feat(audio): implement controlled hw→plughw fallback for ALSA Direct
- Add AlsaDirectError enum for structured error classification - Add BitPerfectMode enum to track runtime bit-perfect status - Implement automatic fallback: hw fails with format error → try plughw - Abort on non-recoverable errors (busy, permissions, invalid params) - Respect ALSA plugin selection from UI settings - Log runtime mode for debugging This allows SMSL-class DACs that fail on hw: to work via plughw: while maintaining bit-perfect guarantees (format conversion only, no resampling). cc
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@@ -411,31 +411,101 @@ impl AlsaBackend {
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/// Try to create direct ALSA stream for hw: devices (bypasses CPAL)
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/// Returns None if device is not a hw: device (should use CPAL instead)
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///
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/// Implements controlled fallback:
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/// 1. Try direct hw access first
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/// 2. If format unsupported, try plughw (format conversion only, no resampling)
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/// 3. Abort on other errors (busy, permissions, etc.)
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pub fn try_create_direct_stream(
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&self,
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config: &BackendConfig,
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) -> Option<Result<super::AlsaDirectStream, String>> {
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) -> Option<Result<(super::AlsaDirectStream, super::backend::BitPerfectMode), String>> {
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let device_id = config.device_id.as_ref()?;
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// Only use direct ALSA for hw: and plughw: devices
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// Only use direct ALSA for hw:/plughw:/front: devices
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if !super::AlsaDirectStream::is_hw_device(device_id) {
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log::info!("[ALSA Backend] Device '{}' is not hw:/plughw:, using CPAL", device_id);
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log::info!("[ALSA Backend] Device '{}' is not hw:/plughw:/front:, using CPAL", device_id);
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return None;
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}
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// Determine the base device path for hw/plughw attempts
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// front:CARD=X,DEV=Y -> extract card name for hw attempts
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let (hw_device, plughw_device) = if device_id.starts_with("front:CARD=") {
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// front:CARD=AMP,DEV=0 -> need to find corresponding hw:X,0
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// For now, try the front: device directly as it's already hardware-direct
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(device_id.to_string(), format!("plug:{}", device_id))
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} else if device_id.starts_with("hw:") {
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(device_id.to_string(), device_id.replace("hw:", "plughw:"))
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} else if device_id.starts_with("plughw:") {
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// Already plughw, try it directly
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(device_id.replace("plughw:", "hw:"), device_id.to_string())
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} else {
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(device_id.to_string(), format!("plug:{}", device_id))
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};
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// Respect ALSA plugin selection from settings
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let try_hw_first = match config.alsa_plugin {
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Some(AlsaPlugin::Hw) => true,
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Some(AlsaPlugin::PlugHw) => false, // Skip hw, go directly to plughw
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Some(AlsaPlugin::Pcm) => {
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log::info!("[ALSA Backend] PCM mode selected, not using direct ALSA");
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return None; // Use CPAL instead
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}
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None => true, // Default: try hw first
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};
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if try_hw_first {
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log::info!(
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"[ALSA Backend] Attempting DIRECT hw stream: {} ({}Hz, {}ch)",
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hw_device,
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config.sample_rate,
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config.channels
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);
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match super::AlsaDirectStream::new(&hw_device, config.sample_rate, config.channels) {
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Ok(stream) => {
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log::info!("[ALSA Backend] ✓ Direct hw stream created successfully");
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return Some(Ok((stream, super::backend::BitPerfectMode::DirectHardware)));
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}
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Err(e) => {
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let error = super::backend::AlsaDirectError::from_alsa_error(&e);
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log::warn!("[ALSA Backend] hw attempt failed: {}", error);
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if !error.allows_plughw_fallback() {
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// Non-recoverable error (busy, permissions, etc.)
