Audio on Android
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
Format of mixer_paths.xml
- ALSA controls are defined by
- Audio routes are defined by
pathelement contains a collection of
ctlelements as well as other
- The path
nameattribute is used to select an audio route.
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/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.
Udev rules can be used to detect if headphones are plugged in, example for this is from xiaomi-santoni
Converting Android configurations using xml2ucm
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.
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
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
$ 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
mixer_path.xml and device config XML are converted to UCM format then an audio route can be set by using
$ alsaucm -c wm8994aif1 set _verb HiFi set _enadev Headphones
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.
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/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
- #1328 Audio added to samsung-p4wifi with udev rules for redirecting audio to headphones when plugging them in
- How to convert Android Audio configurations to alsa/pulseaudio
- pmaports!238 Audio added to samsung-i927, running close to mainline