[ALSA] Add description about power-saving mode

Added the description about power-saving mode on AC97 and HD-audio drivers.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>

1 files changed, 41 insertions, 0 deletions

diff --git a/Documentation/sound/alsa/powersave.txt b/Documentation/sound/alsa/powersave.txt
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+++ b/Documentation/sound/alsa/powersave.txt
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+Notes on Power-Saving Mode
+==========================
+
+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.