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kvm
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Fix reversed stereo channels by querying ALSA channel map 

Query the ALSA channel map (snd_pcm_get_chmap) to detect hardware that
reports non-standard channel ordering (R,L instead of L,R). When detected,
swap channels after capture to ensure correct left/right positioning.

This properly handles hardware quirks (like TC358743 HDMI audio) without
hardcoding device names, making the solution portable and correct.
This commit is contained in:
Alex P 2025-11-21 19:27:01 +02:00
parent 72966389d9
commit 3692cdae83
1 changed files with 31 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

39
internal/audio/c/audio.c
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@ -44,6 +44,7 @@ static snd_pcm_t *pcm_playback_handle = NULL; // INPUT: Client microphone → de
static const char *alsa_capture_device = NULL; static const char *alsa_capture_device = NULL;
static const char *alsa_playback_device = NULL; static const char *alsa_playback_device = NULL;
static bool capture_channels_swapped = false; // True if hardware reports R,L instead of L,R
static OpusEncoder *encoder = NULL; static OpusEncoder *encoder = NULL;
static OpusDecoder *decoder = NULL; static OpusDecoder *decoder = NULL;
@ -466,8 +467,26 @@ static int configure_alsa_device(snd_pcm_t *handle, const char *device_name, uin
err = snd_pcm_prepare(handle); err = snd_pcm_prepare(handle);
if (err < 0) return err; if (err < 0) return err;
if (num_channels == 2) {
snd_pcm_chmap_t *chmap = snd_pcm_get_chmap(handle);
if (chmap != NULL) {
if (chmap->channels == 2) {
bool is_swapped = (chmap->pos[0] == SND_CHMAP_FR && chmap->pos[1] == SND_CHMAP_FL);
if (is_swapped) {
fprintf(stderr, "INFO: %s: Hardware reports swapped channel map (R,L instead of L,R)\n",
device_name);
fflush(stderr);
}
if (actual_frame_size_out && is_swapped) {
*actual_frame_size_out |= 0x8000;
}
}
free(chmap);
}
}
if (actual_rate_out) *actual_rate_out = verified_rate; if (actual_rate_out) *actual_rate_out = verified_rate;
if (actual_frame_size_out) *actual_frame_size_out = hw_frame_size; if (actual_frame_size_out) *actual_frame_size_out &= 0x7FFF;
return 0; return 0;
} }
@ -527,8 +546,8 @@ int jetkvm_audio_capture_init() {
} }
unsigned int actual_rate = 0; unsigned int actual_rate = 0;
uint16_t actual_frame_size = 0; uint16_t actual_frame_size_with_flag = 0;
err = configure_alsa_device(pcm_capture_handle, "capture", capture_channels, &actual_rate, &actual_frame_size); err = configure_alsa_device(pcm_capture_handle, "capture", capture_channels, &actual_rate, &actual_frame_size_with_flag);
if (err < 0) { if (err < 0) {
snd_pcm_t *handle = pcm_capture_handle; snd_pcm_t *handle = pcm_capture_handle;
pcm_capture_handle = NULL; pcm_capture_handle = NULL;
@ -538,11 +557,9 @@ int jetkvm_audio_capture_init() {
return -2; return -2;
} }
// Store hardware-negotiated values capture_channels_swapped = (actual_frame_size_with_flag & 0x8000) != 0;
hardware_sample_rate = actual_rate; hardware_sample_rate = actual_rate;
hardware_frame_size = actual_frame_size; hardware_frame_size = actual_frame_size_with_flag & 0x7FFF;
// Validate hardware frame size
if (hardware_frame_size > 3840) { if (hardware_frame_size > 3840) {
fprintf(stderr, "ERROR: capture: Hardware frame size %u exceeds buffer capacity 3840\n", fprintf(stderr, "ERROR: capture: Hardware frame size %u exceeds buffer capacity 3840\n",
hardware_frame_size); hardware_frame_size);
@ -706,7 +723,13 @@ retry_read:
simd_clear_samples_s16(&pcm_hw_buffer[pcm_rc * capture_channels], remaining_samples); simd_clear_samples_s16(&pcm_hw_buffer[pcm_rc * capture_channels], remaining_samples);
} }
// Resample to 48kHz if needed if (capture_channels_swapped && capture_channels == 2) {
for (uint32_t i = 0; i < hardware_frame_size; i++) {
short temp = pcm_hw_buffer[i * 2];
pcm_hw_buffer[i * 2] = pcm_hw_buffer[i * 2 + 1];
pcm_hw_buffer[i * 2 + 1] = temp;
}
}
short *pcm_to_encode; short *pcm_to_encode;
if (capture_resampler) { if (capture_resampler) {
spx_uint32_t in_len = hardware_frame_size; spx_uint32_t in_len = hardware_frame_size;