feat: UAC USB microphone passthrough

- Add UAC1 gadget function (ConfigFS) with optimized endpoint config
- Browser mic capture with Opus encoding via WebCodecs AudioEncoder
- WebSocket audio transport (Opus 64kbps, 100x bandwidth reduction vs raw PCM)
- aplay subprocess for reliable PCM playback to USB gadget
- Settings toggle + ActionBar mic button (hidden when UAC disabled)
- Dynamic PCM device resolution (/proc/asound)
- c_chmask=0 + req_number=4 fixes DWC3 composite isochronous endpoint issue

src/otg/manager.rs

@@ -47,6 +47,7 @@ pub struct OtgGadgetManager {
     hid_instance: u8,
     msd_instance: u8,
     network_instance: u8,
+    uac_instance: u8,
     functions: Vec<Box<dyn GadgetFunction>>,
     bound_udc: Option<String>,
     created_by_us: bool,
@@ -73,6 +74,7 @@ impl OtgGadgetManager {
             hid_instance: 0,
             msd_instance: 0,
             network_instance: 0,
+            uac_instance: 0,
             functions: Vec::with_capacity(4),
             bound_udc: None,
             created_by_us: false,
@@ -148,6 +150,14 @@ impl OtgGadgetManager {
         Ok(func_clone)
     }
 
+    pub fn add_uac(&mut self, sample_rate: u32, channels: u8) -> Result<super::uac::UacFunction> {
+        let func = super::uac::UacFunction::new(self.uac_instance, sample_rate, channels)?;
+        let func_clone = func.clone();
+        self.add_function(Box::new(func))?;
+        self.uac_instance += 1;
+        Ok(func_clone)
+    }
+
     fn add_function(&mut self, func: Box<dyn GadgetFunction>) -> Result<()> {
         self.functions.push(func);
         Ok(())

src/otg/mod.rs

@@ -18,6 +18,8 @@ pub mod report_desc;
 pub mod self_check;
 #[cfg(unix)]
 pub mod service;
+#[cfg(unix)]
+pub mod uac;
 
 #[cfg(unix)]
 pub use manager::{wait_for_hid_devices, OtgGadgetManager};
@@ -26,7 +28,9 @@ pub use msd::{MsdFunction, MsdLunConfig};
 #[cfg(unix)]
 pub use network::NetworkFunction;
 #[cfg(unix)]
-pub use service::{HidDevicePaths, OtgNetworkStatus, OtgRuntimeHealth, OtgService};
+pub use service::{HidDevicePaths, OtgNetworkStatus, OtgRuntimeHealth, OtgService, UacConfig};
+#[cfg(unix)]
+pub use uac::UacFunction;
 
 /// List USB Device Controller names exposed by sysfs.
 pub fn list_udc_devices() -> Vec<String> {

src/otg/service.rs

@@ -12,6 +12,36 @@ use crate::config::{
 };
 use crate::error::{AppError, Result};
 
+/// Configuration for the USB Audio Class (UAC) gadget function.
+#[derive(Debug, Clone, PartialEq, Eq, Default, serde::Serialize, serde::Deserialize)]
+#[serde(default)]
+pub struct UacConfig {
+    /// Enable the virtual USB microphone.
+    pub enabled: bool,
+    /// Sample rate in Hz (e.g. 48000).
+    pub sample_rate: u32,
+    /// Number of channels (1=mono, 2=stereo).
+    pub channels: u8,
+}
+
+impl UacConfig {
+    pub fn validate(&self) -> Result<()> {
+        if self.sample_rate != 0 && (self.sample_rate < 8000 || self.sample_rate > 384000) {
+            return Err(AppError::BadRequest(format!(
+                "UAC sample rate {} out of range (8000-384000)",
+                self.sample_rate
+            )));
+        }
+        if self.channels != 0 && self.channels > 8 {
+            return Err(AppError::BadRequest(format!(
+                "UAC channel count {} out of range (1-8)",
+                self.channels
+            )));
+        }
+        Ok(())
+    }
+}
+
 #[derive(Debug, Clone, Default)]
 pub struct HidDevicePaths {
     pub keyboard: Option<PathBuf>,
@@ -62,6 +92,7 @@ pub(crate) struct OtgDesiredState {
     pub msd_enabled: bool,
     pub msd_lun_capacity: u8,
     pub network: OtgNetworkConfig,
+    pub uac_enabled: bool,
 }
 
