| Liam Girdwood | eb1a6af | 2006-10-06 18:34:51 +0200 | [diff] [blame^] | 1 | ALSA SoC Layer |
| 2 | ============== |
| 3 | |
| 4 | The overall project goal of the ALSA System on Chip (ASoC) layer is to provide |
| 5 | better ALSA support for embedded system on chip procesors (e.g. pxa2xx, au1x00, |
| 6 | iMX, etc) and portable audio codecs. Currently there is some support in the |
| 7 | kernel for SoC audio, however it has some limitations:- |
| 8 | |
| 9 | * Currently, codec drivers are often tightly coupled to the underlying SoC |
| 10 | cpu. This is not ideal and leads to code duplication i.e. Linux now has 4 |
| 11 | different wm8731 drivers for 4 different SoC platforms. |
| 12 | |
| 13 | * There is no standard method to signal user initiated audio events. |
| 14 | e.g. Headphone/Mic insertion, Headphone/Mic detection after an insertion |
| 15 | event. These are quite common events on portable devices and ofter require |
| 16 | machine specific code to re route audio, enable amps etc after such an event. |
| 17 | |
| 18 | * Current drivers tend to power up the entire codec when playing |
| 19 | (or recording) audio. This is fine for a PC, but tends to waste a lot of |
| 20 | power on portable devices. There is also no support for saving power via |
| 21 | changing codec oversampling rates, bias currents, etc. |
| 22 | |
| 23 | |
| 24 | ASoC Design |
| 25 | =========== |
| 26 | |
| 27 | The ASoC layer is designed to address these issues and provide the following |
| 28 | features :- |
| 29 | |
| 30 | * Codec independence. Allows reuse of codec drivers on other platforms |
| 31 | and machines. |
| 32 | |
| 33 | * Easy I2S/PCM audio interface setup between codec and SoC. Each SoC interface |
| 34 | and codec registers it's audio interface capabilities with the core and are |
| 35 | subsequently matched and configured when the application hw params are known. |
| 36 | |
| 37 | * Dynamic Audio Power Management (DAPM). DAPM automatically sets the codec to |
| 38 | it's minimum power state at all times. This includes powering up/down |
| 39 | internal power blocks depending on the internal codec audio routing and any |
| 40 | active streams. |
| 41 | |
| 42 | * Pop and click reduction. Pops and clicks can be reduced by powering the |
| 43 | codec up/down in the correct sequence (including using digital mute). ASoC |
| 44 | signals the codec when to change power states. |
| 45 | |
| 46 | * Machine specific controls: Allow machines to add controls to the sound card |
| 47 | e.g. volume control for speaker amp. |
| 48 | |
| 49 | To achieve all this, ASoC basically splits an embedded audio system into 3 |
| 50 | components :- |
| 51 | |
| 52 | * Codec driver: The codec driver is platform independent and contains audio |
| 53 | controls, audio interface capabilities, codec dapm definition and codec IO |
| 54 | functions. |
| 55 | |
| 56 | * Platform driver: The platform driver contains the audio dma engine and audio |
| 57 | interface drivers (e.g. I2S, AC97, PCM) for that platform. |
| 58 | |
| 59 | * Machine driver: The machine driver handles any machine specific controls and |
| 60 | audio events. i.e. turing on an amp at start of playback. |
| 61 | |
| 62 | |
| 63 | Documentation |
| 64 | ============= |
| 65 | |
| 66 | The documentation is spilt into the following sections:- |
| 67 | |
| 68 | overview.txt: This file. |
| 69 | |
| 70 | codec.txt: Codec driver internals. |
| 71 | |
| 72 | DAI.txt: Description of Digital Audio Interface standards and how to configure |
| 73 | a DAI within your codec and CPU DAI drivers. |
| 74 | |
| 75 | dapm.txt: Dynamic Audio Power Management |
| 76 | |
| 77 | platform.txt: Platform audio DMA and DAI. |
| 78 | |
| 79 | machine.txt: Machine driver internals. |
| 80 | |
| 81 | pop_clicks.txt: How to minimise audio artifacts. |
| 82 | |
| 83 | clocking.txt: ASoC clocking for best power performance. |