Audio

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Audio on Android

Mixer configuration

This provides a brief overview of how Android audio HALs configure the ALSA mixer.

Android devices can have an assortment mixer configurations such as headphones, speakers, or microphones. For a particular device the audio route configurations are defined in mixer_paths.xml. For an example see the mixer path XML for Nexus 5 (hammerhead).

Android audio HALs make use of tinyalsa and audio_route. Tinyalsa is a small standalone library to interface with ALSA in the Linux kernel. Audio HALs call the audio_route library which loads audio routes from an XML and controls the mixer through tinyalsa.

Format of mixer_paths.xml

  • ALSA controls are defined by ctl elements.
  • Audio routes are defined by path elements. A path element contains a collection of ctl elements as well as other path elements.
  • The path name attribute is used to select an audio route.

Headphone jack

Android kernels use switch_class kernel driver to notify the Android frameworks of headphone jack events. The driver source code can be located at: drivers/switch/switch_class.c or drivers/staging/android/switch/switch_class.c. Note that the switch class driver is an Android specific driver. It has not been upstreamed to the mainline kernel.

In userspace the WiredAccessoryManager monitors the /sys/class/switch/h2w for uevents when the headphone is (un)plugged.

Converting Android configurations using xml2ucm

This section is incomplete. It requires a reference to more complete documentation of the device config XML format.

The audio routes in Linux userspace can be defined in ALSA UCM configuration files. There is a tool called xml2ucm which will convert the Android mixer_paths.xml and a device config XML to UCM files. The xml2ucm tool introduces the device config XML which provides necessary routing information that was hardcoded in the audio HAL on Android.

The mixer_path.xml is commonly found in the Android device repository. (e.g. android_device_lge_hammerhead). You must write the device config XML for your device.

Format of device config XML

A device config XML describes a sound card with use cases and sets of audio routes. In the examples available at the moment there is only one use case named HiFi. Under each <use-case> there are multiple <device> elements. In this context a <device> means audio route(s) which comprise a logical audio device (e.g. Headphone, Headset, Speaker).

Android audio HALs can and do select multiple audio routes at once. For example a headset can have a headphone output route and a microphone input route. To handle this a <device> element (the headset) can contain multiple <path> elements (audio routes). Example device config XML files are provided in the xml2ucm repository.

The ctl-device must be the name of the sound card which can be obtained using aplay. In the following example from the Samsung p4wifi the ctl-device is the sound card named wm8994aif1.

$ aplay -L
null
    Discard all samples (playback) or generate zero samples (capture)
default:CARD=wm8994aif1
    wm8994-aif1, 
    Default Audio Device
sysdefault:CARD=wm8994aif1
    wm8994-aif1, 
    Default Audio Device

Once the mixer_path.xml and device config XML are converted to UCM format then an audio route can be set by using alsaucm. Example:

$ alsaucm -c wm8994aif1 set _verb HiFi set _enadev Headphones

alsactl method

Cascardo reported having success with the following method on Debian, this should work for postmarketOS as well:

I ran a debian chroot on a cyanogenmod, set the volume up, then ran alsactl save on the chroot. booted into debian, then after alsactl restore was run, I had audio working

so, in summary, it's hard to know which mixer settings will make it work, so my strategy was running some android stack that correctly set the mixer, then saving the state with a chroot or any other tool that would allow you to save, copy and restore that state later on a gnu stack

This method may work for non-Android devices as well, as long as you see an audio device in alsamixer. Just boot the normal OS, save the volume settings somehow, then restore them.

WM1811

This audio chip is present in several Samsung devices that are based on the Exynos 4412 platform. There is a configuration for the Samsung Galaxy Note 10.1 available at the opensgn project. Have a look at the files in opensgn/data/n8000/usr/share/alsa/ucm/Midas_WM1811/. In opensgn/data/n8000/etc/rc.local there are the commands to use the configuration:

# restore alsa sinitial tate
/usr/sbin/alsactl restore -f /usr/share/alsa/ucm/Midas_WM1811/init.state

# enable speaker for output and builtin mic for input
/usr/bin/alsaucm -c Midas_WM1811 set _verb HiFi set _enadev Speaker
/usr/bin/alsaucm -c Midas_WM1811 set _verb HiFi set _enadev Mic

For other devices the configuration might need to be adapted. On the Samsung Galaxy Note 8.0 for example, the audio device is named "Kona_WM1811". Replacing all occurrences in file names and file contents of Midas by Kona worked.

See also