audio: Allow streams to change the device-side channels maps.

Fixes #11881.

include/SDL3/SDL_audio.h

@@ -942,7 +942,7 @@ extern SDL_DECLSPEC void SDLCALL SDL_CloseAudioDevice(SDL_AudioDeviceID devid);
  * Binding a stream to a device will set its output format for playback
  * devices, and its input format for recording devices, so they match the
  * device's settings. The caller is welcome to change the other end of the
- * stream's format at any time.
+ * stream's format at any time with SDL_SetAudioStreamFormat().
  *
  * \param devid an audio device to bind a stream to.
  * \param streams an array of audio streams to bind.
@@ -1104,6 +1104,12 @@ extern SDL_DECLSPEC bool SDLCALL SDL_GetAudioStreamFormat(SDL_AudioStream *strea
  * next sound file, and start putting that new data while the previous sound
  * file is still queued, and everything will still play back correctly.
  *
+ * If a stream is bound to a device, then the format of the side of the stream
+ * bound to a device cannot be changed (src_spec for recording devices,
+ * dst_spec for playback devices). Attempts to make a change to this side
+ * will be ignored, but this will not report an error. The other side's format
+ * can be changed.
+ *
  * \param stream the stream the format is being changed.
  * \param src_spec the new format of the audio input; if NULL, it is not
  *                 changed.
@@ -1298,6 +1304,11 @@ extern SDL_DECLSPEC int * SDLCALL SDL_GetAudioStreamOutputChannelMap(SDL_AudioSt
  * race condition hasn't changed the format while this call is setting the
  * channel map.
  *
+ * Unlike attempting to change the stream's format, the input channel map on a
+ * stream bound to a recording device is permitted to change at any time; any
+ * data added to the stream from the device after this call will have the new
+ * mapping, but previously-added data will still have the prior mapping.
+ *
  * \param stream the SDL_AudioStream to change.
  * \param chmap the new channel map, NULL to reset to default.
  * \param count The number of channels in the map.
@@ -1349,6 +1360,13 @@ extern SDL_DECLSPEC bool SDLCALL SDL_SetAudioStreamInputChannelMap(SDL_AudioStre
  * race condition hasn't changed the format while this call is setting the
  * channel map.
  *
+ * Unlike attempting to change the stream's format, the output channel map on
+ * a stream bound to a recording device is permitted to change at any time;
+ * any data added to the stream after this call will have the new mapping, but
+ * previously-added data will still have the prior mapping. When the channel
+ * map doesn't match the hardware's channel layout, SDL will convert the data
+ * before feeding it to the device for playback.
+ *
  * \param stream the SDL_AudioStream to change.
  * \param chmap the new channel map, NULL to reset to default.
  * \param count The number of channels in the map.

src/audio/SDL_audio.c

@@ -281,6 +281,18 @@ bool SDL_AudioSpecsEqual(const SDL_AudioSpec *a, const SDL_AudioSpec *b, const i
     return true;
 }
 
+bool SDL_AudioChannelMapsEqual(int channels, const int *channel_map_a, const int *channel_map_b)
+{
+    if (channel_map_a == channel_map_b) {
+        return true;
+    } else if ((channel_map_a != NULL) != (channel_map_b != NULL)) {
+        return false;
+    } else if (channel_map_a && (SDL_memcmp(channel_map_a, channel_map_b, sizeof (*channel_map_a) * channels) != 0)) {
+        return false;
+    }
+    return true;
+}
+
 
 // Zombie device implementation...
 
@@ -1134,7 +1146,7 @@ bool SDL_PlaybackAudioThreadIterate(SDL_AudioDevice *device)
             SDL_AudioStream *stream = logdev->bound_streams;
 
