drivers: tones: add SDL-based beep generator

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Ryan Turner <ryan@turnrye.com>

meson.build

@@ -356,12 +356,14 @@ linux_src = ['platform/targets/linux/emulator/emulator.c',
              'platform/drivers/audio/file_source.c',
              'platform/drivers/audio/audio_sdl.cpp',
              'platform/targets/linux/platform.c',
+             'platform/drivers/tones/toneGenerator_sdl.cpp',
              'platform/drivers/CPS/cps_io_libc.c',
              'platform/drivers/NVM/posix_file.c']
 
 linux_inc = ['platform/targets/linux',
              'platform/targets/linux/emulator',
-             'platform/drivers/audio']
+             'platform/drivers/audio',
+             'platform/drivers/tones']
 
 linux_def = {'PLATFORM_LINUX': '', 'VP_USE_FILESYSTEM':''}
 

platform/drivers/tones/toneGenerator_sdl.cpp

@@ -0,0 +1,108 @@
+/*
+ * SPDX-FileCopyrightText: Copyright 2020-2026 OpenRTX Contributors
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#include <math.h>
+#include <pthread.h>
+#include "SDL2/SDL.h"
+#include "toneGenerator_sdl.h"
+#include "interfaces/audio.h"
+
+#define BEEP_RATE 48000
+#define BEEP_CHUNK (BEEP_RATE / 50) // 20 ms of samples
+#define BEEP_AMPLITUDE 8000
+
+// Number of extra queued chunks tolerated before a playback sync() blocks.
+#define PLAY_QUEUE_SLACK 2
+
+static pthread_t beepThread;
+static pthread_once_t beepOnce = PTHREAD_ONCE_INIT;
+static pthread_mutex_t beepMutex = PTHREAD_MUTEX_INITIALIZER;
+static int beepActive = 0;
+static uint16_t beepFreq = 0;
+
+static void *beepThreadFunc(void *arg)
+{
+    (void)arg;
+
+    SDL_AudioSpec want;
+    SDL_AudioSpec have;
+    SDL_zero(want);
+    want.freq = BEEP_RATE;
+    want.format = AUDIO_S16SYS;
+    want.channels = 1;
+    want.samples = 512;
+
+    SDL_AudioDeviceID dev = SDL_OpenAudioDevice(NULL, 0, &want, &have, 0);
+    if (dev == 0)
+        return NULL;
+
+    stream_sample_t chunk[BEEP_CHUNK];
+    double phase = 0.0;
+    int playing = 0;
+
+    for (;;) {
+        pthread_mutex_lock(&beepMutex);
+        int active = beepActive;
+        uint16_t freq = beepFreq;
+        pthread_mutex_unlock(&beepMutex);
+
+        if (active != 0) {
+            if (playing == 0) {
+                SDL_ClearQueuedAudio(dev);
+                SDL_PauseAudioDevice(dev, 0);
+                playing = 1;
+            }
+
+            double step = (2.0 * M_PI * (double)freq) / (double)BEEP_RATE;
+            for (size_t i = 0; i < BEEP_CHUNK; i++) {
+                chunk[i] = (stream_sample_t)(BEEP_AMPLITUDE * sin(phase));
+                phase += step;
+                if (phase >= (2.0 * M_PI))
+                    phase -= (2.0 * M_PI);
+            }
+
+            Uint32 chunkBytes = (Uint32)sizeof(chunk);
+            while (SDL_GetQueuedAudioSize(dev)
+                   > (PLAY_QUEUE_SLACK * chunkBytes))
+                SDL_Delay(1);
+
+            SDL_QueueAudio(dev, chunk, chunkBytes);
+        } else {
+            if (playing != 0) {
+                SDL_ClearQueuedAudio(dev);
+                SDL_PauseAudioDevice(dev, 1);
+                playing = 0;
+                phase = 0.0;
+            }
+
+            SDL_Delay(5);
+        }
+    }
+
+    return NULL;
+}
+
+static void beepThreadStart(void)
+{
+    pthread_create(&beepThread, NULL, beepThreadFunc, NULL);
+}
+
+void toneGen_sdl_start(uint16_t freq)
+{
+    pthread_once(&beepOnce, beepThreadStart);
+
+    pthread_mutex_lock(&beepMutex);
+    beepFreq = freq;
+    beepActive = 1;
+    pthread_mutex_unlock(&beepMutex);
+}
+
+void toneGen_sdl_stop(void)
+{
+    pthread_mutex_lock(&beepMutex);
+    beepActive = 0;
+    pthread_mutex_unlock(&beepMutex);
+}

platform/drivers/tones/toneGenerator_sdl.h

@@ -0,0 +1,39 @@
+/*
+ * SPDX-FileCopyrightText: Copyright 2020-2026 OpenRTX Contributors
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#ifndef TONE_GENERATOR_SDL_H
+#define TONE_GENERATOR_SDL_H
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * SDL2-backed beep tone generator for the Linux emulator and WebAssembly
+ * targets. Drives a dedicated audio device on a background thread that
+ * synthesizes a phase-continuous sine wave.
+ */
+
+/**
+ * Start a continuous beep tone at the given frequency on a dedicated output
+ * device. Calling it again while a beep is active retunes the tone.
+ *
+ * @param freq: beep frequency, in Hz.
+ */
+void toneGen_sdl_start(uint16_t freq);
+
+/**
+ * Stop an ongoing beep tone.
+ */
+void toneGen_sdl_stop(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* TONE_GENERATOR_SDL_H */

platform/targets/linux/platform.c

@@ -10,6 +10,7 @@
 #include <stdio.h>
 #include "core/gps.h"
 #include "emulator.h"
+#include "toneGenerator_sdl.h"
 
 /*
  * Create the data structure holding Module17 calibration data to make the
@@ -109,12 +110,12 @@ void platform_ledOff(led_t led)
 
 void platform_beepStart(uint16_t freq)
 {
-    printf("platform_beepStart(%u)\n", freq);
+    toneGen_sdl_start(freq);
 }
 
 void platform_beepStop()
 {
-    printf("platform_beepStop()\n");
+    toneGen_sdl_stop();
 }
 
 datetime_t platform_getCurrentTime()