How to enable audio codec power saving
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How to enable audio codec power saving
The sound device (e.g., AD1981B or AD1892 codecs) requires a noticable amount of power (0.3W on T43). This power can be saved when the device is not in use by disabling it either completely, or partially.
Disabling the device
Disable the entire sound device by unloading modules or, on older kernels, by # echo -n 3 > /sys/class/sound/controlC0/device/power/state (replace 3 by 0 to enable again).
On-demand power control
On-demand power control lets the kernel switch off parts of the audio codec and audio bus that are not in use at the moment. However, this can cause noticeable "click" sounds when enabling/disabling parts of the codec.
AC97 audio subsystem
There's an experimental patch from 2.6.19-rc2, that was later corrected in 2.6.19-rc3, which enables the support of aggressive AC97 power-saving mode in the Linux ALSA drivers. In this mode, the AC97 powerdown register bits are dynamically controlled at each open/close of PCM streams. To allow powering off the analog mixer, make sure all analog inputs (Line, Microphone, CD, etc.) are muted.
It is working without a hitch in an IBM X31 with kernel 2.6.20.
The mode is disabled by default, and activated via power_save=1 option for snd-ac97-codec driver (at run time, use # echo Y > /sys/module/snd_ac97_codec/parameters/power_save).
You may hear a faint click each time the device is powered up (at start of playback) and each time the device is powered down (2 seconds after playback ends).
There are reports that on-demand power control can save 0.2W. Make sure the microphone is muted, though, or its channel won't be turned off.
HDA audio subsystem
If you have one of the newer Intel chips, there is an equivalent feature as of kernel 2.6.24, you can select the CONFIG_SND_HDA_POWER_SAVE option and set CONFIG_SND_HDA_POWER_SAVE_DEFAULT to the number of seconds after which you want the device to time out.
You can also control this on the fly via sysfs:
# echo 10 > /sys/module/snd_hda_intel/parameters/power_save
would give you a timeout of 10 seconds. If you want to save more power you can disable the controller as well:
# echo Y > /sys/module/snd_hda_intel/parameters/power_save_controller
However you might want to disable this if you get annoyed by a "click" sound when the device wakes up again:
# echo N > /sys/module/snd_hda_intel/parameters/power_save_controller
