96 lines
3.7 KiB
Plaintext
96 lines
3.7 KiB
Plaintext
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
|
|
multiple re-usable component drivers :-
|
|
|
|
* Codec class drivers: The codec class driver is platform independent and
|
|
contains audio controls, audio interface capabilities, codec DAPM
|
|
definition and codec IO functions. This class extends to BT, FM and MODEM
|
|
ICs if required. Codec class drivers should be generic code that can run
|
|
on any architecture and machine.
|
|
|
|
* Platform class drivers: The platform class driver includes the audio DMA
|
|
engine driver, digital audio interface (DAI) drivers (e.g. I2S, AC97, PCM)
|
|
and any audio DSP drivers for that platform.
|
|
|
|
* Machine class driver: The machine driver class acts as the glue that
|
|
describes and binds the other component drivers together to form an ALSA
|
|
"sound card device". It handles any machine specific controls and
|
|
machine level audio events (e.g. turning on an amp at start of playback).
|
|
|
|
|
|
Documentation
|
|
=============
|
|
|
|
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.
|
|
|
|
jack.txt: ASoC jack detection.
|
|
|
|
DPCM.txt: Dynamic PCM - Describes DPCM with DSP examples.
|