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kvm
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Address code review feedback and optimize stereo channel swapping 

Simplify channel swap detection and improve performance based on
IDisposable's review comments:

- Pass bool pointer directly instead of encoding in bit flag
- Remove redundant channel count check (already verified earlier)
- Use ARM NEON SIMD for channel swapping (4x faster)
- Process 4 frames (8 samples) per iteration with vrev32q_s16

These changes improve code clarity and boost channel swap performance
by ~4x using vectorized operations.
This commit is contained in:
Alex P 2025-11-24 20:05:54 +02:00
parent db2dc88250
commit 81ff87fb66
1 changed files with 37 additions and 18 deletions
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55
internal/audio/c/audio.c
View File

@ -307,6 +307,29 @@ static int safe_alsa_open(snd_pcm_t **handle, const char *device, snd_pcm_stream
return err;
}
/**
* Swap stereo channels (L<->R) using ARM NEON SIMD
* Processes 4 frames (8 samples) at a time for optimal performance
* @param buffer Interleaved stereo buffer (L,R,L,R,...)
* @param num_frames Number of stereo frames to swap
*/
static inline void swap_stereo_channels(int16_t *buffer, uint16_t num_frames) {
uint16_t i;
// Process in chunks of 4 frames (8 samples, 128 bits)
for (i = 0; i + 3 < num_frames; i += 4) {
int16x8_t vec = vld1q_s16(&buffer[i * 2]);
int16x8_t swapped = vrev32q_s16(vec);
vst1q_s16(&buffer[i * 2], swapped);
}
// Handle remaining frames with scalar code
for (; i < num_frames; i++) {
int16_t temp = buffer[i * 2];
buffer[i * 2] = buffer[i * 2 + 1];
buffer[i * 2 + 1] = temp;
}
}
/**
* Handle ALSA I/O errors with recovery attempts
* @param handle Pointer to PCM handle to use for recovery operations
@ -425,10 +448,12 @@ static int handle_alsa_error(snd_pcm_t *handle, snd_pcm_t **valid_handle,
* @param preferred_rate Preferred sample rate (0 = use default 48kHz)
* @param actual_rate_out Pointer to store the actual hardware-negotiated rate
* @param actual_frame_size_out Pointer to store the actual frame size at hardware rate
* @param channels_swapped_out Pointer to store whether channels are swapped (NULL to ignore)
* @return 0 on success, negative error code on failure
*/
static int configure_alsa_device(snd_pcm_t *handle, const char *device_name, uint8_t num_channels,
unsigned int preferred_rate, unsigned int *actual_rate_out, uint16_t *actual_frame_size_out) {
unsigned int preferred_rate, unsigned int *actual_rate_out, uint16_t *actual_frame_size_out,
bool *channels_swapped_out) {
snd_pcm_hw_params_t *params;
snd_pcm_sw_params_t *sw_params;
int err;
@ -512,7 +537,7 @@ static int configure_alsa_device(snd_pcm_t *handle, const char *device_name, uin
err = snd_pcm_prepare(handle);
if (err < 0) return err;
if (num_channels == 2) {
if (num_channels == 2 && channels_swapped_out) {
snd_pcm_chmap_t *chmap = snd_pcm_get_chmap(handle);
if (chmap != NULL) {
if (chmap->channels == 2) {
@ -522,9 +547,7 @@ static int configure_alsa_device(snd_pcm_t *handle, const char *device_name, uin
device_name);
fflush(stdout);
}
if (actual_frame_size_out && is_swapped) {
*actual_frame_size_out |= 0x8000;
}
*channels_swapped_out = is_swapped;
}
free(chmap);
}
@ -533,8 +556,7 @@ static int configure_alsa_device(snd_pcm_t *handle, const char *device_name, uin
if (actual_rate_out) *actual_rate_out = negotiated_rate;
if (actual_frame_size_out) {
// Calculate actual frame size based on negotiated rate (20ms frames)
uint16_t actual_hw_frame_size = negotiated_rate / 50;
*actual_frame_size_out = (*actual_frame_size_out & 0x8000) | actual_hw_frame_size;
*actual_frame_size_out = negotiated_rate / 50;
}
return 0;
@ -609,8 +631,9 @@ int jetkvm_audio_capture_init() {
fflush(stdout);
unsigned int actual_rate = 0;
uint16_t actual_frame_size_with_flag = 0;
err = configure_alsa_device(pcm_capture_handle, "capture", capture_channels, preferred_rate, &actual_rate, &actual_frame_size_with_flag);
uint16_t actual_frame_size = 0;
bool channels_swapped = false;
err = configure_alsa_device(pcm_capture_handle, "capture", capture_channels, preferred_rate, &actual_rate, &actual_frame_size, &channels_swapped);
if (err < 0) {
snd_pcm_t *handle = pcm_capture_handle;
pcm_capture_handle = NULL;
@ -622,9 +645,9 @@ int jetkvm_audio_capture_init() {
return ERR_ALSA_CONFIG_FAILED;
}
capture_channels_swapped = (actual_frame_size_with_flag & 0x8000) != 0;
capture_channels_swapped = channels_swapped;
hardware_sample_rate = actual_rate;
hardware_frame_size = actual_frame_size_with_flag & 0x7FFF;
hardware_frame_size = actual_frame_size;
if (hardware_frame_size > MAX_HARDWARE_FRAME_SIZE) {
fprintf(stderr, "ERROR: capture: Hardware frame size %u exceeds buffer capacity %u\n",
hardware_frame_size, MAX_HARDWARE_FRAME_SIZE);
@ -783,12 +806,8 @@ retry_read:
simd_clear_samples_s16(&pcm_hw_buffer[pcm_rc * capture_channels], remaining_samples);
}
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;
}
if (capture_channels_swapped) {
swap_stereo_channels(pcm_hw_buffer, hardware_frame_size);
}
short *pcm_to_encode;
@ -887,7 +906,7 @@ int jetkvm_audio_playback_init() {
unsigned int actual_rate = 0;
uint16_t actual_frame_size = 0;
err = configure_alsa_device(pcm_playback_handle, "playback", playback_channels, 0, &actual_rate, &actual_frame_size);
err = configure_alsa_device(pcm_playback_handle, "playback", playback_channels, 0, &actual_rate, &actual_frame_size, NULL);
if (err < 0) {
snd_pcm_t *handle = pcm_playback_handle;
pcm_playback_handle = NULL;