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+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+
+.. _mem2mem:
+
+********************************
+Video Memory-To-Memory Interface
+********************************
+
+A V4L2 memory-to-memory device can compress, decompress, transform, or
+otherwise convert video data from one format into another format, in memory.
+Such memory-to-memory devices set the ``V4L2_CAP_VIDEO_M2M`` or
+``V4L2_CAP_VIDEO_M2M_MPLANE`` capability. Examples of memory-to-memory
+devices are codecs, scalers, deinterlacers or format converters (i.e.
+converting from YUV to RGB).
+
+A memory-to-memory video node acts just like a normal video node, but it
+supports both output (sending frames from memory to the hardware)
+and capture (receiving the processed frames from the hardware into
+memory) stream I/O. An application will have to setup the stream I/O for
+both sides and finally call :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`
+for both capture and output to start the hardware.
+
+Memory-to-memory devices function as a shared resource: you can
+open the video node multiple times, each application setting up their
+own properties that are local to the file handle, and each can use
+it independently from the others. The driver will arbitrate access to
+the hardware and reprogram it whenever another file handler gets access.
+This is different from the usual video node behavior where the video
+properties are global to the device (i.e. changing something through one
+file handle is visible through another file handle).
+
+One of the most common memory-to-memory device is the codec. Codecs
+are more complicated than most and require additional setup for
+their codec parameters. This is done through codec controls.
+See :ref:`codec-controls`. More details on how to use codec memory-to-memory
+devices are given in the following sections.
+
+.. toctree::
+    :maxdepth: 1
+
+    dev-decoder
+    dev-encoder
+    dev-stateless-decoder