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本文提供有关高级Linux声音体系结构（ALSA）的信息，该体系结构为Linux操作系统提供音频功能。

==Purpose==
本文的目的是介绍ALSA框架。

ALSA框架为Linux提供了全面的音频功能，包括音频流的录制和播放，模拟或数字格式，以及路由和混合功能。
ALSA还支持音频中间件，如 [[PulseAudio]], [[GStreamer overview|Gstreamer]]或Android。

== System overview==

[[File:ALSAOverview.png|link=]]

===Component descriptions===

*'''alsa-utils''' (User space)
The ALSA utility package, provided by the Linux community, contains the command line utilities for the ALSA project (aplay, arecord, amixer, alsamixer ...). These tools are useful for controlling soundcards. They also provide an example of ALSA API use, for application implementation.
*'''alsa-lib''' (User space)
The ALSA Library package contains the ALSA library used by programs (for instance alsa-utils programs) requiring an access to the ALSA sound interface. The ALSA library provides a level of abstraction, such as the PCM and control abstractions, over the audio devices provided by the kernel modules. 
*'''ALSA framework''' (Kernel space)
The ALSA core provides an API to implement audio drivers and PCM/control interfaces to expose audio devices on the userland. 
The PCM interface handles the data flow and control. The interface manages controls exported by the ALSA driver (audio path, volumes...).

*'''ASoC framework (ALSA System On Chip)''' (Kernel space)
The aim of the ALSA System on Chip (ASoC) layer<ref>[https://www.kernel.org/doc/html/latest/sound/soc/index.html ASoC layer documentation]</ref> is to improve ALSA support for embedded system-on-chip processors and audio codecs. The ASoC framework provides a DMA engine which interfaces with DMA framework to handle the transfer of audio samples. ASoC also supports the dynamic power management of audio pathes through the DAPM driver. ASoC acts as an ALSA driver, which splits an embedded audio system into three types of platform independent drivers: the CPU DAI, the codec and the machine drivers.

*'''ASoC drivers''' (Kernel space)

ASoC drivers allow the implementation of hardware dependent code for  ASoC driver classes:

:* Codec drivers:
::These drivers are the drivers for the backend audio components. (see Codec peripherals below)

:* CPU DAI drivers:
::There is a specific CPU DAI driver for each STM32 audio peripheral (see CPU DAI peripherals below). Each CPU DAI supports at least one of the following protocols: I2S, PCM, or S/PDIF.

:* Machine drivers:
::The machine drivers describe and bind the CPU DAIs and codec drivers together to create the DAI links and ALSA soundcard. The ASoC framework provides a machine driver implementing a generic soundcard called "audio-graph-card"<ref name="Audio graph card bindings">{{CodeSource | Linux kernel | Documentation/devicetree/bindings/sound/audio-graph-card.txt}}</ref><ref name="Device graph bindings">{{CodeSource | Linux kernel | Documentation/devicetree/bindings/graph.txt}}</ref>. This generic machine driver is used for STM32 MPUs soundcards.

The schematic below, illustrates the general layout of an ASoC soundcard. Refer to [[Soundcard configuration|soundcard configuration]] to see examples of soundcards implementation for STM32 MPUs boards.

[[File:asoc_generic_soundcard.png|link=]]

*'''CPU DAI peripherals''' (Hardware)
:The ST microprocessor peripheral provides the CPU audio interface. The audio internal peripheral list can be found in [[:Category:Audio peripherals|Audio peripherals section]].

*'''Codec peripherals''' (Hardware)
:The codec peripherals are the external (none CPU) hardware audio I/O devices (i.e. audio codec IC, digital microphone, amplifier, simple IO connector ...).

===API descriptions===
* '''User space interface:'''
:The ALSA library reference <ref>[http://www.alsa-project.org/alsa-doc/alsa-lib/ ALSA library API]</ref> documents the userland API library.
* '''Kernel driver interface:'''
:The ALSA kernel documentation <ref>[https://www.kernel.org/doc/html/latest/sound/kernel-api/index.html ALSA and ASoC driver API documentation]</ref> documents the ASOC and ALSA driver APIs.