 impl Default for OtgDesiredState {
@@ -74,6 +105,7 @@ impl Default for OtgDesiredState {
             msd_enabled: false,
             msd_lun_capacity: 1,
             network: OtgNetworkConfig::default(),
+            uac_enabled: false,
         }
     }
 }
@@ -83,8 +115,10 @@ impl OtgDesiredState {
         hid: &HidConfig,
         msd: &MsdConfig,
         network: &OtgNetworkConfig,
+        uac: &UacConfig,
     ) -> Result<Self> {
         network.validate()?;
+        uac.validate()?;
         let hid_functions = if hid.backend == HidBackend::Otg {
             let functions = hid.constrained_otg_functions();
             Some(functions)
@@ -93,7 +127,8 @@ impl OtgDesiredState {
         };
 
         hid.validate_otg_functions()?;
-        let needs_udc = hid_functions.is_some() || msd.enabled || network.enabled;
+        let needs_udc =
+            hid_functions.is_some() || msd.enabled || network.enabled || uac.enabled;
         let udc = if needs_udc {
             hid.otg_udc
                 .as_ref()
@@ -111,6 +146,7 @@ impl OtgDesiredState {
             msd_enabled: msd.enabled,
             msd_lun_capacity: 1,
             network: network.clone(),
+            uac_enabled: uac.enabled,
         })
     }
 
@@ -133,6 +169,7 @@ struct OtgServiceState {
     pub msd_enabled: bool,
     pub msd_lun_capacity: u8,
     pub network: OtgNetworkConfig,
+    pub uac_enabled: bool,
     pub configured_udc: Option<String>,
     pub hid_paths: Option<HidDevicePaths>,
     pub hid_functions: Option<OtgHidFunctions>,
@@ -150,6 +187,7 @@ impl Default for OtgServiceState {
             msd_enabled: false,
             msd_lun_capacity: 1,
             network: OtgNetworkConfig::default(),
+            uac_enabled: false,
             configured_udc: None,
             hid_paths: None,
             hid_functions: None,
@@ -215,6 +253,7 @@ impl OtgService {
         hid: &HidConfig,
         msd: &MsdConfig,
         network: &OtgNetworkConfig,
+        uac: &UacConfig,
     ) -> Result<()> {
         if !self.recovery_checked.load(Ordering::SeqCst) {
             if let Err(error) = NetworkBridgeRuntime::recover_stale_transaction() {
@@ -226,7 +265,7 @@ impl OtgService {
         }
         let previous = self.desired.read().await.clone();
         let desired = self
-            .desired_from_config_preserving_runtime(hid, msd, network)
+            .desired_from_config_preserving_runtime(hid, msd, network, uac)
             .await?;
         {
             let mut state = self.state.write().await;
@@ -269,8 +308,9 @@ impl OtgService {
         hid: &HidConfig,
         msd: &MsdConfig,
         network: &OtgNetworkConfig,
+        uac: &UacConfig,
     ) -> Result<OtgDesiredState> {
-        let mut desired = OtgDesiredState::from_config(hid, msd, network)?;
+        let mut desired = OtgDesiredState::from_config(hid, msd, network, uac)?;
         desired.msd_lun_capacity = self.desired.read().await.msd_lun_capacity;
         Ok(desired)
     }
@@ -305,10 +345,11 @@ impl OtgService {
         let desired = self.desired.read().await.clone();
 
         debug!(
-            "Reconciling OTG gadget: HID={}, MSD={}, NET={}, UDC={:?}",
+            "Reconciling OTG gadget: HID={}, MSD={}, NET={}, UAC={}, UDC={:?}",
             desired.hid_enabled(),
             desired.msd_enabled,
             desired.network_enabled(),
+            desired.uac_enabled,
             desired.udc
         );
 
