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