libcamera: fence: Introduce Fence

Introduce a Fence class which models a synchronization primitive that
allows to notify the availability of a resource.

The Fence is modeled as a wrapper of a UniqueFD instance where
read events are used to signal the Fence. The class can be later
extended to support additional signalling mechanisms.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

include/libcamera/fence.h

@@ -0,0 +1,31 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2021, Google Inc.
+ *
+ * internal/fence.h - Synchronization fence
+ */
+
+#pragma once
+
+#include <libcamera/base/class.h>
+#include <libcamera/base/unique_fd.h>
+
+namespace libcamera {
+
+class Fence
+{
+public:
+	Fence(UniqueFD fd);
+
+	bool isValid() const { return fd_.isValid(); }
+	const UniqueFD &fd() const { return fd_; }
+
+	UniqueFD release() { return std::move(fd_); }
+
+private:
+	LIBCAMERA_DISABLE_COPY_AND_MOVE(Fence)
+
+	UniqueFD fd_;
+};
+
+} /* namespace libcamera */

include/libcamera/meson.build

@@ -6,6 +6,7 @@ libcamera_public_headers = files([
     'camera.h',
     'camera_manager.h',
     'controls.h',
+    'fence.h',
     'framebuffer.h',
     'framebuffer_allocator.h',
     'geometry.h',

src/libcamera/fence.cpp

@@ -0,0 +1,112 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2021, Google Inc.
+ *
+ * fence.cpp - Synchronization fence
+ */
+
+#include "libcamera/fence.h"
+
+namespace libcamera {
+
+/**
+ *
+ * \file libcamera/fence.h
+ * \brief Definition of the Fence class
+ */
+
+/**
+ * \class Fence
+ * \brief Synchronization primitive to manage resources
+ *
+ * The Fence class models a synchronization primitive that can be used by
+ * applications to explicitly synchronize resource usage, and can be shared by
+ * multiple processes.
+ *
+ * Fences are most commonly used in association with frame buffers. A
+ * FrameBuffer can be associated with a Fence so that the library can wait for
+ * the Fence to be signalled before allowing the camera device to actually
+ * access the memory area described by the FrameBuffer.
+ *
+ * \sa Request::addBuffer()
+ *
+ * By using a fence, applications can then synchronize between frame buffer
+ * consumers and producers, as for example a display device and a camera, to
+ * guarantee that a new data transfers only happen once the existing frames have
+ * been displayed.
+ *
+ * A Fence can be realized by different event notification primitives, the most
+ * common of which is represented by waiting for read events to happen on a
+ * <a href="https://www.kernel.org/doc/html/latest/driver-api/sync_file.html">kernel sync file.</a>
+ * This is currently the only mechanism supported by libcamera, but others can
+ * be implemented by extending or subclassing this class and implementing
+ * opportune handling in the core library.
+ *
+ * \internal
+ *
+ * The Fence class is a thin abstraction around a UniqueFD which simply allows
+ * to access it as a const reference or to move its ownership to the caller.
+ *
+ * The usage of the Fence class allows to abstract the underlying
+ * synchronization mechanism in use and implement an interface towards other
+ * library components that will not change when new synchronization primitives
+ * will be added as fences.
+ *
+ * A Fence is constructed with a UniqueFD whose ownership is moved in the Fence.
+ * A FrameBuffer can be associated with a Fence by passing it to the
+ * Request::addBuffer() function, which will move the Fence into the FrameBuffer
+ * itself. Once a Request is queued to the Camera, a preparation phase
+ * guarantees that before actually applying the Request to the hardware, all the
+ * valid fences of the frame buffers in a Request are correctly signalled. Once
+ * a Fence has completed, the library will release the FrameBuffer fence so that
+ * application won't be allowed to access it.
+ *
+ * An optional timeout can be started while waiting for a fence to complete. If
+ * waiting on a Fence fails for whatever reason, the FrameBuffer's fence is not
+ * reset and is made available to application for them to handle it, by
+ * releasing the Fence to correctly close the underlying UniqueFD.
+ *
+ * A failure in waiting for a Fence to complete will result in the Request to
+ * complete in failed state.
+ *
+ * \sa Request::prepare()
+ * \sa PipelineHandler::doQueueRequests()
+ */
+
+/**
+ * \brief Create a Fence
+ * \param[in] fd The fence file descriptor
+ *
+ * The file descriptor ownership is moved to the Fence.
+ */
+Fence::Fence(UniqueFD fd)
+	: fd_(std::move(fd))
+{
+}
+
+/**
+ * \fn Fence::isValid()
+ * \brief Check if a Fence is valid
+ *
+ * A Fence is valid if the file descriptor it wraps is valid.
+ *
+ * \return True if the Fence is valid, false otherwise
+ */
+
+/**
+ * \fn Fence::fd()
+ * \brief Retrieve a constant reference to the file descriptor
+ * \return A const reference to the fence file descriptor
+ */
+
+/**
+ * \fn Fence::release()
+ * \brief Release the ownership of the file descriptor
+ *
+ * Release the ownership of the wrapped file descriptor by returning it to the
+ * caller.
+ *
+ * \return The wrapper UniqueFD
+ */
+
+} /* namespace libcamera */

src/libcamera/meson.build

@@ -15,6 +15,7 @@ libcamera_sources = files([
     'delayed_controls.cpp',
     'device_enumerator.cpp',
     'device_enumerator_sysfs.cpp',
+    'fence.cpp',
     'formats.cpp',
     'framebuffer.cpp',
     'framebuffer_allocator.cpp',