[PATCH RFC v1] ASoC: core: Support for limiting the volume

[Peter Ujfalusi <peter.ujfalusi@nokia.com>]

Add support for the core to limit the maximum volume on an
existing control.
The function will modify the soc_mixer_control.max value
of the given control.
The new value must be lower than the original one (chip maximum)

If there is a need for limiting a gain on a given control,
than machine drivers can do the following in their
snd_soc_dai_link.init function:

snd_soc_limit_volume(codec, "TPA6140A2 Headphone Playback Volume", 21);

This will modify the original 31 (chip maximum) to 21, so user
space will not be able to set the gain higher than this.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
---

Hello Mark, Liam,

So this is my proposed way of doing the volume limiting within the core.
By looking around the code, this should be safe.
With this simple function all machine driver can limit gains on a given
control in a way, that the codec/amp driver does not need to be involved.

What do you think?

---
Peter

 include/sound/soc.h  |    2 ++
 sound/soc/soc-core.c |   38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 0 deletions(-)

diff --git a/include/sound/soc.h b/include/sound/soc.h
index 01e9c66..9f306f0 100644
--- a/include/sound/soc.h
+++ b/include/sound/soc.h
@@ -326,6 +326,8 @@ int snd_soc_get_volsw_s8(struct snd_kcontrol *kcontrol,
 	struct snd_ctl_elem_value *ucontrol);
 int snd_soc_put_volsw_s8(struct snd_kcontrol *kcontrol,
 	struct snd_ctl_elem_value *ucontrol);
+int snd_soc_limit_volume(struct snd_soc_codec *codec,
+	const char *name, int max);
 
 /**
  * struct snd_soc_jack_pin - Describes a pin to update based on jack detection
diff --git a/sound/soc/soc-core.c b/sound/soc/soc-core.c
index d59076e..5968499 100644
--- a/sound/soc/soc-core.c
+++ b/sound/soc/soc-core.c
@@ -2238,6 +2238,44 @@ int snd_soc_put_volsw_s8(struct snd_kcontrol *kcontrol,
 EXPORT_SYMBOL_GPL(snd_soc_put_volsw_s8);
 
 /**
+ * snd_soc_limit_volume - Set new limit to an existing volume control.
+ *
+ * @codec: where to look for the control
+ * @name: Name of the control
+ * @max: new maximum limit
+ *
+ * Return 0 for success, else error.
+ */
+int snd_soc_limit_volume(struct snd_soc_codec *codec,
+	const char *name, int max)
+{
+	struct snd_card *card = codec->card;
+	struct snd_kcontrol *kctl;
+	struct soc_mixer_control *mc;
+	int found = 0;
+	int ret = -EINVAL;
+
+	/* Sanity check for name and max */
+	if (unlikely(!name || max <= 0))
+		return -EINVAL;
+
+	list_for_each_entry(kctl, &card->controls, list) {
+		if (!strncmp(kctl->id.name, name, sizeof(kctl->id.name))) {
+			found = 1;
+			break;
+		}
+	}
+	if (found) {
+		mc = (struct soc_mixer_control *)kctl->private_value;
+		if (max < mc->max)
+			mc->max = max;
+		ret = 0;
+	}
+	return ret;
+}
+EXPORT_SYMBOL_GPL(snd_soc_limit_volume);
+
+/**
  * snd_soc_dai_set_sysclk - configure DAI system or master clock.
  * @dai: DAI
  * @clk_id: DAI specific clock ID
-- 
1.7.1