clocksource: kona: Add basic use of external clock

When an clock is specified in the device tree, enable it and use it to
determine the external clock frequency.
Signed-off-by: NTim Kryger <tim.kryger@linaro.org>
Reviewed-by: NMarkus Mayer <markus.mayer@linaro.org>
Reviewed-by: NMatt Porter <matt.porter@linaro.org>
Reviewed-by: NChristian Daudt <bcm@fixthebug.org>
Acked-by: NDaniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: NChristian Daudt <bcm@fixthebug.org>
Signed-off-by: NOlof Johansson <olof@lixom.net>

drivers/clocksource/bcm_kona_timer.c

@@ -17,6 +17,7 @@
 #include <linux/jiffies.h>
 #include <linux/clockchips.h>
 #include <linux/types.h>
+#include <linux/clk.h>
 
 #include <linux/io.h>
 #include <asm/mach/time.h>
@@ -101,11 +102,18 @@ kona_timer_get_counter(void *timer_base, uint32_t *msw, uint32_t *lsw)
 static void __init kona_timers_init(struct device_node *node)
 {
 	u32 freq;
+	struct clk *external_clk;
 
-	if (!of_property_read_u32(node, "clock-frequency", &freq))
+	external_clk = of_clk_get_by_name(node, NULL);
+
+	if (!IS_ERR(external_clk)) {
+		arch_timer_rate = clk_get_rate(external_clk);
+		clk_prepare_enable(external_clk);
+	} else if (!of_property_read_u32(node, "clock-frequency", &freq)) {
 		arch_timer_rate = freq;
-	else
-		panic("clock-frequency not set in the .dts file");
+	} else {
+		panic("unable to determine clock-frequency");
+	}
 
 	/* Setup IRQ numbers */
 	timers.tmr_irq = irq_of_parse_and_map(node, 0);