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Notes on Power-Saving Mode

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AC97 and HD-audio drivers have the automatic power-saving mode.

This feature is enabled via Kconfig CONFIG_SND_AC97_POWER_SAVE

and CONFIG_SND_HDA_POWER_SAVE options, respectively.

With the automatic power-saving, the driver turns off the codec power

appropriately when no operation is required.  When no applications use

the device and/or no analog loopback is set, the power disablement is

done fully or partially.  It'll save a certain power consumption, thus

good for laptops (even for desktops).

The time-out for automatic power-off can be specified via power_save

module option of snd-ac97-codec and snd-hda-intel modules.  Specify

the time-out value in seconds.  0 means to disable the automatic

power-saving.  The default value of timeout is given via

CONFIG_SND_AC97_POWER_SAVE_DEFAULT and

CONFIG_SND_HDA_POWER_SAVE_DEFAULT Kconfig options.  Setting this to 1

(the minimum value) isn't recommended because many applications try to

reopen the device frequently.  10 would be a good choice for normal

operations.

The power_save option is exported as writable.  This means you can

adjust the value via sysfs on the fly.  For example, to turn on the

automatic power-save mode with 10 seconds, write to

/sys/modules/snd_ac97_codec/parameters/power_save (usually as root):

        # echo 10 > /sys/modules/snd_ac97_codec/parameters/power_save

Note that you might hear click noise/pop when changing the power

state.  Also, it often takes certain time to wake up from the

power-down to the active state.  These are often hardly to fix, so

don't report extra bug reports unless you have a fix patch ;-)

For HD-audio interface, there is another module option,

power_save_controller.  This enables/disables the power-save mode of

the controller side.  Setting this on may reduce a bit more power

consumption, but might result in longer wake-up time and click noise.

Try to turn it off when you experience such a thing too often.