boards/adafruit-itsybitsy-m4: turn off APA102 LED on startup

boards/adafruit-itsybitsy-m4/board.c

@@ -53,3 +53,39 @@ static mtd_spi_nor_t samd51_nor_dev = {
 
 mtd_dev_t *mtd0 = (mtd_dev_t *)&samd51_nor_dev;
 #endif /* MODULE_MTD */
+
+static inline void _toggle(unsigned n)
+{
+    n *= 2;
+    while (--n) {
+        /* This might break if the GPIO driver gets more optimized, but
+         * with the current implementation the toggle is slow enough :)  */
+        gpio_toggle(APA102_PARAM_CLK_PIN);
+    }
+}
+
+void board_init(void)
+{
+    /* if the real driver is used, it will deal with the LED */
+    if (IS_USED(MODULE_APA102)) {
+        return;
+    }
+
+    /* bootloader leaves on an annoyingly bright LED - turn it off manually */
+    gpio_init(APA102_PARAM_DATA_PIN, GPIO_OUT);
+    gpio_init(APA102_PARAM_CLK_PIN, GPIO_OUT);
+
+    /* start frame - 32 zero bits */
+    gpio_clear(APA102_PARAM_DATA_PIN);
+    _toggle(32);
+
+    /* LED frame: 3 start bits (1), 5 alpha bits (0), 24 color bits (0) */
+    gpio_set(APA102_PARAM_DATA_PIN);
+    _toggle(3);
+    gpio_clear(APA102_PARAM_DATA_PIN);
+    _toggle(29); /* 5 alpha + 8 red + 8 green + 8 blue */
+
+    /* end frame - 32 one bits */
+    gpio_set(APA102_PARAM_DATA_PIN);
+    _toggle(32);
+}