http://git.kernel.org/cgit/linux/kernel/git/tiwai/sound.git/tree/Documentation/sound/alsa
/HD-Audio.txt

check the response of hda unsol event when headphone is plugged and
unplugged


Tracepoints
~~~~~~~~~~~
The hd-audio driver gives a few basic tracepoints.
`hda:hda_send_cmd` traces each CORB write while `hda:hda_get_response`
traces the response from RIRB (only when read from the codec driver).
`hda:hda_bus_reset` traces the bus-reset due to fatal error, etc,
`hda:hda_unsol_event` traces the unsolicited events, and
`hda:hda_power_down` and `hda:hda_power_up` trace the power down/up
via power-saving behavior.

Enabling all tracepoints can be done like
------------------------------------------------------------------------
  # echo 1 > /sys/kernel/debug/tracing/events/hda/enable
------------------------------------------------------------------------
then after some commands, you can traces from
/sys/kernel/debug/tracing/trace file.  For example, when you want to
trace what codec command is sent, enable the tracepoint like:
------------------------------------------------------------------------
  # cat /sys/kernel/debug/tracing/trace
  # tracer: nop

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https://bugs.launchpad.net/bugs/1067434

Title:
  Sound output device keeps changing when using headphones

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