clocksource/drivers/tcb_clksrc: Use 32 bit tcb as sched_clock

On newer boards the TC can be read as single 32 bit value without locking.
Thus the clock can be used as reference for sched_clock which is much more
accurate than the jiffies implementation.

Tested on a Atmel SAMA5D2 board.
Signed-off-by: David Engraf <david.engraf@sysgo.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/tcb_clksrc.c

@@ -10,6 +10,7 @@
 #include <linux/io.h>
 #include <linux/platform_device.h>
 #include <linux/atmel_tc.h>
+#include <linux/sched_clock.h>
 
 
 /*
@@ -56,11 +57,16 @@ static u64 tc_get_cycles(struct clocksource *cs)
 	return (upper << 16) | lower;
 }
 
-static u64 tc_get_cycles32(struct clocksource *cs)
+static u32 tc_get_cv32(void)
 {
 	return __raw_readl(tcaddr + ATMEL_TC_REG(0, CV));
 }
 
+static u64 tc_get_cycles32(struct clocksource *cs)
+{
+	return tc_get_cv32();
+}
+
 static struct clocksource clksrc = {
 	.name           = "tcb_clksrc",
 	.rating         = 200,
@@ -69,6 +75,11 @@ static struct clocksource clksrc = {
 	.flags		= CLOCK_SOURCE_IS_CONTINUOUS,
 };
 
+static u64 notrace tc_read_sched_clock(void)
+{
+	return tc_get_cv32();
+}
+
 #ifdef CONFIG_GENERIC_CLOCKEVENTS
 
 struct tc_clkevt_device {
@@ -339,6 +350,9 @@ static int __init tcb_clksrc_init(void)
 		clksrc.read = tc_get_cycles32;
 		/* setup ony channel 0 */
 		tcb_setup_single_chan(tc, best_divisor_idx);
+
+		/* register sched_clock on chips with single 32 bit counter */
+		sched_clock_register(tc_read_sched_clock, 32, divided_rate);
 	} else {
 		/* tclib will give us three clocks no matter what the
 		 * underlying platform supports.