firmware: Some stubs for the audio mixing code.

hackamp/firmware/src/mixer.rs

@@ -1,2 +1,57 @@
 //! Functionality to mix and forward different audio streams.
 #![warn(missing_docs)]
+
+use cortex_m::interrupt;
+
+use super::audio_buffer::AudioBuffer;
+
+/// The mixer connects several upstream audio ports (i.e., USB audio device interface) to one
+/// downstream audio port.
+///
+/// The audio from the upstream ports is mixed and forwarded to the downstream port. The audio from
+/// the downstream port (i.e., microphone) is forwarded to all upstream ports.
+pub struct Mixer {
+    usb1: AudioPort,
+    amp: AudioPort,
+}
+
+impl Mixer {
+    /// Creates a new mixer for the specified audio ports.
+    pub fn new(usb1: AudioPort, amp: AudioPort) -> Mixer {
+        Mixer { usb1, amp }
+    }
+
+    /// Checks whether data can be read from the audio ports and mixes/forwards it as required.
+    ///
+    /// The function returns true if any data was sent to any audio port.
+    pub fn poll(&mut self) -> bool {
+        interrupt::free(|cs| {
+            if self.usb1.in_.can_read(cs) {
+                // TODO
+            }
+            if self.amp.in_.can_read(cs) {
+                // TODO
+            }
+            // TODO
+        });
+        false
+    }
+}
+
+/// The connection between an audio source/sink and the mixer.
+///
+/// The connection is made via two buffers (one for input, one for output).
+pub struct AudioPort {
+    // TODO: Fields and functions to set the volume and to mute the port.
+    in_: &'static AudioBuffer,
+    out: &'static AudioBuffer,
+}
+
+impl AudioPort {
+    /// Creates a new audio port.
+    ///
+    /// The port is initially muted and set to maximum volume.
+    pub fn new(in_: &'static AudioBuffer, out: &'static AudioBuffer) -> AudioPort {
+        AudioPort { in_, out }
+    }
+}