1 .. SPDX-License-Identifier: GFDL-1.1-no-invari 1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2 2
3 .. _mem2mem: 3 .. _mem2mem:
4 4
5 ******************************** 5 ********************************
6 Video Memory-To-Memory Interface 6 Video Memory-To-Memory Interface
7 ******************************** 7 ********************************
8 8
9 A V4L2 memory-to-memory device can compress, d 9 A V4L2 memory-to-memory device can compress, decompress, transform, or
10 otherwise convert video data from one format i 10 otherwise convert video data from one format into another format, in memory.
11 Such memory-to-memory devices set the ``V4L2_C 11 Such memory-to-memory devices set the ``V4L2_CAP_VIDEO_M2M`` or
12 ``V4L2_CAP_VIDEO_M2M_MPLANE`` capability. Exam 12 ``V4L2_CAP_VIDEO_M2M_MPLANE`` capability. Examples of memory-to-memory
13 devices are codecs, scalers, deinterlacers or 13 devices are codecs, scalers, deinterlacers or format converters (i.e.
14 converting from YUV to RGB). 14 converting from YUV to RGB).
15 15
16 A memory-to-memory video node acts just like a 16 A memory-to-memory video node acts just like a normal video node, but it
17 supports both output (sending frames from memo 17 supports both output (sending frames from memory to the hardware)
18 and capture (receiving the processed frames fr 18 and capture (receiving the processed frames from the hardware into
19 memory) stream I/O. An application will have t 19 memory) stream I/O. An application will have to setup the stream I/O for
20 both sides and finally call :ref:`VIDIOC_STREA 20 both sides and finally call :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>`
21 for both capture and output to start the hardw 21 for both capture and output to start the hardware.
22 22
23 Memory-to-memory devices function as a shared 23 Memory-to-memory devices function as a shared resource: you can
24 open the video node multiple times, each appli 24 open the video node multiple times, each application setting up their
25 own properties that are local to the file hand 25 own properties that are local to the file handle, and each can use
26 it independently from the others. The driver w 26 it independently from the others. The driver will arbitrate access to
27 the hardware and reprogram it whenever another 27 the hardware and reprogram it whenever another file handler gets access.
28 This is different from the usual video node be 28 This is different from the usual video node behavior where the video
29 properties are global to the device (i.e. chan 29 properties are global to the device (i.e. changing something through one
30 file handle is visible through another file ha 30 file handle is visible through another file handle).
31 31
32 One of the most common memory-to-memory device 32 One of the most common memory-to-memory device is the codec. Codecs
33 are more complicated than most and require add 33 are more complicated than most and require additional setup for
34 their codec parameters. This is done through c 34 their codec parameters. This is done through codec controls.
35 See :ref:`codec-controls`. More details on how 35 See :ref:`codec-controls`. More details on how to use codec memory-to-memory
36 devices are given in the following sections. 36 devices are given in the following sections.
37 37
38 .. toctree:: 38 .. toctree::
39 :maxdepth: 1 39 :maxdepth: 1
40 40
41 dev-decoder 41 dev-decoder
42 dev-encoder 42 dev-encoder
43 dev-stateless-decoder 43 dev-stateless-decoder
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