MIPS: ralink: Add tty detection

MT7688 has several uarts that can be used for console. There are several
boards in the wild, that use ttyS1 or ttyS2. This patch applies a simply
autodetection routine to figure out which ttyS the bootloader used as
console. The uarts come up in 6 bit mode by default. The bootloader will
have set 8 bit mode on the console. Find that 8bit tty and use it.
Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11459/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/ralink/early_printk.c

@@ -25,11 +25,13 @@
 #define MT7628_CHIP_NAME1	0x20203832
 
 #define UART_REG_TX		0x04
+#define UART_REG_LCR		0x0c
 #define UART_REG_LSR		0x14
 #define UART_REG_LSR_RT2880	0x1c
 
 static __iomem void *uart_membase = (__iomem void *) KSEG1ADDR(EARLY_UART_BASE);
 static __iomem void *chipid_membase = (__iomem void *) KSEG1ADDR(CHIPID_BASE);
+static int init_complete;
 
 static inline void uart_w32(u32 val, unsigned reg)
 {
@@ -47,8 +49,32 @@ static inline int soc_is_mt7628(void)
 		(__raw_readl(chipid_membase) == MT7628_CHIP_NAME1);
 }
 
+static void find_uart_base(void)
+{
+	int i;
+
+	if (!soc_is_mt7628())
+		return;
+
+	for (i = 0; i < 3; i++) {
+		u32 reg = uart_r32(UART_REG_LCR + (0x100 * i));
+
+		if (!reg)
+			continue;
+
+		uart_membase = (__iomem void *) KSEG1ADDR(EARLY_UART_BASE +
+							  (0x100 * i));
+		break;
+	}
+}
+
 void prom_putchar(unsigned char ch)
 {
+	if (!init_complete) {
+		find_uart_base();
+		init_complete = 1;
+	}
+
 	if (IS_ENABLED(CONFIG_SOC_MT7621) || soc_is_mt7628()) {
 		uart_w32(ch, UART_TX);
 		while ((uart_r32(UART_REG_LSR) & UART_LSR_THRE) == 0)