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