libcamera: buffer: Add dmabuf file descriptors

In addition to referencing buffer memory by index, add support to
referencing it using dmabuf file descriptors. This will be used to
reference buffer memory allocated outside of libcamera and import it.

The dmabuf file descriptors are stored in an array in the Buffer class,
and a new Stream::createBuffer() overload is added to construct a buffer
from dmabuf file descriptor.

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>

src/libcamera/buffer.cpp

@@ -269,7 +269,8 @@ void BufferPool::destroyBuffers()
  * for a stream with Stream::createBuffer().
  */
 Buffer::Buffer(unsigned int index, const Buffer *metadata)
-	: index_(index), status_(Buffer::BufferSuccess), request_(nullptr),
+	: index_(index), dmabuf_({ -1, -1, -1 }),
+	  status_(Buffer::BufferSuccess), request_(nullptr),
 	  stream_(nullptr)
 {
 	if (metadata) {
@@ -289,6 +290,16 @@ Buffer::Buffer(unsigned int index, const Buffer *metadata)
  * \return The buffer index
  */
 
+/**
+ * \fn Buffer::dmabufs()
+ * \brief Retrieve the dmabuf file descriptors for all buffer planes
+ *
+ * The dmabufs array contains one dmabuf file descriptor per plane. Unused
+ * entries are set to -1.
+ *
+ * \return The dmabuf file descriptors
+ */
+
 /**
  * \fn Buffer::bytesused()
  * \brief Retrieve the number of bytes occupied by the data in the buffer