1======================= 2ALSA SoC Layer Overview 3======================= 4 5The overall project goal of the ALSA System on Chip (ASoC) layer is to 6provide better ALSA support for embedded system-on-chip processors (e.g. 7pxa2xx, au1x00, iMX, etc) and portable audio codecs. Prior to the ASoC 8subsystem there was some support in the kernel for SoC audio, however it 9had some limitations:- 10 11 * Codec drivers were often tightly coupled to the underlying SoC 12 CPU. This is not ideal and leads to code duplication - for example, 13 Linux had different wm8731 drivers for 4 different SoC platforms. 14 15 * There was no standard method to signal user initiated audio events (e.g. 16 Headphone/Mic insertion, Headphone/Mic detection after an insertion 17 event). These are quite common events on portable devices and often require 18 machine specific code to re-route audio, enable amps, etc., after such an 19 event. 20 21 * Drivers tended to power up the entire codec when playing (or 22 recording) audio. This is fine for a PC, but tends to waste a lot of 23 power on portable devices. There was also no support for saving 24 power via changing codec oversampling rates, bias currents, etc. 25 26 27ASoC Design 28=========== 29 30The ASoC layer is designed to address these issues and provide the following 31features :- 32 33 * Codec independence. Allows reuse of codec drivers on other platforms 34 and machines. 35 36 * Easy I2S/PCM audio interface setup between codec and SoC. Each SoC 37 interface and codec registers its audio interface capabilities with the 38 core and are subsequently matched and configured when the application 39 hardware parameters are known. 40 41 * Dynamic Audio Power Management (DAPM). DAPM automatically sets the codec to 42 its minimum power state at all times. This includes powering up/down 43 internal power blocks depending on the internal codec audio routing and any 44 active streams. 45 46 * Pop and click reduction. Pops and clicks can be reduced by powering the 47 codec up/down in the correct sequence (including using digital mute). ASoC 48 signals the codec when to change power states. 49 50 * Machine specific controls: Allow machines to add controls to the sound card 51 (e.g. volume control for speaker amplifier). 52 53To achieve all this, ASoC basically splits an embedded audio system into 54multiple re-usable component drivers :- 55 56 * Codec class drivers: The codec class driver is platform independent and 57 contains audio controls, audio interface capabilities, codec DAPM 58 definition and codec IO functions. This class extends to BT, FM and MODEM 59 ICs if required. Codec class drivers should be generic code that can run 60 on any architecture and machine. 61 62 * Platform class drivers: The platform class driver includes the audio DMA 63 engine driver, digital audio interface (DAI) drivers (e.g. I2S, AC97, PCM) 64 and any audio DSP drivers for that platform. 65 66 * Machine class driver: The machine driver class acts as the glue that 67 describes and binds the other component drivers together to form an ALSA 68 "sound card device". It handles any machine specific controls and 69 machine level audio events (e.g. turning on an amp at start of playback). 70