ALSA: hda - Add description about patch loading

Signed-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/alsa/HD-Audio.txt

@@ -403,6 +403,52 @@ re-configure based on that state, run like below:
 ------------------------------------------------------------------------
 
 
+Early Patching
+~~~~~~~~~~~~~~
+When CONFIG_SND_HDA_PATCH_LOADER=y is set, you can pass a "patch" as a
+firmware file for modifying the HD-audio setup before initializing the
+codec.  This can work basically like the reconfiguration via sysfs in
+the above, but it does it before the first codec configuration.
+
+The patch file looks like below:
+
+------------------------------------------------------------------------
+  [codec]
+  0x12345678 0xabcd1234 2
+
+  [model]
+  auto
+
+  [pincfg]
+  0x12 0x411111f0
+
+  [verb]
+  0x20 0x500 0x03
+  0x20 0x400 0xff
+
+  [hint]
+  hp_detect = yes
+------------------------------------------------------------------------
+
+The file needs to have a line `[codec]`.  The next line should contain
+three numbers indicating the codec vendor-id (0x12345678 in the
+example), the codec subsystem-id (0xabcd1234) and the address (2) of
+the codec.  The rest patch entries are applied to this specified codec
+until another codec entry is given.
+
+The `[model]` line allows to change the model name of the each codec.
+In the example above, it will be changed to model=auto.
+Note that this overrides the module option.
+
+After the `[pincfg]` line, the contents are parsed as the initial
+default pin-configurations just like `user_pin_configs` sysfs above.
+The values can be shown in user_pin_configs sysfs file, too.
+
+Similarly, the lines after `[verb]` are parsed as `init_verbs`
+sysfs entries, and the lines after `[hint]` are parsed as `hints`
+sysfs entries, respectively.
+
+
 Power-Saving
 ~~~~~~~~~~~~
 The power-saving is a kind of auto-suspend of the device.  When the