libcamera: stream: Map external buffers to indexes

Add and use an operation to assign to Buffer representing external
memory locations an index at queueRequest() time. The index is used to
identify the memory buffer to be queued to the video device once the
buffer will be queued in a Request.

In order to minimize relocations in the V4L2 backend, this method
provides a best-effort caching mechanisms that attempts to reuse
BufferMemory previously mapped to the buffer's dmabuf file descriptors,
if any.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

2 files changed, 8 insertions, 0 deletions

diff --git a/include/libcamera/buffer.h b/include/libcamera/buffer.h
index fc5c7d4c..7b657509 100644
--- a/include/libcamera/buffer.h
+++ b/include/libcamera/buffer.h
@@ -30,6 +30,8 @@ public:
 	unsigned int length() const { return length_; }
 
 private:
+	friend class Stream;
+
 	int mmap();
 	int munmap();
 
diff --git a/include/libcamera/stream.h b/include/libcamera/stream.h
index 1883d9e9..2e619cdf 100644
--- a/include/libcamera/stream.h
+++ b/include/libcamera/stream.h
@@ -85,12 +85,18 @@ public:
 protected:
 	friend class Camera;
 
+	int mapBuffer(const Buffer *buffer);
+	void unmapBuffer(const Buffer *buffer);
+
 	void createBuffers(MemoryType memory, unsigned int count);
 	void destroyBuffers();
 
 	BufferPool bufferPool_;
 	StreamConfiguration configuration_;
 	MemoryType memoryType_;
+
+private:
+	std::vector<std::pair<std::array<int, 3>, unsigned int>> bufferCache_;
 };
 
 } /* namespace libcamera */