From cba5b76bb3896a780f68b71cc1c0e31172b5b3e3 Mon Sep 17 00:00:00 2001
From: vicrodh <103399283+vicrodh@users.noreply.github.com>
Date: Mon, 13 Apr 2026 19:17:59 -0600
Subject: [PATCH] fix(audio): fallback to plughw instead of CPAL when DAC rate
 unsupported (#288)
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When /proc/asound reports that the selected hw: device does not
natively support the track's sample rate (e.g., 96 kHz track on a
48 kHz-only Vanatoo Transparent Zero), the previous code returned
None from try_create_direct_stream. The caller then dispatched to
CPAL's create_output_stream, where the CPAL ALSA host could not
locate the device by its /proc/asound name and failed with the
misleading error 'Device not found'.

The same function already has a plughw: fallback path further down
that wraps the hw device with ALSA's plug plugin — the latter
resamples in ALSA space and is the correct behavior for rate
mismatches. This change makes the /proc/asound pre-check set a
local flag that skips the hw attempt, falling through to the
existing plughw branch instead of returning None.

Sound now comes out. Bit-perfect is lost (by necessity) and the
mode reported back to the player is PluginFallback, which lets the
UI surface the degradation (wired up in Fix 4).

cc
---
 crates/qbz-audio/src/alsa_backend.rs | 34 +++++++++++++++++++---------
 1 file changed, 23 insertions(+), 11 deletions(-)

diff --git a/crates/qbz-audio/src/alsa_backend.rs b/crates/qbz-audio/src/alsa_backend.rs
index f7e7976af..f73f40d50 100644
--- a/crates/qbz-audio/src/alsa_backend.rs
+++ b/crates/qbz-audio/src/alsa_backend.rs
@@ -564,33 +564,45 @@ impl AlsaBackend {
         // before opening the PCM device. This avoids the "device busy" issue
         // where ALSA Direct opens the device, fails to configure the rate, and
         // leaves the device in a state CPAL can't open afterwards.
+        //
+        // When the rate is unsupported, we skip the hw: attempt and fall through
+        // to the plughw: path below, which lets ALSA auto-resample in kernel/
+        // userspace. Falling back to CPAL is not an option because CPAL's device
+        // enumeration does not reliably expose raw hw: devices by the same name
+        // the UI stored (front:CARD=X,DEV=Y vs hw:CARD=X,DEV=Y), and the lookup
+        // fails with a misleading "Device not found" error. See issue #288.
+        let mut hw_rate_unsupported = false;
         if let Some(card_name) = extract_card_name_from_device(device_id) {
             if let Some(hw_rates) = get_hw_supported_rates(&card_name) {
                 if !hw_rates.contains(&config.sample_rate) {
                     log::info!(
-                        "[ALSA Backend] Hardware rates for '{}': {:?}. {}Hz not supported, skipping ALSA Direct",
+                        "[ALSA Backend] Hardware rates for '{}': {:?}. {}Hz not supported natively — falling back to plughw for software resample",
                         card_name, hw_rates, config.sample_rate
                     );
-                    return None;
+                    hw_rate_unsupported = true;
+                } else {
+                    log::info!(
+                        "[ALSA Backend] Hardware confirms support for {}Hz (card '{}', rates: {:?})",
+                        config.sample_rate,
+                        card_name,
+                        hw_rates
+                    );
                 }
-                log::info!(
-                    "[ALSA Backend] Hardware confirms support for {}Hz (card '{}', rates: {:?})",
-                    config.sample_rate,
-                    card_name,
-                    hw_rates
-                );
             }
         }
 
-        // Respect ALSA plugin selection from settings
+        // Respect ALSA plugin selection from settings. When /proc/asound already
+        // told us the hw device won't accept the rate, skip straight to plughw
+        // even if the user prefers Hw — plughw with resample is the only path
+        // that produces sound.
         let try_hw_first = match config.alsa_plugin {
-            Some(AlsaPlugin::Hw) => true,
+            Some(AlsaPlugin::Hw) => !hw_rate_unsupported,
             Some(AlsaPlugin::PlugHw) => false, // Skip hw, go directly to plughw
             Some(AlsaPlugin::Pcm) => {
                 log::info!("[ALSA Backend] PCM mode selected, not using direct ALSA");
                 return None; // Use CPAL instead
             }
-            None => true, // Default: try hw first
+            None => !hw_rate_unsupported, // Default: try hw first if rate is supported
         };
 
         if try_hw_first {