serial: 8250_dw: add ability to handle the peripheral clock

First try to find the named clock variants then fall back to the already
existing handling of a nameless declared baudclk.

This also adds the missing documentation for this already existing variant.
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/snps-dw-apb-uart.txt

@@ -4,7 +4,15 @@ Required properties:
 - compatible : "snps,dw-apb-uart"
 - reg : offset and length of the register set for the device.
 - interrupts : should contain uart interrupt.
+
+Clock handling:
+The clock rate of the input clock needs to be supplied by one of
 - clock-frequency : the input clock frequency for the UART.
+- clocks : phandle to the input clock
+
+The supplying peripheral clock can also be handled, needing a second property
+- clock-names: tuple listing input clock names.
+	Required elements: "baudclk", "apb_pclk"
 
 Optional properties:
 - reg-shift : quantity to shift the register offsets by.  If this property is
@@ -23,3 +31,26 @@ Example:
 		reg-shift = <2>;
 		reg-io-width = <4>;
 	};
+
+Example with one clock:
+
+	uart@80230000 {
+		compatible = "snps,dw-apb-uart";
+		reg = <0x80230000 0x100>;
+		clocks = <&baudclk>;
+		interrupts = <10>;
+		reg-shift = <2>;
+		reg-io-width = <4>;
+	};
+
+Example with two clocks:
+
+	uart@80230000 {
+		compatible = "snps,dw-apb-uart";
+		reg = <0x80230000 0x100>;
+		clocks = <&baudclk>, <&apb_pclk>;
+		clock-names = "baudclk", "apb_pclk";
+		interrupts = <10>;
+		reg-shift = <2>;
+		reg-io-width = <4>;
+	};

drivers/tty/serial/8250/8250_dw.c

@@ -59,6 +59,7 @@ struct dw8250_data {
 	int			last_mcr;
 	int			line;
 	struct clk		*clk;
+	struct clk		*pclk;
 	struct uart_8250_dma	dma;
 };
 
@@ -359,10 +360,25 @@ static int dw8250_probe(struct platform_device *pdev)
 		return -ENOMEM;
 
 	data->usr_reg = DW_UART_USR;
-	data->clk = devm_clk_get(&pdev->dev, NULL);
+	data->clk = devm_clk_get(&pdev->dev, "baudclk");
+	if (IS_ERR(data->clk))
+		data->clk = devm_clk_get(&pdev->dev, NULL);
 	if (!IS_ERR(data->clk)) {
-		clk_prepare_enable(data->clk);
-		uart.port.uartclk = clk_get_rate(data->clk);
+		err = clk_prepare_enable(data->clk);
+		if (err)
+			dev_warn(&pdev->dev, "could not enable optional baudclk: %d\n",
+				 err);
+		else
+			uart.port.uartclk = clk_get_rate(data->clk);
+	}
+
+	data->pclk = devm_clk_get(&pdev->dev, "apb_pclk");
+	if (!IS_ERR(data->pclk)) {
+		err = clk_prepare_enable(data->pclk);
+		if (err) {
+			dev_err(&pdev->dev, "could not enable apb_pclk\n");
+			return err;
+		}
 	}
 
 	data->dma.rx_chan_id = -1;
@@ -408,6 +424,9 @@ static int dw8250_remove(struct platform_device *pdev)
 
 	serial8250_unregister_port(data->line);
 
+	if (!IS_ERR(data->pclk))
+		clk_disable_unprepare(data->pclk);
+
 	if (!IS_ERR(data->clk))
 		clk_disable_unprepare(data->clk);
 
@@ -445,6 +464,9 @@ static int dw8250_runtime_suspend(struct device *dev)
 	if (!IS_ERR(data->clk))
 		clk_disable_unprepare(data->clk);
 
+	if (!IS_ERR(data->pclk))
+		clk_disable_unprepare(data->pclk);
+
 	return 0;
 }
 
@@ -452,6 +474,9 @@ static int dw8250_runtime_resume(struct device *dev)
 {
 	struct dw8250_data *data = dev_get_drvdata(dev);
 
+	if (!IS_ERR(data->pclk))
+		clk_prepare_enable(data->pclk);
+
 	if (!IS_ERR(data->clk))
 		clk_prepare_enable(data->clk);