             // We should have updated this elsewhere if the format changed!
-            SDL_assert(SDL_AudioSpecsEqual(&stream->dst_spec, &device->spec, stream->dst_chmap, device->chmap));
+            SDL_assert(SDL_AudioSpecsEqual(&stream->dst_spec, &device->spec, NULL, NULL));
 
             const int br = SDL_GetAtomicInt(&logdev->paused) ? 0 : SDL_GetAudioStreamDataAdjustGain(stream, device_buffer, buffer_size, logdev->gain);
             if (br < 0) {  // Probably OOM. Kill the audio device; the whole thing is likely dying soon anyhow.
@@ -1143,6 +1155,12 @@ bool SDL_PlaybackAudioThreadIterate(SDL_AudioDevice *device)
             } else if (br < buffer_size) {
                 SDL_memset(device_buffer + br, device->silence_value, buffer_size - br);  // silence whatever we didn't write to.
             }
+
+            // generally channel maps will line up, but if the audio stream's chmap has been explicitly changed, do a final swizzle to device layout.
+            if ((br > 0) && (!SDL_AudioChannelMapsEqual(device->spec.channels, stream->dst_chmap, device->chmap))) {
+                ConvertAudio(br / SDL_AUDIO_FRAMESIZE(device->spec), device_buffer, device->spec.format, device->spec.channels, NULL,
+                             device_buffer, device->spec.format, device->spec.channels, device->chmap, NULL, 1.0f);
+            }
         } else {  // need to actually mix (or silence the buffer)
             float *final_mix_buffer = (float *) ((device->spec.format == SDL_AUDIO_F32) ? device_buffer : device->mix_buffer);
             const int needed_samples = buffer_size / SDL_AUDIO_BYTESIZE(device->spec.format);
@@ -1170,7 +1188,7 @@ bool SDL_PlaybackAudioThreadIterate(SDL_AudioDevice *device)
 
                 for (SDL_AudioStream *stream = logdev->bound_streams; stream; stream = stream->next_binding) {
                     // We should have updated this elsewhere if the format changed!
-                    SDL_assert(SDL_AudioSpecsEqual(&stream->dst_spec, &outspec, stream->dst_chmap, device->chmap));
+                    SDL_assert(SDL_AudioSpecsEqual(&stream->dst_spec, &outspec, NULL, NULL));
 
                     /* this will hold a lock on `stream` while getting. We don't explicitly lock the streams
                        for iterating here because the binding linked list can only change while the device lock is held.
@@ -1181,6 +1199,11 @@ bool SDL_PlaybackAudioThreadIterate(SDL_AudioDevice *device)
                         failed = true;
                         break;
                     } else if (br > 0) {  // it's okay if we get less than requested, we mix what we have.
+                        // generally channel maps will line up, but if the audio stream's chmap has been explicitly changed, do a final swizzle to device layout.
+                        if (!SDL_AudioChannelMapsEqual(device->spec.channels, stream->dst_chmap, device->chmap)) {
+                            ConvertAudio(br / SDL_AUDIO_FRAMESIZE(device->spec), device->work_buffer, device->spec.format, device->spec.channels, NULL,
+                                         device->work_buffer, device->spec.format, device->spec.channels, device->chmap, NULL, 1.0f);
+                        }
                         MixFloat32Audio(mix_buffer, (float *) device->work_buffer, br);
                     }
                 }
@@ -1303,11 +1326,20 @@ bool SDL_RecordingAudioThreadIterate(SDL_AudioDevice *device)
                     SDL_assert(stream->src_spec.channels == device->spec.channels);
                     SDL_assert(stream->src_spec.freq == device->spec.freq);
 
+                    void *final_buf = output_buffer;
+
+                    // generally channel maps will line up, but if the audio stream's chmap has been explicitly changed, do a final swizzle to stream layout.
+                    if (!SDL_AudioChannelMapsEqual(device->spec.channels, stream->src_chmap, device->chmap)) {
+                        final_buf = device->mix_buffer;  // this is otherwise unused on recording devices, so it makes convenient scratch space here.
+                        ConvertAudio(br / SDL_AUDIO_FRAMESIZE(device->spec), output_buffer, device->spec.format, device->spec.channels, NULL,
+                                     final_buf, device->spec.format, device->spec.channels, stream->src_chmap, NULL, 1.0f);
+                    }
+
                     /* this will hold a lock on `stream` while putting. We don't explicitly lock the streams
                        for iterating here because the binding linked list can only change while the device lock is held.
                        (we _do_ lock the stream during binding/unbinding to make sure that two threads can't try to bind
                        the same stream to different devices at the same time, though.) */
-                    if (!SDL_PutAudioStreamData(stream, output_buffer, br)) {
+                    if (!SDL_PutAudioStreamData(stream, final_buf, br)) {
                         // oh crud, we probably ran out of memory. This is possibly an overreaction to kill the audio device, but it's likely the whole thing is going down in a moment anyhow.
                         failed = true;
                         break;

src/audio/SDL_audiocvt.c

@@ -154,7 +154,7 @@ static void SwizzleAudio(const int num_frames, void *dst, const void *src, int c
 {
     const int bitsize = (int) SDL_AUDIO_BITSIZE(fmt);
 
