ALSA Jack Software Injection¶
Simple Introduction On Jack Injection¶
Here jack injection means users could inject plugin or plugout events to the audio jacks through debugfs interface, it is helpful to validate ALSA userspace changes. For example, we change the audio profile switching code in the pulseaudio, and we want to verify if the change works as expected and if the change introduce the regression, in this case, we could inject plugin or plugout events to an audio jack or to some audio jacks, we don’t need to physically access the machine and plug/unplug physical devices to the audio jack.
In this design, an audio jack doesn’t equal to a physical audio jack.
Sometimes a physical audio jack contains multi functions, and the
ALSA driver creates multi jack_kctl for a snd_jack, here the
snd_jack represents a physical audio jack and the jack_kctl
represents a function, for example a physical jack has two functions:
headphone and mic_in, the ALSA ASoC driver will build 2 jack_kctl
for this jack. The jack injection is implemented based on the
jack_kctl instead of snd_jack.
To inject events to audio jacks, we need to enable the jack injection
via sw_inject_enable first, once it is enabled, this jack will not
change the state by hardware events anymore, we could inject plugin or
plugout events via jackin_inject and check the jack state via
status, after we finish our test, we need to disable the jack
injection via sw_inject_enable too, once it is disabled, the jack
state will be restored according to the last reported hardware events
and will change by future hardware events.
The Layout of Jack Injection Interface¶
If users enable the SND_JACK_INJECTION_DEBUG in the kernel, the audio jack injection interface will be created as below:
$debugfs_mount_dir/sound
|-- card0
|-- |-- HDMI_DP_pcm_10_Jack
|-- |-- |-- jackin_inject
|-- |-- |-- kctl_id
|-- |-- |-- mask_bits
|-- |-- |-- status
|-- |-- |-- sw_inject_enable
|-- |-- |-- type
...
|-- |-- HDMI_DP_pcm_9_Jack
|--     |-- jackin_inject
|--     |-- kctl_id
|--     |-- mask_bits
|--     |-- status
|--     |-- sw_inject_enable
|--     |-- type
|-- card1
    |-- HDMI_DP_pcm_5_Jack
    |-- |-- jackin_inject
    |-- |-- kctl_id
    |-- |-- mask_bits
    |-- |-- status
    |-- |-- sw_inject_enable
    |-- |-- type
    ...
    |-- Headphone_Jack
    |-- |-- jackin_inject
    |-- |-- kctl_id
    |-- |-- mask_bits
    |-- |-- status
    |-- |-- sw_inject_enable
    |-- |-- type
    |-- Headset_Mic_Jack
        |-- jackin_inject
        |-- kctl_id
        |-- mask_bits
        |-- status
        |-- sw_inject_enable
        |-- type
The Explanation Of The Nodes¶
- kctl_id
- read-only, get jack_kctl->kctl’s id - sound/card1/Headphone_Jack# cat kctl_id Headphone Jack 
- mask_bits
- read-only, get jack_kctl’s supported events mask_bits - sound/card1/Headphone_Jack# cat mask_bits 0x0001 HEADPHONE(0x0001) 
- status
- read-only, get jack_kctl’s current status 
- headphone unplugged: - sound/card1/Headphone_Jack# cat status Unplugged 
- headphone plugged: - sound/card1/Headphone_Jack# cat status Plugged 
- type
- read-only, get snd_jack’s supported events from type (all supported events on the physical audio jack) - sound/card1/Headphone_Jack# cat type 0x7803 HEADPHONE(0x0001) MICROPHONE(0x0002) BTN_3(0x0800) BTN_2(0x1000) BTN_1(0x2000) BTN_0(0x4000) 
- sw_inject_enable
- read-write, enable or disable injection 
- injection disabled: - sound/card1/Headphone_Jack# cat sw_inject_enable Jack: Headphone Jack Inject Enabled: 0 
- injection enabled: - sound/card1/Headphone_Jack# cat sw_inject_enable Jack: Headphone Jack Inject Enabled: 1 
- to enable jack injection: - sound/card1/Headphone_Jack# echo 1 > sw_inject_enable 
- to disable jack injection: - sound/card1/Headphone_Jack# echo 0 > sw_inject_enable 
- jackin_inject
- write-only, inject plugin or plugout 
- to inject plugin: - sound/card1/Headphone_Jack# echo 1 > jackin_inject 
- to inject plugout: - sound/card1/Headphone_Jack# echo 0 > jackin_inject