==Configuration==

*'''Kernel Configuration'''

The ALSA/ASoC and the audio graph card must be enabled in the kernel configuration, as shown below, to enable the sound support. On top of this, the user has to activate the CPU and Codec drivers according to the chosen hardware. The user can use Linux [[Menuconfig or how to configure kernel | Menuconfig tool]] to select the required drivers: 

<pre>
[*] Device Drivers
    [*] Sound card support
        [*] Advanced Linux Sound Architecture
            [*] ALSA for SoC audio support
                STMicroelectronics STM32 SOC audio support
	             [ ] STM32 SAI interface (Serial Audio Interface) support
	             [ ] STM32 I2S interface (SPI/I2S block) support
	             [ ] STM32 S/PDIF receiver (SPDIFRX) support
                CODEC drivers
	             [ ] ...
                [*] ASoC Audio Graph sound card support
</pre>

*'''Device tree configuration'''

The audio sub-system is configured through the soundcard configuration in the [[Device tree|device tree]]. The [[Soundcard configuration|soundcard configuration]] article describes the soundcards available for STM32MPUs on various board. This article details how to configure the [[:Category:Audio peripherals|audio peripherals]] used to implement the soundcards.

==How to use==

The alsa-utils pakage provides a set of utilities to manage audio devices in the Linux kernel: [https://linux.die.net/man/1/aplay aplay], [https://linux.die.net/man/1/arecord arecord], [https://linux.die.net/man/1/amixer amixer], [https://linux.die.net/man/1/iecset iecset] and [https://linux.die.net/man/1/alsactl alsactl]. An overview of these utilities is given below:

=== Playback ===
*List playback devices
 '''Board $>''' aplay -l

*Play a wav file on card [X] device [Y] 
 '''Board $>''' aplay -D hw:[X],[Y] <filename.wav>

*Play a wav file or a generated signal on card [X] device [Y]
 '''Board $>''' speaker-test -D hw:[X],[Y]

Refer to [[How to play audio]] article, to find examples of playback use cases on STM32MPU boards.

=== Record ===
*List record devices
 '''Board $>''' arecord -l
*Capture audio from card [X] device [Y]
 '''Board $>''' arecord -D hw:[X],[Y] -f dat <filename.wav>

Refer to [[How to record audio]] article, to find examples of record use cases for the STM32MPU boards.

=== Controls ===
*List card [X] controls
 '''Board $>''' amixer -c [X] controls
*Set card [X] controls [Y] to value [Z]
 '''Board $>''' amixer -c [X] cset name='[Y]' '[Z]'
*Store soundcard [X] controls state
 '''Board $>''' alsactl store [X]
*Restore soundcard [X] controls state
 '''Board $>''' alsactl restore [X]

Refer to [[Soundcard configuration]] article to find examples of control configuration for the STM32MPU boards.

=== IEC controls ===
*List iec958 parameters
 '''Board $>''' iecset -h
*Set card [X] iec958 parameter [Y] to value [Z]
 '''Board $>''' iecset -c [X] cset [Y] [Z]
*Dump card [X] iec958 value
 '''Board $>''' iecset -c [X] -x

==How to trace and debug the framework==
This chapter introduces tools useful for debugging and monitoring the audio framework and drivers. It comes as an extension to the [[Linux_tracing,_monitoring_and_debugging]] article.

=== How to monitor ===
This section introduces ALSA framework monitoring methods. Refer to [[:Category:Linux monitoring tools|Linux monitoring tools]] articles for further information.

==== Procfs filesystem ====
The ALSA '''asound''' directory<ref>[https://www.kernel.org/doc/html/latest/sound/designs/procfile.html ALSA proc files]</ref> in [[Pseudo_filesystem|procfs]] file system, provides a lot of information on sound cards. The PCM proc files, provides useful PCM substream debugging information, such as hardware/software parameters, stream status and buffer information.

Examples:
:* List PCM audio devices:
<div style="margin-left: 2em;">
 '''Board $>''' cat /proc/{{highlight|asound}}/pcm
</div>
:* Get hardware parameters of a PCM audio device (device "0" of card "0" here):
<div style="margin-left: 2em;">
 '''Board $>''' cat /proc/{{highlight|asound}}/card0/pcm0p/sub0/hw_params
</div>

==== Debugfs filesystem ====
The '''asoc''' directory in [[Debugfs|debugfs]] file system provides information on sound card components.

Examples:
:* List DAIs
<div style="margin-left: 2em;">
 '''Board $>''' cat /sys/kernel/debug/{{highlight|asoc}}/dais

</div>
:* List DAPMs of "xxx.audio-controller" CPU DAI of "{{highlight|STM32MP1-EV}}" soundcard
<div style="margin-left: 2em;">
 '''Board $>''' ls /sys/kernel/debug/{{highlight|asoc}}/{{highlight|STM32MP1-EV}}/xxx.audio-controller/dapm
</div>

=== How to trace ===
This section introduces tracing methods for ALSA framework. Refer to [[:Category:Linux_tracing_tools|Linux_tracing_tools]] articles to go further.

