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fix(alsa): fall back to hw:CARD=<name>,DEV=<n> when aliased id can't open

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Minimal distros (the reporter's Raspberry Pi OS case in issue #331) ship
a trimmed /etc/asound.conf that defines only `!default` and doesn't pull
in the broader alias set most desktop distros provide. A card like the
HifiBerry Digi2 Pro — S/PDIF-only, so its selectable PCM is
`iec958:CARD=sndrpihifiberry,DEV=0` — then fails to open even though
the card is present and usable via the raw `hw:` device.

When CPAL's primary lookup by id returns None AND /proc/asound confirms
the card exists, rebuild the id as `hw:CARD=<name>,DEV=<n>` and retry
the lookup. The raw `hw:` PCM is exposed by the kernel driver for every
registered card, so it's a safe last resort — same card, same channel
routing, just skipping whichever userspace alias layer was missing.

Log both the alias failure and the fallback at warn/info so users can
see what happened without needing to diagnose ALSA config by hand.
Original error path is preserved when no fallback is possible (unknown
id shape, card not in /proc/asound, or hw: equally unavailable).

cc
This commit is contained in:
vicrodh committed 2026-04-20 20:12:39 -06:00
1 parent 05f0d69fcb
commit 927fabbd78
1 file changed
+84 -17
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crates/qbz-audio/src/alsa_backend.rs
+84 -17
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@@ -271,6 +271,33 @@ fn find_card_number_by_name(short_name: &str) -> Option<String> {
.map(|c| c.number.clone())
}
/// Build a `hw:CARD=<name>,DEV=<n>` fallback id from an aliased device id.
/// Returns None for ids that don't carry a `CARD=<name>,DEV=<n>` shape
/// (e.g. `default`, `hw:0,0`, unknown formats).
///
/// The raw `hw:` PCM is defined by the kernel driver for every card the
/// system lists in /proc/asound, which makes it a safe last-resort when
/// higher-level aliases like `iec958:` or `front:` aren't declared in the
/// user's asound.conf (issue #331 — minimal Raspberry Pi OS installs don't
/// ship a config entry for `iec958:CARD=<name>`, so the HifiBerry Digi2
/// Pro's only selectable id failed to open even though the card was
/// present and usable via `hw:`).
fn build_hw_fallback_id(device_id: &str) -> Option<String> {
if !(device_id.starts_with("front:CARD=")
|| device_id.starts_with("sysdefault:CARD=")
|| device_id.starts_with("iec958:CARD=")
|| device_id.starts_with("hdmi:CARD="))
{
return None;
}
let after_card = device_id.split("CARD=").nth(1)?;
let mut parts = after_card.splitn(2, ',');
let card_name = parts.next()?.to_string();
let dev_part = parts.next().unwrap_or("DEV=0");
let dev_num = dev_part.strip_prefix("DEV=").unwrap_or("0");
Some(format!("hw:CARD={},DEV={}", card_name, dev_num))
}
/// Extract card name from an ALSA device ID.
/// `front:CARD=C20,DEV=0` -> `"C20"`, `hw:0,0` -> card 0 short name, etc.
///
@@ -919,7 +946,8 @@ impl AudioBackend for AlsaBackend {
// don't chase ghosts wondering why their DAC "disappeared".
let device = if let Some(device_id) = &config.device_id {
log::info!("[ALSA Backend] Looking for device: {}", device_id);
self.host
let primary = self
.host
.output_devices()
.map_err(|e| format!("Failed to enumerate devices: {}", e))?
.find(|d| {
@@ -927,23 +955,62 @@ impl AudioBackend for AlsaBackend {
.ok()
.map(|desc| desc.name() == device_id.as_str())
.unwrap_or(false)
})
.ok_or_else(|| {
let proc_found = extract_card_name_from_device(device_id)
.and_then(|card| get_hw_supported_rates(&card))
.is_some();
if proc_found {
format!(
"Device '{}' is present in /proc/asound but CPAL cannot open it (usually a sample-rate/format mismatch — track rate {}Hz, or an ALSA name format mismatch). Try the plughw plugin in audio settings.",
device_id, config.sample_rate
)
} else {
format!(
"Device '{}' not found by the ALSA backend (disconnected, renamed, or handled by another app)",
device_id
)
});
// Fallback: if the primary alias didn't resolve but /proc/asound
// shows the card is present, retry with the raw hw:CARD=<name>,
// DEV=<n> PCM. The kernel driver always exposes that for any
// registered card, which covers minimal distro configs where
// iec958:/hdmi:/front: aliases aren't declared in asound.conf
// (issue #331 — HifiBerry Digi2 Pro on Raspberry Pi OS).
let resolved = match primary {
Some(d) => Some(d),
None => match build_hw_fallback_id(device_id) {
Some(hw_id)
if extract_card_name_from_device(device_id)
.and_then(|card| get_hw_supported_rates(&card))
.is_some() =>
{
log::warn!(
"[ALSA Backend] '{}' not resolvable by ALSA (alias likely missing in asound.conf); trying fallback '{}'",
device_id,
hw_id
);
let found = self
.host
.output_devices()
.map_err(|e| format!("Failed to enumerate devices: {}", e))?
.find(|d| {
d.description()
.ok()
.map(|desc| desc.name() == hw_id.as_str())
.unwrap_or(false)
});
if found.is_some() {
log::info!("[ALSA Backend] Using fallback device: {}", hw_id);
}
found
}
})?
_ => None,
},
};
resolved.ok_or_else(|| {
let proc_found = extract_card_name_from_device(device_id)
.and_then(|card| get_hw_supported_rates(&card))
.is_some();
if proc_found {
format!(
"Device '{}' is present in /proc/asound but CPAL cannot open it (usually a sample-rate/format mismatch — track rate {}Hz, or an ALSA name format mismatch). Try the plughw plugin in audio settings.",
device_id, config.sample_rate
)
} else {
format!(
"Device '{}' not found by the ALSA backend (disconnected, renamed, or handled by another app)",
device_id
)
}
})?
} else {
log::info!("[ALSA Backend] Using default device");
self.host