serial: 8250_fintek: Enable high speed mode on Fintek F81866

Fintek F81866 supports baud rates higher than 115200 but needs to raise
it's clock speed from 1.84 to 14.76 MHz.
This is eight times faster, so gives 921600 as resulting baud_base.

F81866 clock register 0xf2:
    Bit 7-2    reserved
    Bit 1-0    00: 1.8432MHz
               01: 18.432MHz
               10: 24MHz
               11: 14.769MHz

Signed-off-by: Lukas Redlinger <rel+kernel@agilox.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_fintek.c

@@ -61,6 +61,12 @@
  * The IRQ setting mode of F81866 is not the same with F81216 series.
  *	Level/Low: IRQ_MODE0:0, IRQ_MODE1:0
  *	Edge/High: IRQ_MODE0:1, IRQ_MODE1:0
+ *
+ * Clock speeds for UART (register F2h)
+ * 00: 1.8432MHz.
+ * 01: 18.432MHz.
+ * 10: 24MHz.
+ * 11: 14.769MHz.
  */
 #define F81866_IRQ_MODE		0xf0
 #define F81866_IRQ_SHARE	BIT(0)
@@ -72,6 +78,13 @@
 #define F81866_LDN_LOW		0x10
 #define F81866_LDN_HIGH		0x16
 
+#define F81866_UART_CLK 0xF2
+#define F81866_UART_CLK_MASK (BIT(1) | BIT(0))
+#define F81866_UART_CLK_1_8432MHZ 0
+#define F81866_UART_CLK_14_769MHZ (BIT(1) | BIT(0))
+#define F81866_UART_CLK_18_432MHZ BIT(0)
+#define F81866_UART_CLK_24MHZ BIT(1)
+
 struct fintek_8250 {
 	u16 pid;
 	u16 base_port;
@@ -256,8 +269,26 @@ static void fintek_8250_set_max_fifo(struct fintek_8250 *pdata)
 	}
 }
 
-static int probe_setup_port(struct fintek_8250 *pdata, u16 io_address,
-			  unsigned int irq)
+static void fintek_8250_goto_highspeed(struct uart_8250_port *uart,
+			      struct fintek_8250 *pdata)
+{
+	sio_write_reg(pdata, LDN, pdata->index);
+
+	switch (pdata->pid) {
+	case CHIP_ID_F81866: /* set uart clock for high speed serial mode */
+		sio_write_mask_reg(pdata, F81866_UART_CLK,
+			F81866_UART_CLK_MASK,
+			F81866_UART_CLK_14_769MHZ);
+
+			uart->port.uartclk = 921600 * 16;
+		break;
+	default: /* leave clock speed untouched */
+		break;
+	}
+}
+
+static int probe_setup_port(struct fintek_8250 *pdata,
+					struct uart_8250_port *uart)
 {
 	static const u16 addr[] = {0x4e, 0x2e};
 	static const u8 keys[] = {0x77, 0xa0, 0x87, 0x67};
@@ -284,18 +315,20 @@ static int probe_setup_port(struct fintek_8250 *pdata, u16 io_address,
 				sio_write_reg(pdata, LDN, k);
 				aux = sio_read_reg(pdata, IO_ADDR1);
 				aux |= sio_read_reg(pdata, IO_ADDR2) << 8;
-				if (aux != io_address)
+				if (aux != uart->port.iobase)
 					continue;
 
 				pdata->index = k;
 
-				irq_data = irq_get_irq_data(irq);
+				irq_data = irq_get_irq_data(uart->port.irq);
 				if (irq_data)
 					level_mode =
 						irqd_is_level_type(irq_data);
 
 				fintek_8250_set_irq_mode(pdata, level_mode);
 				fintek_8250_set_max_fifo(pdata);
+				fintek_8250_goto_highspeed(uart, pdata);
+
 				fintek_8250_exit_key(addr[i]);
 
 				return 0;
@@ -330,7 +363,7 @@ int fintek_8250_probe(struct uart_8250_port *uart)
 	struct fintek_8250 *pdata;
 	struct fintek_8250 probe_data;
 
-	if (probe_setup_port(&probe_data, uart->port.iobase, uart->port.irq))
+	if (probe_setup_port(&probe_data, uart))
 		return -ENODEV;
 
 	pdata = devm_kzalloc(uart->port.dev, sizeof(*pdata), GFP_KERNEL);