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 are 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 on the kernel for SoC audio, however it has some limitations:-
- Currently, codec drivers is often tightly coupled to the underlying SoC CPU. This is not a ideal and leads to code duplication i.e. Linux now have 4 different wm8731 drivers for 4 different SoC platform S.
- There is no standard method to signal user initiated audio events. e.g. headphone/mic insertion, headphone/mic detection after an insertion event. These is quite common events on portable devices and ofter require machine specific code to re route audio, enable amps E TC after such an event.
- Current drivers tend to power up the entire codec when playing (or recording) audio. This was 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 is subsequently matched and co nfigured when the application HW params is 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 a embedded audio system into 3 components:-
- Codec driver:the CODEC Driver is platform independent and contains audio controls, audio interface capabilities, CODEC DP M 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 T Hat platform.
- Machine Driver:the Machine driver handles no machine specific controls and audio events. i.e. Turing on amp at start of playback.
ASOC is still very much work in progress and currently the only supported platform are the PXA2XX. Support for other platforms would follow soon with the IMX and au1x00 SoC ' s (as I had 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:-
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 is in progress ....
The current ASOC snapshot (version 0.11.6) is released on 15th September 2006 and can is found at http://www.rpsys.net/op enzaurus/patches/alsa/
[Quote] ALSA SoC Layer