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log::error!("[ALSA Backend] Cannot fallback - error type: {:?}", error);
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return Some(Err(format!(
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"ALSA Direct failed: {}. Device may be in use or inaccessible.",
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error
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)));
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}
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log::info!("[ALSA Backend] Format unsupported on hw, trying plughw fallback...");
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}
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}
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}
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// Try plughw fallback (format conversion only)
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log::info!(
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"[ALSA Backend] Creating DIRECT ALSA stream for hw: device: {} ({}Hz, {}ch)",
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device_id,
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"[ALSA Backend] Attempting plughw stream: {} ({}Hz, {}ch)",
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plughw_device,
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config.sample_rate,
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config.channels
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);
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// Create direct ALSA stream (bypasses rodio/CPAL)
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Some(super::AlsaDirectStream::new(
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device_id,
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config.sample_rate,
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config.channels,
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))
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match super::AlsaDirectStream::new(&plughw_device, config.sample_rate, config.channels) {
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Ok(stream) => {
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log::info!("[ALSA Backend] ✓ plughw stream created (bit-perfect with format conversion)");
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Some(Ok((stream, super::backend::BitPerfectMode::PluginFallback)))
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}
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Err(e) => {
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log::error!("[ALSA Backend] plughw fallback also failed: {}", e);
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Some(Err(format!(
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"Bit-perfect playback could not be established. hw failed, plughw failed: {}",
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e
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)))
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}
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}
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}
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}
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@@ -117,6 +117,91 @@ pub struct BackendConfig {
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/// Result type for backend operations
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pub type BackendResult<T> = Result<T, String>;
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/// ALSA Direct stream error classification
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/// Used to determine if fallback to plughw is appropriate
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#[derive(Debug, Clone)]
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pub enum AlsaDirectError {
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/// PCM format not supported by hardware (can fallback to plughw)
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UnsupportedFormat(String),
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/// Device is busy/in use by another application
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DeviceBusy(String),
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/// Permission denied to access device
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PermissionDenied(String),
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/// Invalid parameters (channels, sample rate)
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InvalidParams(String),
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/// Device not found
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DeviceNotFound(String),
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/// Generic/unknown error
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Other(String),
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}
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impl std::fmt::Display for AlsaDirectError {
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fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
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match self {
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AlsaDirectError::UnsupportedFormat(msg) => write!(f, "Unsupported format: {}", msg),
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AlsaDirectError::DeviceBusy(msg) => write!(f, "Device busy: {}", msg),
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AlsaDirectError::PermissionDenied(msg) => write!(f, "Permission denied: {}", msg),
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AlsaDirectError::InvalidParams(msg) => write!(f, "Invalid parameters: {}", msg),
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AlsaDirectError::DeviceNotFound(msg) => write!(f, "Device not found: {}", msg),
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AlsaDirectError::Other(msg) => write!(f, "{}", msg),
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}
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}
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}
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impl AlsaDirectError {
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/// Check if this error allows fallback to plughw
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pub fn allows_plughw_fallback(&self) -> bool {
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matches!(self, AlsaDirectError::UnsupportedFormat(_))
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}
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/// Create from raw ALSA error message
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pub fn from_alsa_error(msg: &str) -> Self {
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let msg_lower = msg.to_lowercase();
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if msg_lower.contains("no supported audio format")
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|| msg_lower.contains("format")
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|| msg_lower.contains("s24_3le")
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|| msg_lower.contains("s24le")
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|| msg_lower.contains("sample format")
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{
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AlsaDirectError::UnsupportedFormat(msg.to_string())
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} else if msg_lower.contains("busy")
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|| msg_lower.contains("resource temporarily unavailable")
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|| msg_lower.contains("device or resource busy")
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{
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AlsaDirectError::DeviceBusy(msg.to_string())
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} else if msg_lower.contains("permission")
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|| msg_lower.contains("access denied")
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|| msg_lower.contains("operation not permitted")
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{
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AlsaDirectError::PermissionDenied(msg.to_string())
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} else if msg_lower.contains("not found")
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|| msg_lower.contains("no such")
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|| msg_lower.contains("doesn't exist")
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{
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AlsaDirectError::DeviceNotFound(msg.to_string())
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} else if msg_lower.contains("invalid")
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|| msg_lower.contains("channels")
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|| msg_lower.contains("rate")
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{
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AlsaDirectError::InvalidParams(msg.to_string())
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} else {
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AlsaDirectError::Other(msg.to_string())
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}
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}
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}
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/// Runtime mode for bit-perfect status tracking
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#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
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pub enum BitPerfectMode {
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/// Direct hardware access (hw:), guaranteed bit-perfect
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DirectHardware,
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/// Plugin hardware fallback (plughw:), bit-perfect with format conversion only
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PluginFallback,
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/// Not using bit-perfect path (pcm, pipewire, pulse)
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Disabled,
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}
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/// Audio backend trait
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///
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/// All audio backends must implement this trait to provide
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@@ -451,7 +451,10 @@ fn try_init_stream_with_backend(
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// Downcast backend to AlsaBackend to access try_create_direct_stream
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if let Some(alsa_backend) = backend.as_any().downcast_ref::<crate::audio::alsa_backend::AlsaBackend>() {
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if let Some(result) = alsa_backend.try_create_direct_stream(&config) {
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return Some(result.map(|stream| StreamType::AlsaDirect(Arc::new(stream))));
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return Some(result.map(|(stream, mode)| {
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log::info!("ALSA Direct stream created with mode: {:?}", mode);
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StreamType::AlsaDirect(Arc::new(stream))
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}));
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}
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}
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}
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