On AT91 there is no architecture specific sched_clock() implementation,
so the default fallback is used. This fallback uses the jiffie counter
as sched_clock().
There is NO standard clocksource available that is accurate enough,
except the TC-based clocksource implementation. Therefor this
implementation is used as base for the sched_clock(). This clocksource
offers sub-millisecond time-stamping. (< 200 ns)

Signed-off-by: Remy Bohmer <redacted>
---
 arch/arm/Kconfig                 |    1 +
 arch/arm/kernel/sched_clock.c    |    7 +++++++
 drivers/clocksource/tcb_clksrc.c |   23 +++++++++++++++++++++++
 3 files changed, 31 insertions(+), 0 deletions(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 9adc278..e0563a7 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig

@@ -296,6 +296,7 @@ config ARCH_AT91
 	select HAVE_CLK
 	select CLKDEV_LOOKUP
 	select ARM_PATCH_PHYS_VIRT if MMU
+	select HAVE_SCHED_CLOCK if ATMEL_TCB_CLKSRC
 	help
 	  This enables support for systems based on the Atmel AT91RM9200,
 	  AT91SAM9 and AT91CAP9 processors.

diff --git a/arch/arm/kernel/sched_clock.c b/arch/arm/kernel/sched_clock.c
index 9a46370..f9028e4 100644
--- a/arch/arm/kernel/sched_clock.c
+++ b/arch/arm/kernel/sched_clock.c

@@ -20,6 +20,13 @@ static void (*sched_clock_update_fn)(void);
 
 static void sched_clock_poll(unsigned long wrap_ticks)
 {
+	/*
+	 * The sched_clock_update_fn may be initialised AFTER first call to
+	 * time_init()
+	 */
+	if (unlikely(!sched_clock_update_fn))
+		return;