-    bool has_null_mappings = false;
+    bool has_null_mappings = false;  // !!! FIXME: calculate this when setting the channel map instead.
     for (int i = 0; i < channels; i++) {
         if (map[i] == -1) {
             has_null_mappings = true;
@@ -257,6 +257,11 @@ void ConvertAudio(int num_frames,
     const int dst_bitsize = (int) SDL_AUDIO_BITSIZE(dst_format);
     const int dst_sample_frame_size = (dst_bitsize / 8) * dst_channels;
 
+    const bool chmaps_match = (src_channels == dst_channels) && SDL_AudioChannelMapsEqual(src_channels, src_map, dst_map);
+    if (chmaps_match) {
+        src_map = dst_map = NULL;  // NULL both these out so we don't do any unnecessary swizzling.
+    }
+
     /* Type conversion goes like this now:
         - swizzle through source channel map to "standard" layout.
         - byteswap to CPU native format first if necessary.
@@ -635,8 +640,6 @@ bool SetAudioStreamChannelMap(SDL_AudioStream *stream, const SDL_AudioSpec *spec
         // already have this map, don't allocate/copy it again.
     } else if (SDL_ChannelMapIsBogus(chmap, channels)) {
         result = SDL_SetError("Invalid channel mapping");
-    } else if ((isinput != -1) && stream->bound_device && (!!isinput == !!stream->bound_device->physical_device->recording)) {
-        // quietly refuse to change the format of the end currently bound to a device.
     } else {
         if (SDL_ChannelMapIsDefault(chmap, channels)) {
             chmap = NULL;  // just apply a default mapping.
@@ -661,12 +664,12 @@ bool SetAudioStreamChannelMap(SDL_AudioStream *stream, const SDL_AudioSpec *spec
 
 bool SDL_SetAudioStreamInputChannelMap(SDL_AudioStream *stream, const int *chmap, int channels)
 {
-    return SetAudioStreamChannelMap(stream, &stream->src_spec, &stream->src_chmap, chmap, channels, true);
+    return SetAudioStreamChannelMap(stream, &stream->src_spec, &stream->src_chmap, chmap, channels, 1);
 }
 
 bool SDL_SetAudioStreamOutputChannelMap(SDL_AudioStream *stream, const int *chmap, int channels)
 {
-    return SetAudioStreamChannelMap(stream, &stream->dst_spec, &stream->dst_chmap, chmap, channels, false);
+    return SetAudioStreamChannelMap(stream, &stream->dst_spec, &stream->dst_chmap, chmap, channels, 0);
 }
 
 int *SDL_GetAudioStreamInputChannelMap(SDL_AudioStream *stream, int *count)

src/audio/SDL_sysaudio.h

@@ -125,9 +125,13 @@ extern void ConvertAudio(int num_frames,
 
 // Compare two SDL_AudioSpecs, return true if they match exactly.
 // Using SDL_memcmp directly isn't safe, since potential padding might not be initialized.
-// either channel maps can be NULL for the default (and both should be if you don't care about them).
+// either channel map can be NULL for the default (and both should be if you don't care about them).
 extern bool SDL_AudioSpecsEqual(const SDL_AudioSpec *a, const SDL_AudioSpec *b, const int *channel_map_a, const int *channel_map_b);
 
+// See if two channel maps match
+// either channel map can be NULL for the default (and both should be if you don't care about them).
+extern bool SDL_AudioChannelMapsEqual(int channels, const int *channel_map_a, const int *channel_map_b);
+
 // allocate+copy a channel map.
 extern int *SDL_ChannelMapDup(const int *origchmap, int channels);