refactor: make cel_serial_read non-blocking

Drop timeout_ms; read now returns immediately with whatever
data is available (0 if none), so callers don't block the rest
of their loop waiting on serial I/O.

telemetry's poll loop now sleeps interval_ms itself between
empty reads via a small sleep_ms helper.

celrs/serial.c

@@ -37,12 +37,11 @@ void cel_serial_close(cel_serial_port* port) {
     free(port);
 }
 
-size_t cel_serial_read(cel_serial_port* port, uint8_t* buf, size_t len, int timeout_ms) {
+size_t cel_serial_read(cel_serial_port* port, uint8_t* buf, size_t len) {
     (void)port;
     (void)buf;
     (void)len;
-    (void)timeout_ms;
+    /* TODO: platform-specific non-blocking read */
-    /* TODO: platform-specific read */
     return 0;
 }
 

celrs/serial.h

@@ -15,8 +15,9 @@ cel_serial_port* cel_serial_open(char const* path, int baud_rate);
 /* Close and free the serial port */
 void cel_serial_close(cel_serial_port* port);
 
-/* Read up to len bytes. Returns bytes read, 0 on timeout/error. */
+/* Read up to len bytes without blocking.
-size_t cel_serial_read(cel_serial_port* port, uint8_t* buf, size_t len, int timeout_ms);
+ * Returns bytes read immediately available, or 0 if none/error. */
+size_t cel_serial_read(cel_serial_port* port, uint8_t* buf, size_t len);
 
 /* Write data. Returns bytes written, 0 on error. */
 size_t cel_serial_write(cel_serial_port* port, uint8_t const* buf, size_t len);

tests/test_serial.c

@@ -34,7 +34,7 @@ void test_read_returns_zero_stub(void) {
     TEST_ASSERT_NOT_NULL(port);
     uint8_t buf[16];
     /* Stub implementation returns 0 */
-    size_t n = cel_serial_read(port, buf, sizeof(buf), 100);
+    size_t n = cel_serial_read(port, buf, sizeof(buf));
     TEST_ASSERT_EQUAL_UINT(0, n);
     cel_serial_close(port);
 }

tools/telemetry.c

@@ -5,6 +5,12 @@
 #include <stdlib.h>
 #include <string.h>
 
+#ifdef _WIN32
+#include <windows.h>
+#else
+#include <time.h>
+#endif
+
 /* TX power index to dBm mapping (ELRS standard) */
 static int const s_tx_power_dbm[] = {
     0, 20, 26, 30, 32, 34, 36, 38,
@@ -31,6 +37,15 @@ static int telemetry_parse_link(int16_t* rssi, uint8_t* link_quality,
     return 0;
 }
 
+static void sleep_ms(int ms) {
+#ifdef _WIN32
+    Sleep((DWORD)ms);
+#else
+    struct timespec ts = {.tv_sec = ms / 1000, .tv_nsec = (ms % 1000) * 1000000L};
+    nanosleep(&ts, NULL);
+#endif
+}
+
 static void print_usage(char const* prog) {
     printf("Usage: %s --port <serial_port> [--baudrate <rate>] [interval_ms]\n", prog);
     printf("       %s --list\n", prog);
@@ -106,8 +121,11 @@ int main(int argc, char const* argv[]) {
     int frames = 0, errors = 0;
 
     for (int i = 0; i < 20; i++) { /* read up to 20 telemetry frames */
-        size_t n = cel_serial_read(port, buf, sizeof(buf), interval_ms);
+        size_t n = cel_serial_read(port, buf, sizeof(buf));
-        if (n == 0) continue;
+        if (n == 0) {
+            sleep_ms(interval_ms);
+            continue;
+        }
 
         cel_crsf_frame frame;
         if (cel_crsf_frame_parse(&frame, buf, n) != 0) {