Based on kernel version 4.2. Page generated on 2015-09-09 12:15 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 53 multiple re-usable component drivers :- 54 55 * Codec class drivers: The codec class driver is platform independent and 56 contains audio controls, audio interface capabilities, codec DAPM 57 definition and codec IO functions. This class extends to BT, FM and MODEM 58 ICs if required. Codec class drivers should be generic code that can run 59 on any architecture and machine. 60 61 * Platform class drivers: The platform class driver includes the audio DMA 62 engine driver, digital audio interface (DAI) drivers (e.g. I2S, AC97, PCM) 63 and any audio DSP drivers for that platform. 64 65 * Machine class driver: The machine driver class acts as the glue that 66 decribes and binds the other component drivers together to form an ALSA 67 "sound card device". It handles any machine specific controls and 68 machine level audio events (e.g. turning on an amp at start of playback). 69 70 71 Documentation 72 ============= 73 74 The documentation is spilt into the following sections:- 75 76 overview.txt: This file. 77 78 codec.txt: Codec driver internals. 79 80 DAI.txt: Description of Digital Audio Interface standards and how to configure 81 a DAI within your codec and CPU DAI drivers. 82 83 dapm.txt: Dynamic Audio Power Management 84 85 platform.txt: Platform audio DMA and DAI. 86 87 machine.txt: Machine driver internals. 88 89 pop_clicks.txt: How to minimise audio artifacts. 90 91 clocking.txt: ASoC clocking for best power performance. 92 93 jack.txt: ASoC jack detection. 94 95 DPCM.txt: Dynamic PCM - Describes DPCM with DSP examples.