clocksource: dw_apb_timer_of: add clock-handling

Add the possibility to get the clock-frequency from a timer clock instead
of specifying it as dt property. Additionally also add the possibility
to also define a controlling periphal clock for the timer block.

The clock-frequency property is kept to act as fallback if no clocks
are specified.

Signed-off-by: Heiko Stuebner <[e-mail]>
Acked-by: Jamie Iles <[e-mail]>

Documentation/devicetree/bindings/rtc/dw-apb.txt

···
 - reg: physical base address of the controller and length of memory mapped
   region.
 - interrupts: IRQ line for the timer.
+- either clocks+clock-names or clock-frequency properties
+
+Optional properties:
+- clocks	: list of clock specifiers, corresponding to entries in
+		  the clock-names property;
+- clock-names	: should contain "timer" and "pclk" entries, matching entries
+		  in the clocks property.
 - clock-frequency: The frequency in HZ of the timer.
 - clock-freq: For backwards compatibility with picoxcell
+
+If using the clock specifiers, the pclk clock is optional, as not all
+systems may use one.
+
 
 Example:
 
···
 				interrupts = <0 169 4>;
 				clock-frequency = <200000000>;
 				reg = <0xffd00000 0x1000>;
+			};
+
+		timer3: timer@ffe00000 {
+				compatible = "snps,dw-apb-timer-osc";
+				interrupts = <0 170 4>;
+				reg = <0xffe00000 0x1000>;
+				clocks = <&timer_clk>, <&timer_pclk>;
+				clock-names = "timer", "pclk";
 			};

drivers/clocksource/dw_apb_timer_of.c

···
 #include <linux/of.h>
 #include <linux/of_address.h>
 #include <linux/of_irq.h>
+#include <linux/clk.h>
 
 #include <asm/mach/time.h>
 #include <asm/sched_clock.h>
···
 static void timer_get_base_and_rate(struct device_node *np,
 				    void __iomem **base, u32 *rate)
 {
+	struct clk *timer_clk;
+	struct clk *pclk;
+
 	*base = of_iomap(np, 0);
 
 	if (!*base)
 		panic("Unable to map regs for %s", np->name);
 
+	/*
+	 * Not all implementations use a periphal clock, so don't panic
+	 * if it's not present
+	 */
+	pclk = of_clk_get_by_name(np, "pclk");
+	if (!IS_ERR(pclk))
+		if (clk_prepare_enable(pclk))
+			pr_warn("pclk for %s is present, but could not be activated\n",
+				np->name);
+
+	timer_clk = of_clk_get_by_name(np, "timer");
+	if (IS_ERR(timer_clk))
+		goto try_clock_freq;
+
+	if (!clk_prepare_enable(timer_clk)) {
+		*rate = clk_get_rate(timer_clk);
+		return;
+	}
+
+try_clock_freq:
 	if (of_property_read_u32(np, "clock-freq", rate) &&
 		of_property_read_u32(np, "clock-frequency", rate))
-		panic("No clock-frequency property for %s", np->name);
+		panic("No clock nor clock-frequency property for %s", np->name);
 }
 
 static void add_clockevent(struct device_node *event_timer)