==== Dynamic traces ====
ALSA framework and driver debug traces can be added to the kernel logs by using the [[How_to_use_the_kernel_dynamic_debug|dynamic debug]] mechanism.
* Example: Activate dynamic trace for SAI Linux driver, and print the traces to the console:

 '''Board $>''' echo -n 'file stm32_sai.c +p; file stm32_sai_sub.c +p' > /sys/kernel/debug/dynamic_debug/control; 
 '''Board $>''' dmesg -n8;

==== Tracing filesystem ====
The Linux kernel offers a [[Pseudo_filesystem|tracefs]] filesystem, provided with Linux kernel tracing framework. ALSA and ASoC have their own tracepoints in this tracing filesystem:
* '''asoc''' entry for ASoC, provides the DAPM, jack and bias level tracepoints.<ref name="tracepoints">{{CodeSource | Linux kernel | Documentation/trace/tracepoint-analysis.rst}}</ref>
* '''snd_pcm''' entry for ALSA, provides the PCM buffers and PCM hardware parameters tracepoints<ref name="tracepoints"></ref><ref name="tracepoints_pcm">[https://www.kernel.org/doc/html/latest/sound/designs/tracepoints.html ALSA tracepoints]</ref>.

===== Activate DAPM traces =====
Prerequisite: the CONFIG_FUNCTION_TRACER configuration must first be enabled in the [[Menuconfig or how to configure kernel | Linux kernel configuration]]
* Enable trace<ref name="tracepoints"></ref>
 '''Board $>''' echo '1' > /sys/kernel/debug/tracing/events/{{highlight|asoc}}/enable
* Check log:
 '''Board $>''' cat /sys/kernel/debug/tracing/trace

===== Activate PCM hardware parameter traces =====
Prerequisite: the CONFIG_FUNCTION_TRACER and CONFIG_SND_DEBUG configurations must first be enabled in the [[Menuconfig or how to configure kernel | Linux kernel configuration]]
* Enable trace<ref name="tracepoints"></ref>
 '''Board $>''' echo '1' > /sys/kernel/debug/tracing/events/{{highlight|snd_pcm}}/enable
* Check log:
 '''Board $>''' cat /sys/kernel/debug/tracing/trace

===== Activate PCM buffer state traces (PCM ring buffer overrun/underrun debugging) =====
Prerequisite: the CONFIG_FUNCTION_TRACER, CONFIG_SND_DEBUG, CONFIG_SND_DEBUG_VERBOSE and SND_PCM_XRUN_DEBUG configurations must first be enabled in the [[Menuconfig or how to configure kernel | Linux kernel configuration]]
* Set XRUN trace verbosity<ref>[http://www.alsa-project.org/main/index.php/XRUN_Debug XRUN Debug]</ref>
 # Enable basic debugging and stack dump
 '''Board $>''' echo 3 > /proc/asound/card0/pcm0p/xrun_debug
* Enable trace<ref name="tracepoints"></ref>
 '''Board $>''' echo '1' > /sys/kernel/debug/tracing/events/{{highlight|snd_pcm}}/enable
* Check log:
 '''Board $>''' cat /sys/kernel/debug/tracing/trace

=== How to debug ===
Refer to the [[:Category:Linux debugging tools|Linux debugging tools]] articles.

==Source code location==

===User space===

* [http://git.alsa-project.org/?p=alsa-lib.git;a=summary alsa-lib sources]
* [http://git.alsa-project.org/?p=alsa-utils.git;a=summary alsa-utils sources]
	
===Kernel space===

* {{CodeSource | Linux kernel | sound/core | ALSA core}}
* {{CodeSource | Linux kernel | sound/soc | ASoC core}}
* {{CodeSource | Linux kernel | sound/soc/stm | ASoC STM32 drivers}}
* {{CodeSource | Linux kernel | sound/soc/codecs | ASoC codecs drivers}}

==References==
<references />

<noinclude>
[[Category:ALSA]]
{{PublicationRequestId | 10644 | 2019-02-06 | StephenG}}
{{ArticleBasedOnModel | Framework overview article model}}
</noinclude>