x86_64: allow serial port baud rate to be configured

arch/x86_64/include/arch/serial.h

@@ -18,7 +18,7 @@ extern char serial_recv_byte(int device);
 
 extern int serial_rcvd(int device);
 
-extern void serialcon_init(void);
+extern void serialcon_init(int baud);
 
 #ifdef __cplusplus
 }

arch/x86_64/init.c

@@ -48,7 +48,7 @@ int ml_init(uintptr_t arg)
 
 	bootstrap_cpu_init();
 	vgacon_init();
-	serialcon_init();
+	serialcon_init(115200);
 	clock_calibrate(500);
 
 	print_kernel_banner();

arch/x86_64/serial.c

@@ -49,12 +49,44 @@ void serialcon_write(struct console *con, const char *s, unsigned int len)
 	}
 }
 
-static void init_serial_port(int port)
+static int get_baud_divisor(int baud)
 {
+	int freq = 115200;
+	int best_baud = -1;
+	int best_div = -1;
+
+	for (int i = 1; i < 254; i++) {
+		int this_baud = freq / i;
+
+		if (this_baud == baud) {
+			return i;
+		}
+
+		if (best_baud == -1) {
+			best_baud = this_baud;
+			best_div = i;
+			continue;
+		}
+
+		if (this_baud < baud && best_baud > baud) {
+			/* TODO pick divisor that gives the closest baud rate */
+			return best_div;
+		}
+
+		best_baud = this_baud;
+		best_div = i;
+	}
+
+	return best_div;
+}
+
+static void init_serial_port(int port, int baud)
+{
+	int baud_div = get_baud_divisor(baud);
 	outportb(port + 1, 0x00);     // Disable all interrupts
 	outportb(port + 3, 0x80);     // Enable DLAB (set baud rate divisor)
-	outportb(port + 0, 0x03);    // Set divisor to 3 (lo byte) 38400 baud
-	outportb(port + 1, 0x00);    //                  (hi byte)
+	outportb(port + 0, baud_div); // Set divisor
+	outportb(port + 1, 0x00);
 	outportb(port + 3, 0x03);     // 8 bits, no parity, one stop bit
 	outportb(port + 2, 0xC7);     // Enable FIFO, clear them, with 14-byte threshold
 	outportb(port + 4, 0x0B);     // IRQs enabled, RTS/DSR set
@@ -85,8 +117,8 @@ static struct console serialcon = {
 	.c_lock = SPIN_LOCK_INIT,
 };
 
-void serialcon_init(void)
+void serialcon_init(int baud)
 {
-	init_serial_port(COM1);
+	init_serial_port(COM1, baud);
 	console_register(&serialcon);
 }