Documentation/sound/designs/powersave.rst

   1 ==========================
   2 Notes on Power-Saving Mode
   3 ==========================
   4
   5 AC97 and HD-audio drivers have the automatic power-saving mode.
   6 This feature is enabled via Kconfig ``CONFIG_SND_AC97_POWER_SAVE``
   7 and ``CONFIG_SND_HDA_POWER_SAVE`` options, respectively.
   8
   9 With the automatic power-saving, the driver turns off the codec power
  10 appropriately when no operation is required.  When no applications use
  11 the device and/or no analog loopback is set, the power disablement is
  12 done fully or partially.  It'll save a certain power consumption, thus
  13 good for laptops (even for desktops).
  14
  15 The time-out for automatic power-off can be specified via ``power_save``
  16 module option of snd-ac97-codec and snd-hda-intel modules.  Specify
  17 the time-out value in seconds.  0 means to disable the automatic
  18 power-saving.  The default value of timeout is given via
  19 ``CONFIG_SND_AC97_POWER_SAVE_DEFAULT`` and
  20 ``CONFIG_SND_HDA_POWER_SAVE_DEFAULT`` Kconfig options.  Setting this to 1
  21 (the minimum value) isn't recommended because many applications try to
  22 reopen the device frequently.  10 would be a good choice for normal
  23 operations.
  24
  25 The ``power_save`` option is exported as writable.  This means you can
  26 adjust the value via sysfs on the fly.  For example, to turn on the
  27 automatic power-save mode with 10 seconds, write to
  28 ``/sys/modules/snd_ac97_codec/parameters/power_save`` (usually as root):
  29 ::
  30
  31         # echo 10 > /sys/modules/snd_ac97_codec/parameters/power_save
  32
  33
  34 Note that you might hear click noise/pop when changing the power
  35 state.  Also, it often takes certain time to wake up from the
  36 power-down to the active state.  These are often hardly to fix, so
  37 don't report extra bug reports unless you have a fix patch ;-)
  38
  39 For HD-audio interface, there is another module option,
  40 power_save_controller.  This enables/disables the power-save mode of
  41 the controller side.  Setting this on may reduce a bit more power
  42 consumption, but might result in longer wake-up time and click noise.
  43 Try to turn it off when you experience such a thing too often.