Wrap header with ifdef for C++ compatibility

* Add clock sync comment for the curious reader

* While here remove extra semicolon

* Fix indentation

software/libdvi/dvi.h

@@ -1,6 +1,10 @@
 #ifndef _DVI_H
 #define _DVI_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #define N_TMDS_LANES 3
 #define TMDS_SYNC_LANE 0 // blue!
 
@@ -69,4 +73,8 @@ void dvi_scanbuf_main_16bpp(struct dvi_inst *inst);
 void dvi_framebuf_main_8bpp(struct dvi_inst *inst);
 void dvi_framebuf_main_16bpp(struct dvi_inst *inst);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif

software/libdvi/dvi_serialiser.c

@@ -63,6 +63,8 @@ void dvi_serialiser_enable(struct dvi_serialiser_cfg *cfg, bool enable) {
 	for (int i = 0; i < N_TMDS_LANES; ++i)
 		mask |= 1u << (cfg->sm_tmds[i] + PIO_CTRL_SM_ENABLE_LSB);
 	if (enable) {
+		// The DVI spec allows for phase offset between clock and data links.
+		// So PWM and PIO do not need to be synchronised perfectly.
 		hw_set_bits(&cfg->pio->ctrl, mask);
 		pwm_set_enabled(pwm_gpio_to_slice_num(cfg->pins_clk), true);
 	}

software/libdvi/dvi_timing.c

@@ -217,7 +217,7 @@ static uint32_t __attribute__((aligned(8))) __dvi_const(empty_scanline_tmds)[6]
 void dvi_timing_state_init(struct dvi_timing_state *t) {
 	t->v_ctr = 0;
 	t->v_state = DVI_STATE_FRONT_PORCH;
-};
+}
 
 void __dvi_func(dvi_timing_state_advance)(const struct dvi_timing *t, struct dvi_timing_state *s) {
 		s->v_ctr++;
@@ -252,7 +252,7 @@ static void _set_data_cb(dma_cb_t *cb, const struct dvi_lane_dma_cfg *dma_cfg,
 	channel_config_set_chain_to(&cb->c, dma_cfg->chan_ctrl);
 	// Note we never send a null trigger, so IRQ_QUIET is an IRQ suppression flag
 	channel_config_set_irq_quiet(&cb->c, !irq_on_finish);
-};
+}
 
 void dvi_setup_scanline_for_vblank(const struct dvi_timing *t, const struct dvi_lane_dma_cfg dma_cfg[],
 		bool vsync_asserted, struct dvi_scanline_dma_list *l) {

software/libdvi/tmds_encode.c

@@ -191,7 +191,7 @@ static void tmds_encode_symbols(uint8_t pixel, uint32_t* negative_balance_sym, u
 	}
 
 	int imbalance = byte_imbalance(sym & 0xFF);
-  if (imbalance == 0) {
+	if (imbalance == 0) {
 		if ((sym & 0x100) == 0) sym ^= 0x2ff;
 		*positive_balance_sym = sym;
 		*negative_balance_sym = sym;