0001 =======================
0002 ALSA SoC Layer Overview
0003 =======================
0004
0005 The overall project goal of the ALSA System on Chip (ASoC) layer is to
0006 provide better ALSA support for embedded system-on-chip processors (e.g.
0007 pxa2xx, au1x00, iMX, etc) and portable audio codecs. Prior to the ASoC
0008 subsystem there was some support in the kernel for SoC audio, however it
0009 had some limitations:-
0010
0011 * Codec drivers were often tightly coupled to the underlying SoC
0012 CPU. This is not ideal and leads to code duplication - for example,
0013 Linux had different wm8731 drivers for 4 different SoC platforms.
0014
0015 * There was no standard method to signal user initiated audio events (e.g.
0016 Headphone/Mic insertion, Headphone/Mic detection after an insertion
0017 event). These are quite common events on portable devices and often require
0018 machine specific code to re-route audio, enable amps, etc., after such an
0019 event.
0020
0021 * Drivers tended to power up the entire codec when playing (or
0022 recording) audio. This is fine for a PC, but tends to waste a lot of
0023 power on portable devices. There was also no support for saving
0024 power via changing codec oversampling rates, bias currents, etc.
0025
0026
0027 ASoC Design
0028 ===========
0029
0030 The ASoC layer is designed to address these issues and provide the following
0031 features :-
0032
0033 * Codec independence. Allows reuse of codec drivers on other platforms
0034 and machines.
0035
0036 * Easy I2S/PCM audio interface setup between codec and SoC. Each SoC
0037 interface and codec registers its audio interface capabilities with the
0038 core and are subsequently matched and configured when the application
0039 hardware parameters are known.
0040
0041 * Dynamic Audio Power Management (DAPM). DAPM automatically sets the codec to
0042 its minimum power state at all times. This includes powering up/down
0043 internal power blocks depending on the internal codec audio routing and any
0044 active streams.
0045
0046 * Pop and click reduction. Pops and clicks can be reduced by powering the
0047 codec up/down in the correct sequence (including using digital mute). ASoC
0048 signals the codec when to change power states.
0049
0050 * Machine specific controls: Allow machines to add controls to the sound card
0051 (e.g. volume control for speaker amplifier).
0052
0053 To achieve all this, ASoC basically splits an embedded audio system into
0054 multiple re-usable component drivers :-
0055
0056 * Codec class drivers: The codec class driver is platform independent and
0057 contains audio controls, audio interface capabilities, codec DAPM
0058 definition and codec IO functions. This class extends to BT, FM and MODEM
0059 ICs if required. Codec class drivers should be generic code that can run
0060 on any architecture and machine.
0061
0062 * Platform class drivers: The platform class driver includes the audio DMA
0063 engine driver, digital audio interface (DAI) drivers (e.g. I2S, AC97, PCM)
0064 and any audio DSP drivers for that platform.
0065
0066 * Machine class driver: The machine driver class acts as the glue that
0067 describes and binds the other component drivers together to form an ALSA
0068 "sound card device". It handles any machine specific controls and
0069 machine level audio events (e.g. turning on an amp at start of playback).
