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