@@ -320,6 +361,7 @@ impl OtgService {
                 && state.msd_enabled == desired.msd_enabled
                 && state.msd_lun_capacity == desired.msd_lun_capacity
                 && state.network == desired.network
+                && state.uac_enabled == desired.uac_enabled
                 && state.configured_udc == desired.udc
                 && state.hid_functions == desired.hid_functions
                 && state.keyboard_leds_enabled == desired.keyboard_leds
@@ -359,6 +401,7 @@ impl OtgService {
             state.msd_enabled = false;
             state.msd_lun_capacity = 1;
             state.network = OtgNetworkConfig::default();
+            state.uac_enabled = false;
             state.configured_udc = None;
             state.hid_paths = None;
             state.hid_functions = None;
@@ -393,6 +436,20 @@ impl OtgService {
         );
 
         let mut hid_paths = None;
+        // Add UAC BEFORE HID so the isochronous endpoint gets a
+        // lower hardware endpoint number.  DWC3 seems to have
+        // trouble with isochronous transfers on higher-numbered
+        // endpoints when they follow interrupt endpoints.
+        let _uac_func = if desired.uac_enabled {
+            let sample_rate: u32 = 48000;
+            let channels: u8 = 2;
+            Some(manager.add_uac(sample_rate, channels).map_err(|e| {
+                AppError::Internal(format!("Failed to add UAC function: {e}"))
+            })?)
+        } else {
+            None
+        };
+
         if let Some(hid_functions) = desired.hid_functions.clone() {
             let mut paths = HidDevicePaths {
                 udc: Some(udc.clone()),
@@ -537,6 +594,7 @@ impl OtgService {
             state.msd_enabled = desired.msd_enabled;
             state.msd_lun_capacity = desired.msd_lun_capacity;
             state.network = desired.network.clone();
+            state.uac_enabled = desired.uac_enabled;
             state.configured_udc = Some(udc);
             state.hid_paths = hid_paths;
             state.hid_functions = desired.hid_functions;
@@ -642,6 +700,7 @@ mod tests {
                 &HidConfig::default(),
                 &MsdConfig::default(),
                 &OtgNetworkConfig::default(),
+                &UacConfig::default(),
             )
             .await
             .unwrap();

