

	static ThreadData *current();

private:
	friend class Thread;
	friend class ThreadMain;

	Thread *thread_;
	bool running_;
	pid_t tid_;

	Mutex mutex_;

	std::atomic<EventDispatcher *> dispatcher_;

	std::condition_variable cv_;
	std::atomic<bool> exit_;
	int exitCode_;

	MessageQueue messages_;
};

/**
 * \brief Thread wrapper for the main thread
 */
class ThreadMain : public Thread
{
public:
	ThreadMain()
	{
		data_->running_ = true;
	}

protected:
	void run() override
	{
		LOG(Thread, Fatal) << "The main thread can't be restarted";
	}
};

static thread_local ThreadData *currentThreadData = nullptr;
static ThreadMain mainThread;

/**
 * \brief Retrieve thread-local internal data for the current thread
 * \return The thread-local internal data for the current thread
 */
ThreadData *ThreadData::current()
{
	if (currentThreadData)
		return currentThreadData;

	/*
	 * The main thread doesn't receive thread-local data when it is
	 * started, set it here.
	 */
	ThreadData *data = mainThread.data_;
	data->tid_ = syscall(SYS_gettid);
	currentThreadData = data;
	return data;
}

/**
 * \typedef Mutex
 * \brief An alias for std::mutex
 */

/**
 * \typedef MutexLocker
 * \brief An alias for std::unique_lock<std::mutex>
 */

/**
 * \class Thread
 * \brief A thread of execution
 *
 * The Thread class is a wrapper around std::thread that handles integration
 * with the Object, Signal and EventDispatcher classes.
 *
 * Thread instances by default run an event loop until the exit() method is
 * called. A custom event dispatcher may be installed with
 * setEventDispatcher(), otherwise a poll-based event dispatcher is used. This
 * behaviour can be overriden by overloading the run() method.
 *
 * \context This class is \threadsafe.
 */

/**
 * \brief Create a thread
 */
Thread::Thread()
{
	data_ = new ThreadData;
	data_->thread_ = this;
}

Thread::~Thread()
{
	delete data_->dispatcher_.load(std::memory_order_relaxed);
	delete data_;
}

/**
 * \brief Start the thread
 */
void Thread::start()
{
	MutexLocker locker(data_->mutex_);

	if (data_->running_)
		return;

	data_->running_ = true;
	data_->exitCode_ = -1;
	data_->exit_.store(false, std::memory_order_relaxed);

	thread_ = std::thread(&Thread::startThread, this);
}

void Thread::startThread()
{
	struct ThreadCleaner {
		ThreadCleaner(Thread *thread, void (Thread::*cleaner)())
			: thread_(thread), cleaner_(cleaner)
		{
		}
		~ThreadCleaner()
		{
			(thread_->*cleaner_)();
		}

		Thread *thread_;
		void (Thread::*cleaner_)();
	};

	/*
	 * Make sure the thread is cleaned up even if the run method exits
	 * abnormally (for instance via a direct call to pthread_cancel()).
	 */
	thread_local ThreadCleaner cleaner(this, &Thread::finishThread);

	data_->tid_ = syscall(SYS_gettid);
	currentThreadData = data_;

	run();
}

/**
 * \brief Enter the event loop
 *
 * This method enter an event loop based on the event dispatcher instance for
 * the thread, and blocks until the exit() method is called. It is meant to be
 * called within the thread from the run() method and shall not be called
 * outside of the thread.
 *
 * \return The exit code passed to the exit() method
 */
int Thread::exec()
{
	MutexLocker locker(data_->mutex_);

	EventDispatcher *dispatcher = eventDispatcher();

	locker.unlock();

	while (!data_->exit_.load(std::memory_order_acquire))
		dispatcher->processEvents();

	locker.lock();

	return data_->exitCode_;
}

/**
 * \brief Main method of the thread
 *
 * When the thread is started with start(), it calls this method in the context
 * of the new thread. The run() method can be overloaded to perform custom
 * work. When this method returns the thread execution is stopped, and the \ref
 * finished signal is emitted.
 *
 * The base implementation just calls exec().
 */
void Thread::run()
{
	exec();
}

void Thread::finishThread()
{
	data_->mutex_.lock();
	data_->running_ = false;
	data_->mutex_.unlock();

	finished.emit(this);
	data_->cv_.notify_all();
}

/**
 * \brief Stop the thread's event loop
 * \param[in] code The exit code
 *
 * This method interrupts the event loop started by the exec() method, causing
 * exec() to return \a code.
 *
 * Calling exit() on a thread that reimplements the run() method and doesn't
 * call exec() will likely have no effect.
 */
void Thread::exit(int code)
{
	data_->exitCode_ = code;
	data_->exit_.store(true, std::memory_order_release);

	EventDispatcher *dispatcher = data_->dispatcher_.load(std::memory_order_relaxed);
	if (!dispatcher)
		return;

	dispatcher->interrupt();
}

/**
 * \brief Wait for the thread to finish
 * \param[in] duration Maximum wait duration
 *
 * This function waits until the thread finishes or the \a duration has
 * elapsed, whichever happens first. If \a duration is equal to
 * utils::duration::max(), the wait never times out. If the thread is not
 * running the function returns immediately.
 *
 * \return True if the thread has finished, or false if the wait timed out
 */
bool Thread::wait(utils::duration duration)
{
	bool finished = true;

	{
		MutexLocker locker(data_->mutex_);

		if (duration == utils::duration::max())
			data_->cv_.wait(locker, [&]() { return !data_->running_; });
		else
			finished = data_->cv_.wait_for(locker, duration,
						       [&]() { return !data_->running_; });
	}

	if (thread_.joinable())
		thread_.join();

	return finished;
}

/**
 * \brief Check if the thread is running
 *
 * A Thread instance is considered as running once the underlying thread has
 * started. This method guarantees that it returns true after the start()