ALSA: hda - Add documentation for tracepoints

check the current value.  If it's non-zero, the feature is turned on.

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, 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

  #

  #       TASK-PID    CPU#    TIMESTAMP  FUNCTION

  #          | |       |          |         |

         <...>-7807  [002] 105147.774889: hda_send_cmd: [0:0] val=e3a019

         <...>-7807  [002] 105147.774893: hda_send_cmd: [0:0] val=e39019

         <...>-7807  [002] 105147.999542: hda_send_cmd: [0:0] val=e3a01a

         <...>-7807  [002] 105147.999543: hda_send_cmd: [0:0] val=e3901a

         <...>-26764 [001] 349222.837143: hda_send_cmd: [0:0] val=e3a019

         <...>-26764 [001] 349222.837148: hda_send_cmd: [0:0] val=e39019

         <...>-26764 [001] 349223.058539: hda_send_cmd: [0:0] val=e3a01a

         <...>-26764 [001] 349223.058541: hda_send_cmd: [0:0] val=e3901a

------------------------------------------------------------------------

Here `[0:0]` indicates the card number and the codec address, and

`val` shows the value sent to the codec, respectively.  The value is

a packed value, and you can decode it via hda-decode-verb program

included in hda-emu package below.  For example, the value e3a019 is

to set the left output-amp value to 25.

------------------------------------------------------------------------

  % hda-decode-verb 0xe3a019

  raw value = 0x00e3a019

  cid = 0, nid = 0x0e, verb = 0x3a0, parm = 0x19

  raw value: verb = 0x3a0, parm = 0x19

  verbname = set_amp_gain_mute

  amp raw val = 0xa019

  output, left, idx=0, mute=0, val=25

------------------------------------------------------------------------

Development Tree

~~~~~~~~~~~~~~~~

The latest development codes for HD-audio are found on sound git tree: