[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 <[e-mail]>
Signed-off-by: Jaroslav Kysela <[e-mail]>

Documentation/sound/alsa/ALSA-Configuration.txt

···
 		codecs (for debugging only)
     enable_msi	- Enable Message Signaled Interrupt (MSI) (default = off)
     power_save	- Automatic power-saving timtout (in second, 0 =
-		disable, default = 10)
+		disable)
     power_save_controller - Reset HD-audio controller in power-saving mode
 		(default = on)
 
···
     subsystem ID (output of "lspci -nv") to ALSA BTS or alsa-devel
     ML (see the section "Links and Addresses").
 
-    When CONFIG_SND_HDA_POWER_SAVE is set, two options, power_save and
-    power_save_controller become available.  power_save specifies the
-    time to turn off the power automatically at idle status.  When
-    power_save_controller is true, the controller is also turned off.
-    This might result in more obvious click noise at turning on/off.
+    power_save and power_save_controller options are for power-saving
+    mode.  See powersave.txt for details.
 
     Note 2: If you get click noises on output, try the module option
 	    position_fix=1 or 2.  position_fix=1 will use the SD_LPIB

Documentation/sound/alsa/powersave.txt

···
+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.