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
- 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
- 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.
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
- 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:-
- WM9713, WM9714
- WM9712, WM9711
- AC97 codecs (supported via ac97_codec.c)
- Example Baseband modem <--> WM9713 driver
- Example Bluetooth Codec <--> WM8753 driver
- 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