libcamera: buffer: Create a MappedBuffer

Provide a MappedFrameBuffer helper class which will map
all of the Planes within a FrameBuffer and provide CPU addressable
pointers for those planes.

The MappedFrameBuffer implements the interface of the MappedBuffer
allowing other buffer types to be constructed of the same form, with a
common interface and cleanup.

This allows MappedBuffer instances to be created from Camera3Buffer types.

Mappings are removed upon destruction.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

include/libcamera/internal/buffer.h

@@ -0,0 +1,47 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2020, Google Inc.
+ *
+ * buffer.h - Internal buffer handling
+ */
+#ifndef __LIBCAMERA_INTERNAL_BUFFER_H__
+#define __LIBCAMERA_INTERNAL_BUFFER_H__
+
+#include <sys/mman.h>
+#include <vector>
+
+#include <libcamera/buffer.h>
+#include <libcamera/span.h>
+
+namespace libcamera {
+
+class MappedBuffer
+{
+public:
+	using Plane = Span<uint8_t>;
+
+	~MappedBuffer();
+
+	MappedBuffer(MappedBuffer &&other);
+	MappedBuffer &operator=(MappedBuffer &&other);
+
+	bool isValid() const { return error_ == 0; }
+	int error() const { return error_; }
+	const std::vector<Plane> &maps() const { return maps_; }
+
+protected:
+	MappedBuffer();
+
+	int error_;
+	std::vector<Plane> maps_;
+};
+
+class MappedFrameBuffer : public MappedBuffer
+{
+public:
+	MappedFrameBuffer(const FrameBuffer *buffer, int flags);
+};
+
+} /* namespace libcamera */
+
+#endif /* __LIBCAMERA_INTERNAL_BUFFER_H__ */

src/libcamera/buffer.cpp

@@ -6,6 +6,7 @@
  */
 
 #include <libcamera/buffer.h>
+#include "libcamera/internal/buffer.h"
 
 #include <errno.h>
 #include <string.h>
@@ -15,8 +16,11 @@
 #include "libcamera/internal/log.h"
 
 /**
- * \file buffer.h
+ * \file libcamera/buffer.h
  * \brief Buffer handling
+ *
+ * \file libcamera/internal/buffer.h
+ * \brief Internal buffer handling support
  */
 
 namespace libcamera {
@@ -290,4 +294,149 @@ int FrameBuffer::copyFrom(const FrameBuffer *src)
 	return 0;
 }
 
+/**
+ * \class MappedBuffer
+ * \brief Provide an interface to support managing memory mapped buffers
+ *
+ * The MappedBuffer interface provides access to a set of MappedPlanes which
+ * are available for access by the CPU.
+ *
+ * This class is not meant to be constructed directly, but instead derived
+ * classes should be used to implement the correct mapping of a source buffer.
+ *
+ * This allows treating CPU accessible memory through a generic interface
+ * regardless of whether it originates from a libcamera FrameBuffer or other
+ * source.
+ */
+
+/**
+ * \typedef MappedBuffer::Plane
+ * \brief A mapped region of memory accessible to the CPU
+ *
+ * The MappedBuffer::Plane uses the Span interface to describe the mapped memory
+ * region.
+ */
+
+/**
+ * \brief Construct an empty MappedBuffer
+ */
+MappedBuffer::MappedBuffer()
+	: error_(0)
+{
+}
+
+/**
+ * \brief Move constructor, construct the MappedBuffer with the contents of \a
+ * other using move semantics
+ * \param[in] other The other MappedBuffer
+ *
+ * Moving a MappedBuffer moves the mappings contained in the \a other to the new
+ * MappedBuffer and invalidates the \a other.
+ *
+ * No mappings are unmapped or destroyed in this process.
+ */
+MappedBuffer::MappedBuffer(MappedBuffer &&other)
+{
+	*this = std::move(other);
+}
+
+/**
+ * \brief Move assignment operator, replace the mappings with those of \a other
+* \param[in] other The other MappedBuffer
+ *
+ * Moving a MappedBuffer moves the mappings contained in the \a other to the new
+ * MappedBuffer and invalidates the \a other.
+ *
+ * No mappings are unmapped or destroyed in this process.
+ */
+MappedBuffer &MappedBuffer::operator=(MappedBuffer &&other)
+{
+	error_ = other.error_;
+	maps_ = std::move(other.maps_);
+	other.error_ = -ENOENT;
+
+	return *this;
+}
+
+MappedBuffer::~MappedBuffer()
+{
+	for (Plane &map : maps_)
+		munmap(map.data(), map.size());
+}
+
+/**
+ * \fn MappedBuffer::isValid()
+ * \brief Check if the MappedBuffer instance is valid
+ * \return True if the MappedBuffer has valid mappings, false otherwise
+ */
+
+/**
+ * \fn MappedBuffer::error()
+ * \brief Retrieve the map error status
+ *
+ * This function retrieves the error status from the MappedBuffer.
+ * The error status is a negative number as defined by errno.h. If
+ * no error occurred, this function returns 0.
+ *
+ * \return The map error code
+ */
+
+/**
+ * \fn MappedBuffer::maps()
+ * \brief Retrieve the mapped planes
+ *
+ * This function retrieves the successfully mapped planes stored as a vector
+ * of Span<uint8_t> to provide access to the mapped memory.
+ *
+ * \return A vector of the mapped planes
+ */
+
+/**
+ * \var MappedBuffer::error_
+ * \brief Stores the error value if present
+ *
+ * MappedBuffer derived classes shall set this to a negative value as defined
+ * by errno.h if an error occured during the mapping process.
+ */
+
+/**
+ * \var MappedBuffer::maps_
+ * \brief Stores the internal
+ *
+ * MappedBuffer derived classes shall store the mappings they create in this
+ * vector which is parsed during destruct to unmap any memory mappings which
+ * completed successfully.
+ */
+
+/**
+ * \class MappedFrameBuffer
+ * \brief Map a FrameBuffer using the MappedBuffer interface
+ */
+
+/**
+ * \brief Map all planes of a FrameBuffer
+ * \param[in] buffer FrameBuffer to be mapped
+ * \param[in] flags Protection flags to apply to map
+ *
+ * Construct an object to map a frame buffer for CPU access.
+ * The flags are passed directly to mmap and should be either PROT_READ,
+ * PROT_WRITE, or a bitwise-or combination of both.
+ */
+MappedFrameBuffer::MappedFrameBuffer(const FrameBuffer *buffer, int flags)
+{
+	maps_.reserve(buffer->planes().size());
+
+	for (const FrameBuffer::Plane &plane : buffer->planes()) {
+		void *address = mmap(nullptr, plane.length, flags,
+				     MAP_SHARED, plane.fd.fd(), 0);
+		if (address == MAP_FAILED) {
+			error_ = -errno;
+			LOG(Buffer, Error) << "Failed to mmap plane";
+			break;
+		}
+
+		maps_.emplace_back(static_cast<uint8_t *>(address), plane.length);
+	}
+}
+
 } /* namespace libcamera */