Figure out how to play both mono and stereo .wav files through the headphones on a Freescale MCIMX6Q-SDB running a stock core-image-minimal with only added recipe alsa-utils.

Some /sys/kernel/debug/ files to check out later:

• /sys/kernel/debug/asoc/<cardname>/<codecname>/dapm/*
• /sys/kernel/debug/regmap/<address>/registers

• fully-updated poky and meta-freescale layers
• MACHINE = imx6qdlsabresd
• IMAGE_INSTALL_append = “ alsa-utils”
• EXTRA_IMAGE_FEATURES = “package-management debug-tweaks”

Part of alsa-utils package, in /usr/share/sounds/alsa/ (all mono files):

Front_Center.wav  Front_Right.wav   Rear_Center.wav   Rear_Right.wav    Side_Right.wav
Front_Left.wav    Noise.wav         Rear_Left.wav     Side_Left.wav

Also manually copied under /home/root/:

-rw-r--r--    1 root     root         53544 Oct  4 17:42 pcm0808m.wav
-rw-r--r--    1 root     root        107042 Oct  4 17:42 pcm0808s.wav
-rw-r--r--    1 root     root        107042 Oct  4 17:42 pcm1608m.wav
-rw-r--r--    1 root     root        214040 Oct  4 17:42 pcm1608s.wav

...
Advanced Linux Sound Architecture Driver Initialized.
...
wm8962 0-001a: customer id 0 revision D
fsl-asrc 2034000.asrc: driver registered
input: WM8962 Beep Generator as /devices/soc0/soc/2100000.aips-bus/21a0000.i2c/i2c-0/0-001a/input/input5
imx-wm8962 sound: wm8962 <-> 202c000.ssi mapping ok
imx-wm8962 sound: snd-soc-dummy-dai <-> 2034000.asrc mapping ok
imx-wm8962 sound: wm8962 <-> 202c000.ssi mapping ok
imx-audio-hdmi sound-hdmi: hdmi-hifi <-> soc:hdmi_audio@00120000 mapping ok
...
wm8962-supply: disabling
SW4: disabling
ALSA device list:
  #0: wm8962-audio
  #1: imx-hdmi-soc
...

With no other sound or CODEC configuration, first, turn up headphone volume:

# amixer cset name="Headphone Volume" 100,100
numid=24,iface=MIXER,name='Headphone Volume'
  ; type=INTEGER,access=rw---R--,values=2,min=0,max=127,step=0
  : values=100,100
  | dBscale-min=-121.00dB,step=1.00dB,mute=1

In loop, play all mono files in /usr/share/sounds/alsa/, they all play on the left headphone only (which is to be expected):

# for w in *.wav ; do
> aplay $w
> sleep 1
> done

For .wav files under /home/root/, mono files play only on left HP channel, stereo .wav files play on both channels, so this verifies that both HP channels work, at least with stereo files.

Copy more sample files to target:

-rw-r--r--    1 root     root         53544 Oct  3  2017 pcm0808m.wav
-rw-r--r--    1 root     root        107042 Oct  3  2017 pcm0808s.wav
-rw-r--r--    1 root     root        107042 Oct  3  2017 pcm1608m.wav
-rw-r--r--    1 root     root        214040 Oct  3  2017 pcm1608s.wav

Mono files play only on left, stereo files play on both.

By default:

# amixer cget name="DAC L/R Swap Switch"
numid=20,iface=MIXER,name='DAC L/R Swap Switch'
  ; type=BOOLEAN,access=rw------,values=1
  : values=off

Switch, so mono files play on right headphone:

# amixer cset name="DAC L/R Swap Switch" on
numid=20,iface=MIXER,name='DAC L/R Swap Switch'
  ; type=BOOLEAN,access=rw------,values=1
  : values=on

Turn off for now.

By default, both HP channels bypass the HP mixers and get their output directly from the DAC:

root@imx6qdlsabresd:~# amixer get "HPOUTL PGA"
Simple mixer control 'HPOUTL PGA',0
  Capabilities: enum
  Items: 'DAC' 'Mixer'
  Item0: 'DAC'
root@imx6qdlsabresd:~# amixer get "HPOUTR PGA"
Simple mixer control 'HPOUTR PGA',0
  Capabilities: enum
  Items: 'DAC' 'Mixer'
  Item0: 'DAC'
root@imx6qdlsabresd:~#

For the left channel, we can use the mixer and just feed the left channel into the mixer, effectively getting the same result:

# amixer set "HPOUTL PGA" Mixer
Simple mixer control 'HPOUTL PGA',0
  Capabilities: enum
  Items: 'DAC' 'Mixer'
  Item0: 'Mixer'

This effectively kills the left output channel until we set the mixer properly:

# amixer sget "HPMIXL DACL"
Simple mixer control 'HPMIXL DACL',0
  Capabilities: pswitch pswitch-joined
  Playback channels: Mono
  Mono: Playback [off]

# amixer sset "HPMIXL DACL" on
Simple mixer control 'HPMIXL DACL',0
  Capabilities: pswitch pswitch-joined
  Playback channels: Mono
  Mono: Playback [on]