1ALSA SoC Layer
2==============
3
4The overall project goal of the ALSA System on Chip (ASoC) layer is to
5provide better ALSA support for embedded system-on-chip processors (e.g.
6pxa2xx, au1x00, iMX, etc) and portable audio codecs.  Prior to the ASoC
7subsystem there was some support in the kernel for SoC audio, however it
8had some limitations:-
9
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.
13
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.
19
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.
24
25
26ASoC Design
27===========
28
29The ASoC layer is designed to address these issues and provide the following
30features :-
31
32  * Codec independence. Allows reuse of codec drivers on other platforms
33    and machines.
34
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.
39
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.
44
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.
48
49  * Machine specific controls: Allow machines to add controls to the sound card
50    (e.g. volume control for speaker amplifier).
51
52To achieve all this, ASoC basically splits an embedded audio system into 3
53components :-
54
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.
58
59  * Platform driver: The platform driver contains the audio DMA engine and audio
60    interface drivers (e.g. I2S, AC97, PCM) for that platform.
61
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).
64
65
66Documentation
67=============
68
69The documentation is spilt into the following sections:-
70
71overview.txt: This file.
72
73codec.txt: Codec driver internals.
74
75DAI.txt: Description of Digital Audio Interface standards and how to configure
76a DAI within your codec and CPU DAI drivers.
77
78dapm.txt: Dynamic Audio Power Management
79
80platform.txt: Platform audio DMA and DAI.
81
82machine.txt: Machine driver internals.
83
84pop_clicks.txt: How to minimise audio artifacts.
85
86clocking.txt: ASoC clocking for best power performance.
87