libcamera: buffer: Add an accessor to the BufferMemory

Buffer instances reference memory, which is modelled internally by a
BufferMemory instance. Store a pointer to the BufferMemory in the Buffer
class, and populate it when the buffer is queued to the camera through a
request. This is useful for applications to access the buffer memory in
the buffer or request completion handler.

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

src/libcamera/buffer.cpp

@@ -300,6 +300,17 @@ Buffer::Buffer(unsigned int index, const Buffer *metadata)
  * \return The dmabuf file descriptors
  */
 
+/**
+ * \fn Buffer::mem()
+ * \brief Retrieve the BufferMemory this buffer is associated with
+ *
+ * The association between the buffer and a BufferMemory instance is valid from
+ * the time the request containing this buffer is queued to a camera to the end
+ * of that request's completion handler.
+ *
+ * \return The BufferMemory this buffer is associated with
+ */
+
 /**
  * \fn Buffer::bytesused()
  * \brief Retrieve the number of bytes occupied by the data in the buffer