Add volume range config

This helps with mopidy#3

README.rst

@@ -55,11 +55,19 @@ The following configuration values are available:
   Other typical values includes ``PCM``. Run the command ``amixer scontrols``
   to list available controls on your system.
 
+- ``alsamixer/volmin`` and ``alsamixer/volmax``: Map the Mopidy volume control range to
+  a different range. Values are in the range 0-100. Use this if the default range (0 - 100)
+  is too wide, resulting in a small usable range for Mopidy's volume control.
+  For example try ``volmin = 30`` and ``volmin = 70`` to map Mopidy's volume control to the middle
+  of ALSA's volume range.
+
 Example ``alsamixer`` section from the Mopidy configuration file::
 
     [alsamixer]
     card = 1
     control = PCM
+    volmin = 0
+    volmin = 100
 
 
 Project resources

mopidy_alsamixer/__init__.py

@@ -22,6 +22,8 @@ def get_config_schema(self):
         schema = super(Extension, self).get_config_schema()
         schema['card'] = config.Integer(minimum=0)
         schema['control'] = config.String()
+        schema['volmin'] = config.Integer(minimum=0, maximum=100)
+        schema['volmax'] = config.Integer(minimum=0, maximum=100)
         return schema
 
     def setup(self, registry):

mopidy_alsamixer/mixer.py

@@ -23,6 +23,8 @@ def __init__(self, config):
         self.config = config
         self.card = self.config['alsamixer']['card']
         self.control = self.config['alsamixer']['control']
+        self.volmin = self.config['alsamixer']['volmin']
+        self.volmax = self.config['alsamixer']['volmax']
 
         known_cards = alsaaudio.cards()
         if self.card >= len(known_cards):
@@ -70,13 +72,15 @@ def get_volume(self):
         if not channels:
             return None
         elif channels.count(channels[0]) == len(channels):
-            return int(channels[0])
+            unadjusted_volume = (channels[0] - self.volmin) * 100.0 / (self.volmax - self.volmin)
+            return int(unadjusted_volume)
         else:
             # Not all channels have the same volume
             return None
 
     def set_volume(self, volume):
-        self._mixer.setvolume(volume)
+        adjusted_volume = self.volmin + volume * (self.volmax - self.volmin) / 100.0
+        self._mixer.setvolume(int(adjusted_volume))
         return True
 
     def get_mute(self):