tty: serial: 8250_mtk: Add support for bus clock

The mtk 8250 needs two clocks, one for providing the baudrate and
one that needs to be enabled for register accesses. The latter has
not been supported, this patch adds support for it. It is optional
for now since not all SoCs provide a bus clock.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/mtk-uart.txt

@@ -14,7 +14,14 @@ Required properties:
 
 - interrupts: A single interrupt specifier.
 
-- clocks: Clock driving the hardware.
+- clocks : Must contain an entry for each entry in clock-names.
+  See ../clocks/clock-bindings.txt for details.
+- clock-names:
+  - "baud": The clock the baudrate is derived from
+  - "bus": The bus clock for register accesses (optional)
+
+For compatibility with older device trees an unnamed clock is used for the
+baud clock if the baudclk does not exist. Do not use this for new designs.
 
 Example:
 
@@ -22,5 +29,6 @@ Example:
 		compatible = "mediatek,mt6589-uart", "mediatek,mt6577-uart";
 		reg = <0x11006000 0x400>;
 		interrupts = <GIC_SPI 51 IRQ_TYPE_LEVEL_LOW>;
-		clocks = <&uart_clk>;
+		clocks = <&uart_clk>, <&bus_clk>;
+		clock-names = "baud", "bus";
 	};

drivers/tty/serial/8250/8250_mtk.c

@@ -34,6 +34,7 @@
 struct mtk8250_data {
 	int			line;
 	struct clk		*uart_clk;
+	struct clk		*bus_clk;
 };
 
 static void
@@ -120,6 +121,7 @@ static int mtk8250_runtime_suspend(struct device *dev)
 	struct mtk8250_data *data = dev_get_drvdata(dev);
 
 	clk_disable_unprepare(data->uart_clk);
+	clk_disable_unprepare(data->bus_clk);
 
 	return 0;
 }
@@ -135,6 +137,12 @@ static int mtk8250_runtime_resume(struct device *dev)
 		return err;
 	}
 
+	err = clk_prepare_enable(data->bus_clk);
+	if (err) {
+		dev_warn(dev, "Can't enable bus clock\n");
+		return err;
+	}
+
 	return 0;
 }
 
@@ -153,13 +161,24 @@ mtk8250_do_pm(struct uart_port *port, unsigned int state, unsigned int old)
 static int mtk8250_probe_of(struct platform_device *pdev, struct uart_port *p,
 			   struct mtk8250_data *data)
 {
-	data->uart_clk = devm_clk_get(&pdev->dev, NULL);
+	data->uart_clk = devm_clk_get(&pdev->dev, "baud");
 	if (IS_ERR(data->uart_clk)) {
-		dev_warn(&pdev->dev, "Can't get uart clock\n");
-		return PTR_ERR(data->uart_clk);
+		/*
+		 * For compatibility with older device trees try unnamed
+		 * clk when no baud clk can be found.
+		 */
+		data->uart_clk = devm_clk_get(&pdev->dev, NULL);
+		if (IS_ERR(data->uart_clk)) {
+			dev_warn(&pdev->dev, "Can't get uart clock\n");
+			return PTR_ERR(data->uart_clk);
+		}
+
+		return 0;
 	}
 
-	p->uartclk = clk_get_rate(data->uart_clk);
+	data->bus_clk = devm_clk_get(&pdev->dev, "bus");
+	if (IS_ERR(data->bus_clk))
+		return PTR_ERR(data->bus_clk);
 
 	return 0;
 }
@@ -204,6 +223,7 @@ static int mtk8250_probe(struct platform_device *pdev)
 	uart.port.regshift = 2;
 	uart.port.private_data = data;
 	uart.port.set_termios = mtk8250_set_termios;
+	uart.port.uartclk = clk_get_rate(data->uart_clk);
 
 	/* Disable Rate Fix function */
 	writel(0x0, uart.port.membase +