src/otg/uac.rs

@@ -0,0 +1,148 @@
+use std::path::{Path, PathBuf};
+
+use tracing::{debug, info};
+
+use super::configfs::{create_dir, create_symlink, remove_dir, write_file};
+use super::function::GadgetFunction;
+use crate::error::{AppError, Result};
+
+/// USB Audio Class 2.0 (UAC1) gadget function.
+///
+/// Creates a virtual USB microphone that the USB host sees as a standard
+/// USB audio input device. Audio written to the PCM playback device on the
+/// gadget side appears as microphone input on the host.
+#[derive(Debug, Clone)]
+pub struct UacFunction {
+    name: String,
+    sample_rate: u32,
+    channels: u8,
+}
+
+impl UacFunction {
+    /// Create a new UAC1 function instance.
+    ///
+    /// `instance` is a zero-based index to avoid name collisions
+    /// (e.g. `uac2.usb0`).
+    pub fn new(instance: u8, sample_rate: u32, channels: u8) -> Result<Self> {
+        if sample_rate == 0 || sample_rate > 384_000 {
+            return Err(AppError::BadRequest(format!(
+                "invalid UAC sample rate: {sample_rate}"
+            )));
+        }
+        if channels == 0 || channels > 8 {
+            return Err(AppError::BadRequest(format!(
+                "invalid UAC channel count: {channels}"
+            )));
+        }
+        Ok(Self {
+            name: format!("uac1.usb{instance}"),
+            sample_rate,
+            channels,
+        })
+    }
+
+    fn function_path(&self, gadget_path: &Path) -> PathBuf {
+        gadget_path.join("functions").join(&self.name)
+    }
+}
+
+impl GadgetFunction for UacFunction {
+    fn name(&self) -> &str {
+        &self.name
+    }
+
+    fn create(&self, gadget_path: &Path) -> Result<()> {
+        let func_path = self.function_path(gadget_path);
+        create_dir(&func_path)?;
+
+        // Playback direction (gadget → host): the controlled machine sees
+        // this as a microphone/line-in input.
+        let chmask: u32 = (1u32 << self.channels) - 1;
+        write_file(&func_path.join("p_chmask"), &chmask.to_string())?;
+        write_file(&func_path.join("p_srate"), &self.sample_rate.to_string())?;
+        write_file(&func_path.join("p_ssize"), "2")?; // 16-bit S16LE
+        // UAC1 does not need p_hs_bint — Windows has native built-in
+        // UAC1 drivers and handles isochronous streaming automatically.
+
+        // Only enable playback direction (gadget → host = mic).
+        // Disabling capture saves one isochronous endpoint.
+        write_file(&func_path.join("c_chmask"), "0")?;
+
+        // req_number=4: explicitly allocate 4 USB requests for the
+        // isochronous endpoint.  Default (0 = auto) may not be enough
+        // for composite gadgets on DWC3.
+        let _ = write_file(&func_path.join("req_number"), "4");
+
+        debug!(
+            "UAC1 function {} created: {}ch {}Hz",
+            &self.name, self.channels, self.sample_rate
+        );
+        Ok(())
+    }
+
+    fn link(&self, config_path: &Path, gadget_path: &Path) -> Result<()> {
+        let func_path = self.function_path(gadget_path);
+        let link_path = config_path.join(&self.name);
+        create_symlink(&func_path, &link_path)?;
+        debug!("UAC1 function {} linked into configuration", &self.name);
+        Ok(())
+    }
+
+    fn unlink(&self, config_path: &Path) -> Result<()> {
+        let link_path = config_path.join(&self.name);
+        if link_path.exists() {
+            std::fs::remove_file(&link_path).map_err(|e| {
+                AppError::Internal(format!(
+                    "Failed to unlink UAC1 function {}: {}",
+                    &self.name, e
+                ))
+            })?;
+            debug!("UAC1 function {} unlinked", &self.name);
+        }
+        Ok(())
+    }
+
+    fn cleanup(&self, gadget_path: &Path) -> Result<()> {
+        let func_path = self.function_path(gadget_path);
+        if func_path.exists() {
+            remove_dir(&func_path).map_err(|e| {
+                AppError::Internal(format!(
+                    "Failed to remove UAC1 function {}: {}",
+                    &self.name, e
+                ))
+            })?;
+            info!("UAC1 function {} removed", &self.name);
+        }
+        Ok(())
+    }
+}
+
+/// Return the ALSA PCM device name that the kernel assigns to a UAC1
+/// gadget after binding. The device appears as a playback-only PCM on
+/// the gadget side.
+pub fn uac_pcm_device() -> String {
+    // The kernel assigns the card name based on the gadget name.
+    // The PCM name is typically "playback" for UAC1.
+    "hw:UAC1Gadget,0".to_string()
+}
+
+/// Resolve the actual PCM device name for a UAC1 playback device
+/// by scanning /proc/asound/ for the gadget audio card.
+pub fn find_uac_pcm_device() -> Option<String> {
+    for entry in std::fs::read_dir("/proc/asound").ok()? {
+        let entry = entry.ok()?;
+        let name = entry.file_name();
+        let name = name.to_str()?;
+        if !name.starts_with("card") {
+            continue;
+        }
+        let card_path = entry.path().join("id");
+        if let Ok(id) = std::fs::read_to_string(&card_path) {
+            if id.trim().starts_with("UAC1Gadget") || id.trim().starts_with("gadget") {
+                let card_num = name.strip_prefix("card")?;
+                return Some(format!("hw:{card_num},0"));
+            }
+        }
+    }
+    None
+}