[libcamera-devel] [PATCH v5 03/11] libcamera: fence: Introduce Fence

[Laurent Pinchart]

Hi Jacopo,

Thank you for the patch.

On Fri, Dec 10, 2021 at 09:52:31PM +0100, Jacopo Mondi wrote:
> 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 at jmondi.org>
> Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> ---
>  include/libcamera/fence.h     |  31 ++++++++++
>  include/libcamera/meson.build |   1 +
>  src/libcamera/fence.cpp       | 112 ++++++++++++++++++++++++++++++++++
>  src/libcamera/meson.build     |   1 +
>  4 files changed, 145 insertions(+)
>  create mode 100644 include/libcamera/fence.h
>  create mode 100644 src/libcamera/fence.cpp
> 
> diff --git a/include/libcamera/fence.h b/include/libcamera/fence.h
> new file mode 100644
> index 000000000000..c0c916c264b4
> --- /dev/null
> +++ b/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 */
> diff --git a/include/libcamera/meson.build b/include/libcamera/meson.build
> index 5f42977c034b..8c2cae00d877 100644
> --- a/include/libcamera/meson.build
> +++ b/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',
> diff --git a/src/libcamera/fence.cpp b/src/libcamera/fence.cpp
> new file mode 100644
> index 000000000000..3fb0e9c5281a
> --- /dev/null
> +++ b/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>

Missing . at the end of the line.

> + * 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 */
> diff --git a/src/libcamera/meson.build b/src/libcamera/meson.build
> index 2e54cc04edc7..dc1cc52a427a 100644
> --- a/src/libcamera/meson.build
> +++ b/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',

-- 
Regards,

Laurent Pinchart