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Documentation / sound / alsa / soc / overview.txt

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Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.

1	ALSA SoC Layer
2	==============
4	The overall project goal of the ALSA System on Chip (ASoC) layer is to
5	provide better ALSA support for embedded system-on-chip processors (e.g.
6	pxa2xx, au1x00, iMX, etc) and portable audio codecs.  Prior to the ASoC
7	subsystem there was some support in the kernel for SoC audio, however it
8	had some limitations:-
10	  * Codec drivers were often tightly coupled to the underlying SoC
11	    CPU. This is not ideal and leads to code duplication - for example,
12	    Linux had different wm8731 drivers for 4 different SoC platforms.
14	  * There was no standard method to signal user initiated audio events (e.g.
15	    Headphone/Mic insertion, Headphone/Mic detection after an insertion
16	    event). These are quite common events on portable devices and often require
17	    machine specific code to re-route audio, enable amps, etc., after such an
18	    event.
20	  * Drivers tended to power up the entire codec when playing (or
21	    recording) audio. This is fine for a PC, but tends to waste a lot of
22	    power on portable devices. There was also no support for saving
23	    power via changing codec oversampling rates, bias currents, etc.
26	ASoC Design
27	===========
29	The ASoC layer is designed to address these issues and provide the following
30	features :-
32	  * Codec independence. Allows reuse of codec drivers on other platforms
33	    and machines.
35	  * Easy I2S/PCM audio interface setup between codec and SoC. Each SoC
36	    interface and codec registers its audio interface capabilities with the
37	    core and are subsequently matched and configured when the application
38	    hardware parameters are known.
40	  * Dynamic Audio Power Management (DAPM). DAPM automatically sets the codec to
41	    its minimum power state at all times. This includes powering up/down
42	    internal power blocks depending on the internal codec audio routing and any
43	    active streams.
45	  * Pop and click reduction. Pops and clicks can be reduced by powering the
46	    codec up/down in the correct sequence (including using digital mute). ASoC
47	    signals the codec when to change power states.
49	  * Machine specific controls: Allow machines to add controls to the sound card
50	    (e.g. volume control for speaker amplifier).
52	To achieve all this, ASoC basically splits an embedded audio system into 3
53	components :-
55	  * Codec driver: The codec driver is platform independent and contains audio
56	    controls, audio interface capabilities, codec DAPM definition and codec IO
57	    functions.
59	  * Platform driver: The platform driver contains the audio DMA engine and audio
60	    interface drivers (e.g. I2S, AC97, PCM) for that platform.
62	  * Machine driver: The machine driver handles any machine specific controls and
63	    audio events (e.g. turning on an amp at start of playback).
66	Documentation
67	=============
69	The documentation is spilt into the following sections:-
71	overview.txt: This file.
73	codec.txt: Codec driver internals.
75	DAI.txt: Description of Digital Audio Interface standards and how to configure
76	a DAI within your codec and CPU DAI drivers.
78	dapm.txt: Dynamic Audio Power Management
80	platform.txt: Platform audio DMA and DAI.
82	machine.txt: Machine driver internals.
84	pop_clicks.txt: How to minimise audio artifacts.
86	clocking.txt: ASoC clocking for best power performance.
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