Based on kernel version 3.9. Page generated on 2013-05-02 23:14 EST.
1 ALSA SoC Layer 2 ============== 3 4 The overall project goal of the ALSA System on Chip (ASoC) layer is to 5 provide better ALSA support for embedded system-on-chip processors (e.g. 6 pxa2xx, au1x00, iMX, etc) and portable audio codecs. Prior to the ASoC 7 subsystem there was some support in the kernel for SoC audio, however it 8 had some limitations:- 9 10 * Codec drivers were often tightly coupled to the underlying SoC 11 CPU. This is not ideal and leads to code duplication - for example, 12 Linux had different wm8731 drivers for 4 different SoC platforms. 13 14 * There was no standard method to signal user initiated audio events (e.g. 15 Headphone/Mic insertion, Headphone/Mic detection after an insertion 16 event). These are quite common events on portable devices and often require 17 machine specific code to re-route audio, enable amps, etc., after such an 18 event. 19 20 * Drivers tended to power up the entire codec when playing (or 21 recording) audio. This is fine for a PC, but tends to waste a lot of 22 power on portable devices. There was also no support for saving 23 power via changing codec oversampling rates, bias currents, etc. 24 25 26 ASoC Design 27 =========== 28 29 The ASoC layer is designed to address these issues and provide the following 30 features :- 31 32 * Codec independence. Allows reuse of codec drivers on other platforms 33 and machines. 34 35 * Easy I2S/PCM audio interface setup between codec and SoC. Each SoC 36 interface and codec registers its audio interface capabilities with the 37 core and are subsequently matched and configured when the application 38 hardware parameters are known. 39 40 * Dynamic Audio Power Management (DAPM). DAPM automatically sets the codec to 41 its minimum power state at all times. This includes powering up/down 42 internal power blocks depending on the internal codec audio routing and any 43 active streams. 44 45 * Pop and click reduction. Pops and clicks can be reduced by powering the 46 codec up/down in the correct sequence (including using digital mute). ASoC 47 signals the codec when to change power states. 48 49 * Machine specific controls: Allow machines to add controls to the sound card 50 (e.g. volume control for speaker amplifier). 51 52 To achieve all this, ASoC basically splits an embedded audio system into 3 53 components :- 54 55 * Codec driver: The codec driver is platform independent and contains audio 56 controls, audio interface capabilities, codec DAPM definition and codec IO 57 functions. 58 59 * Platform driver: The platform driver contains the audio DMA engine and audio 60 interface drivers (e.g. I2S, AC97, PCM) for that platform. 61 62 * Machine driver: The machine driver handles any machine specific controls and 63 audio events (e.g. turning on an amp at start of playback). 64 65 66 Documentation 67 ============= 68 69 The documentation is spilt into the following sections:- 70 71 overview.txt: This file. 72 73 codec.txt: Codec driver internals. 74 75 DAI.txt: Description of Digital Audio Interface standards and how to configure 76 a DAI within your codec and CPU DAI drivers. 77 78 dapm.txt: Dynamic Audio Power Management 79 80 platform.txt: Platform audio DMA and DAI. 81 82 machine.txt: Machine driver internals. 83 84 pop_clicks.txt: How to minimise audio artifacts. 85 86 clocking.txt: ASoC clocking for best power performance.