Fix surround sound channel mapping on ALSA

app/streaming/audio/audio.cpp

@@ -59,23 +59,27 @@ int Session::arInit(int /* audioConfiguration */,
 
     SDL_memcpy(&s_ActiveSession->m_AudioConfig, opusConfig, sizeof(*opusConfig));
 
+    s_ActiveSession->m_AudioRenderer = s_ActiveSession->createAudioRenderer();
+    if (s_ActiveSession->m_AudioRenderer == nullptr) {
+        return -1;
+    }
+
+    // Allow the audio renderer to adjust the channel mapping to fit its
+    // preferred channel order
+    s_ActiveSession->m_AudioRenderer->adjustOpusChannelMapping(&s_ActiveSession->m_AudioConfig);
+
     s_ActiveSession->m_OpusDecoder =
             opus_multistream_decoder_create(opusConfig->sampleRate,
                                             opusConfig->channelCount,
                                             opusConfig->streams,
                                             opusConfig->coupledStreams,
-                                            opusConfig->mapping,
+                                            s_ActiveSession->m_AudioConfig.mapping,
                                             &error);
     if (s_ActiveSession->m_OpusDecoder == NULL) {
+        delete s_ActiveSession->m_AudioRenderer;
         SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,
                      "Failed to create decoder: %d",
                      error);
-        return -1;
-    }
-
-    s_ActiveSession->m_AudioRenderer = s_ActiveSession->createAudioRenderer();
-    if (s_ActiveSession->m_AudioRenderer == nullptr) {
-        opus_multistream_decoder_destroy(s_ActiveSession->m_OpusDecoder);
         return -2;
     }
 

app/streaming/audio/renderers/portaudiorenderer.cpp

@@ -194,6 +194,42 @@ int PortAudioRenderer::detectAudioConfiguration() const
     }
 }
 
+void PortAudioRenderer::adjustOpusChannelMapping(OPUS_MULTISTREAM_CONFIGURATION* opusConfig) const
+{
+    const OPUS_MULTISTREAM_CONFIGURATION origConfig = *opusConfig;
+
+    const PaDeviceInfo* deviceInfo = Pa_GetDeviceInfo(Pa_GetDefaultOutputDevice());
+    if (deviceInfo == nullptr) {
+        SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,
+                     "Pa_GetDeviceInfo() failed");
+        return;
+    }
+
+    const PaHostApiInfo* hostApiInfo = Pa_GetHostApiInfo(deviceInfo->hostApi);
+    if (hostApiInfo == nullptr) {
+        SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,
+                     "Pa_GetHostApiInfo() failed");
+        return;
+    }
+
+    SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION,
+                "PortAudio host API: %s",
+                hostApiInfo->name);
+
+    if (hostApiInfo->type == paALSA) {
+        // The default mapping array has is: FL-FR-C-LFE-RL-RR
+        // ALSA expects: FL-FR-RL-RR-C-LFE
+        opusConfig->mapping[0] = origConfig.mapping[0];
+        opusConfig->mapping[1] = origConfig.mapping[1];
+        if (opusConfig->channelCount == 6) {
+            opusConfig->mapping[2] = origConfig.mapping[4];
+            opusConfig->mapping[3] = origConfig.mapping[5];
+            opusConfig->mapping[4] = origConfig.mapping[2];
+            opusConfig->mapping[5] = origConfig.mapping[3];
+        }
+    }
+}
+
 int PortAudioRenderer::paStreamCallback(const void*, void* output, unsigned long frameCount, const PaStreamCallbackTimeInfo*, PaStreamCallbackFlags, void* userData)
 {
     auto me = reinterpret_cast<PortAudioRenderer*>(userData);

app/streaming/audio/renderers/portaudiorenderer.h

@@ -24,6 +24,8 @@ public:
 
     virtual int detectAudioConfiguration() const;
 
+    virtual void adjustOpusChannelMapping(OPUS_MULTISTREAM_CONFIGURATION* opusConfig) const;
+
     static int paStreamCallback(const void *input,
                                 void *output,
                                 unsigned long frameCount,

app/streaming/audio/renderers/renderer.h

@@ -10,6 +10,8 @@ class IAudioRenderer
 public:
     virtual ~IAudioRenderer() {}
 
+    virtual void adjustOpusChannelMapping(OPUS_MULTISTREAM_CONFIGURATION* opusConfig) const = 0;
+
     virtual bool prepareForPlayback(const OPUS_MULTISTREAM_CONFIGURATION* opusConfig) = 0;
 
     virtual void submitAudio(short* audioBuffer, int audioSize) = 0;

app/streaming/audio/renderers/sdl.h

@@ -23,6 +23,8 @@ public:
 
     virtual int detectAudioConfiguration() const;
 
+    virtual void adjustOpusChannelMapping(OPUS_MULTISTREAM_CONFIGURATION* opusConfig) const;
+
 private:
     SDL_AudioDeviceID m_AudioDevice;
     Uint32 m_ChannelCount;

app/streaming/audio/renderers/sdlaud.cpp

@@ -51,6 +51,11 @@ int SdlAudioRenderer::detectAudioConfiguration() const
     }
 }
 
+void SdlAudioRenderer::adjustOpusChannelMapping(OPUS_MULTISTREAM_CONFIGURATION*) const
+{
+    // The default mapping is fine for SDL
+}
+
 bool SdlAudioRenderer::testAudio(int audioConfiguration) const
 {
     SDL_AudioSpec want, have;