[ALSA] add STAC9708 output bias mixer control

AC97 Codec
This adds a mixer control for an undocumented bit of the STAC9708
that somehow affects the analog output.  This should help reducing
the distortion at high output levels on ymfpci and SBLive cards.
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>

sound/pci/ac97/ac97_patch.c

@@ -392,9 +392,36 @@ int patch_sigmatel_stac9700(ac97_t * ac97)
 	return 0;
 }
 
+static int snd_ac97_stac9708_put_bias(snd_kcontrol_t *kcontrol, snd_ctl_elem_value_t *ucontrol)
+{
+	ac97_t *ac97 = snd_kcontrol_chip(kcontrol);
+	int err;
+
+	down(&ac97->page_mutex);
+	snd_ac97_write(ac97, AC97_SIGMATEL_BIAS1, 0xabba);
+	err = snd_ac97_update_bits(ac97, AC97_SIGMATEL_BIAS2, 0x0010,
+				   (ucontrol->value.integer.value[0] & 1) << 4);
+	snd_ac97_write(ac97, AC97_SIGMATEL_BIAS1, 0);
+	up(&ac97->page_mutex);
+	return err;
+}
+
+static const snd_kcontrol_new_t snd_ac97_stac9708_bias_control = {
+	.iface = SNDRV_CTL_ELEM_IFACE_MIXER,
+	.name = "Sigmatel Output Bias Switch",
+	.info = snd_ac97_info_volsw,
+	.get = snd_ac97_get_volsw,
+	.put = snd_ac97_stac9708_put_bias,
+	.private_value = AC97_SINGLE_VALUE(AC97_SIGMATEL_BIAS2, 4, 1, 0),
+};
+
 static int patch_sigmatel_stac9708_specific(ac97_t *ac97)
 {
+	int err;
+
 	snd_ac97_rename_vol_ctl(ac97, "Headphone Playback", "Sigmatel Surround Playback");
+	if ((err = patch_build_controls(ac97, &snd_ac97_stac9708_bias_control, 1)) < 0)
+		return err;
 	return patch_sigmatel_stac97xx_specific(ac97);
 }