From http://www.rpsys.net/openzaurus/patches/alsa/info.html

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. Currently there is some support in the kernel for SoC audio, however it has some limitations:-

• Currently, codec drivers are often tightly coupled to the underlying SoC cpu. This is not ideal and leads to code duplication i.e. Linux now has 4 different wm8731 drivers for 4 different SoC platforms.
   
• There is 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 ofter require machine specific code to re route audio, enable amps etc after such an event.
   
• Current drivers tend 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 is 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 it‘s audio interface capabilities with the core and are subsequently matched and configured when the application hw params are known.
   
• Dynamic Audio Power Management (DAPM). DAPM automatically sets the codec to it‘s 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 amp.

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 dpm 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. i.e. turing on an amp at start of playback.

ASoC is still very much work in progress and currently the only supported platform is the pxa2xx. Support for other platforms will follow soon with the iMX and au1x00 SoC‘s (as I have both boards). Atm, codec drivers exist for:-

• AK4535
• UCB1380
• WM8753
• WM8731
• WM8750
• WM8971
• WM8974
• WM9713, WM9714
• WM9712, WM9711
• AC97 codecs (supported via ac97_codec.c)

In progress:-

• WM8772

Also included:-

• Example Baseband modem <--> WM9713 driver
• Example Bluetooth Codec <--> WM8753 driver

Supported machines:-

• Mainstone II  (AC97, I2S and PCM codecs)
• Zaurus SL-C7x0: Corgi, Shepherd, Husky, Boxer
• Zaurus SL-Cxx00: Akita, Spitz, Borzoi, Terrier
• Zaurus SL-6000x: Tosa
• Zaurus SL-5600: Poodle
• Others are in progress.......

The current ASoC snapshot (version 0.11.6) was released on 15th September 2006 and can be found at http://www.rpsys.net/openzaurus/patches/alsa/

[Quote] ALSA SoC Layer