Add option for linear volume scaling with alsa.

README.md

@@ -39,7 +39,7 @@ password = PASS
 backend = alsa
 device = alsa_audio_device # Given by `aplay -L`
 mixer = PCM
-volume-control = alsa # or softvol
+volume-control = alsa # or alsa_linear, or softvol
 onevent = command_run_on_playback_events
 device_name = name_in_spotify_connect
 bitrate = 96|160|320

src/alsa_mixer.rs

@@ -5,6 +5,7 @@ use alsa;
 pub struct AlsaMixer {
     pub device: String,
     pub mixer: String,
+    pub linear_scaling: bool,
 }
 
 impl AlsaMixer {
@@ -17,8 +18,12 @@ impl AlsaMixer {
         let (min, max) = elem.get_playback_volume_range();
 
         let volume_steps = (max - min) as f64;
-        let normalised_volume =
-            (f64::from(volume).log(65535.0) * volume_steps).floor() as i64 + min;
+        let normalised_volume = if self.linear_scaling {
+            ((f64::from(volume) / f64::from(u16::max_value())) * f64::from(volume_steps)) as i64
+                + min
+        } else {
+            (f64::from(volume).log(f64::from(u16::max_value())) * volume_steps).floor() as i64 + min
+        };
 
         elem.set_playback_volume_all(normalised_volume)?;
         Ok(())
@@ -30,6 +35,7 @@ impl Mixer for AlsaMixer {
         AlsaMixer {
             device: "default".to_string(),
             mixer: "Master".to_string(),
+            linear_scaling: false,
         }
     }
     fn start(&self) {}

src/cli.rs

@@ -46,7 +46,7 @@ pub fn command_line_argument_options() -> Options {
     opts.optopt(
         "",
         "volume-control",
-        "Possible values are alsa or softvol.",
+        "Possible values are alsa, alsa_linear, and softvol.",
         "CONTROLLER",
     );
     opts.optflag("v", "verbose", "Add debug information to log.");

src/config.rs

@@ -22,7 +22,7 @@ use hostname;
 const CONFIG_FILE: &str = "spotifyd.conf";
 
 pub enum VolumeController {
-    Alsa,
+    Alsa { linear: bool },
     SoftVol,
 }
 
@@ -36,7 +36,8 @@ impl FromStr for VolumeController {
     type Err = ();
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         match &*s.to_uppercase() {
-            "ALSA" => Ok(VolumeController::Alsa),
+            "ALSA" => Ok(VolumeController::Alsa { linear: false }),
+            "ALSA_LINEAR" => Ok(VolumeController::Alsa { linear: true }),
             "SOFTVOL" => Ok(VolumeController::SoftVol),
             _ => Err(()),
         }

src/setup.rs

@@ -28,14 +28,15 @@ pub fn initial_state(handle: Handle, matches: &Matches) -> main_loop::MainLoopSt
     let local_audio_device = config.audio_device.clone();
     let local_mixer = config.mixer.clone();
     let mut mixer = match config.volume_controller {
-        config::VolumeController::Alsa => {
+        config::VolumeController::Alsa { linear } => {
             info!("Using alsa volume controller.");
             Box::new(move || {
                 Box::new(alsa_mixer::AlsaMixer {
                     device: local_audio_device
                         .clone()
                         .unwrap_or_else(|| "default".to_string()),
                     mixer: local_mixer.clone().unwrap_or_else(|| "Master".to_string()),
+                    linear_scaling: linear,
                 }) as Box<mixer::Mixer>
             }) as Box<FnMut() -> Box<Mixer>>
         }