Fix HDMI audio sample rate detection for non-48kHz sources

Query TC358743 HDMI receiver for detected audio sample rate before
initializing ALSA capture device. This fixes distortion issues when
HDMI sources send 44.1kHz audio (e.g., Armbian SBC) instead of 48kHz.

Previously, the code always requested 48kHz from ALSA, but in I2S slave
mode, the RV1106 I2S controller receives whatever clock rate the TC358743
master provides. This caused a sample rate mismatch where ALSA thought
it was 48kHz but hardware was actually running at 44.1kHz, resulting in
incorrect SpeexDSP resampling and audio distortion.

Changes:
- Add V4L2 ioctl to query TC358743's audio_sampling_rate control
- Use detected rate when configuring ALSA (falls back to 48kHz if unavailable)
- SpeexDSP resampler now gets correct input rate (44.1k, 48k, etc.)
- Supports all HDMI audio sample rates: 32k, 44.1k, 48k, 88.2k, 96k, etc.

Author: pennycoders

internal/audio/c/audio.c

@@ -30,10 +30,19 @@
 #include <signal.h>
 #include <pthread.h>
 #include <stdatomic.h>
+#include <fcntl.h>
+#include <sys/ioctl.h>
+#include <linux/videodev2.h>
 
 // ARM NEON SIMD optimizations (Cortex-A7 accelerates buffer operations, with scalar fallback)
 #include <arm_neon.h>
 
+// TC358743 V4L2 control IDs for audio
+#ifndef V4L2_CID_USER_TC35874X_BASE
+#define V4L2_CID_USER_TC35874X_BASE (V4L2_CID_USER_BASE + 0x10a0)
+#endif
+#define TC35874X_CID_AUDIO_SAMPLING_RATE (V4L2_CID_USER_TC35874X_BASE + 0)
+
 // RV1106 (Cortex-A7) has 64-byte cache lines
 #define CACHE_LINE_SIZE 64
 #define SIMD_ALIGN __attribute__((aligned(16)))
@@ -209,6 +218,54 @@ static volatile sig_atomic_t playback_initialized = 0;
 
 // ALSA UTILITY FUNCTIONS
 
+/**
+ * Query TC358743 HDMI receiver for detected audio sample rate
+ * Reads the hardware-detected sample rate from V4L2 control
+ * @return detected sample rate (44100, 48000, etc.) or 0 if detection fails
+ */
+static unsigned int get_hdmi_audio_sample_rate(void) {
+	// TC358743 is a V4L2 subdevice at /dev/v4l-subdev2
+	int fd = open("/dev/v4l-subdev2", O_RDWR);
+	if (fd < 0) {
+		fprintf(stderr, "WARNING: Could not open /dev/v4l-subdev2 to query HDMI audio sample rate: %s\n", strerror(errno));
+		fflush(stderr);
+		return 0;
+	}
+
+	// Use extended controls API for custom V4L2 controls
+	struct v4l2_ext_control ext_ctrl = {0};
+	ext_ctrl.id = TC35874X_CID_AUDIO_SAMPLING_RATE;
+
+	struct v4l2_ext_controls ext_ctrls = {0};
+	ext_ctrls.ctrl_class = V4L2_CTRL_CLASS_USER;
+	ext_ctrls.count = 1;
+	ext_ctrls.controls = &ext_ctrl;
+
+	if (ioctl(fd, VIDIOC_G_EXT_CTRLS, &ext_ctrls) == -1) {
+		fprintf(stderr, "WARNING: Could not query TC358743 audio sample rate control: %s (errno=%d)\n", strerror(errno), errno);
+		fflush(stderr);
+		close(fd);
+		return 0;
+	}
+
+	close(fd);
+
+	unsigned int detected_rate = (unsigned int)ext_ctrl.value;
+	fprintf(stdout, "DEBUG: TC358743 control read returned: %u Hz (error_idx=%u)\n", detected_rate, ext_ctrls.error_idx);
+	fflush(stdout);
+
+	if (detected_rate == 0) {
+		fprintf(stdout, "INFO: TC358743 reports 0 Hz (no signal or rate not detected yet)\n");
+		fflush(stdout);
+		return 0;  // No signal or rate not detected
+	}
+
+	fprintf(stdout, "INFO: TC358743 detected HDMI audio sample rate: %u Hz\n", detected_rate);
+	fflush(stdout);
+
+	return detected_rate;
+}
+
 /**
  * Open ALSA device with exponential backoff retry
  * @return 0 on success, negative error code on failure
@@ -365,12 +422,13 @@ static int handle_alsa_error(snd_pcm_t *handle, snd_pcm_t **valid_handle,
  * @param handle ALSA PCM handle
  * @param device_name Device name for logging
  * @param num_channels Number of channels (1=mono, 2=stereo)
+ * @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
  * @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 *actual_rate_out, uint16_t *actual_frame_size_out) {
+                                 unsigned int preferred_rate, unsigned int *actual_rate_out, uint16_t *actual_frame_size_out) {
 	snd_pcm_hw_params_t *params;
 	snd_pcm_sw_params_t *sw_params;
 	int err;
@@ -410,8 +468,8 @@ static int configure_alsa_device(snd_pcm_t *handle, const char *device_name, uin
 		return err;
 	}
 
-	// Try to set 48kHz first (preferred), then let hardware negotiate
-	unsigned int requested_rate = opus_sample_rate;
+	// Use preferred rate if specified, otherwise default to 48kHz
+	unsigned int requested_rate = (preferred_rate > 0) ? preferred_rate : opus_sample_rate;
 	err = snd_pcm_hw_params_set_rate_near(handle, params, &requested_rate, 0);
 	if (err < 0) return err;
 
@@ -540,9 +598,19 @@ int jetkvm_audio_capture_init() {
 		return ERR_ALSA_OPEN_FAILED;
 	}
 
+	// Query TC358743 for detected HDMI audio sample rate
+	unsigned int preferred_rate = get_hdmi_audio_sample_rate();
+	if (preferred_rate > 0) {
+		fprintf(stdout, "INFO: Using TC358743 detected sample rate: %u Hz\n", preferred_rate);
+	} else {
+		fprintf(stdout, "INFO: TC358743 sample rate not detected, using default 48kHz\n");
+		preferred_rate = 0;  // Will default to 48kHz
+	}
+	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, &actual_rate, &actual_frame_size_with_flag);
+	err = configure_alsa_device(pcm_capture_handle, "capture", capture_channels, preferred_rate, &actual_rate, &actual_frame_size_with_flag);
 	if (err < 0) {
 		snd_pcm_t *handle = pcm_capture_handle;
 		pcm_capture_handle = NULL;
@@ -819,7 +887,7 @@ 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, &actual_rate, &actual_frame_size);
+	err = configure_alsa_device(pcm_playback_handle, "playback", playback_channels, 0, &actual_rate, &actual_frame_size);
 	if (err < 0) {
 		snd_pcm_t *handle = pcm_playback_handle;
 		pcm_playback_handle = NULL;