[PATCH/RFC] powerpc: Fix decrementer setup on 1GHz boards

[Stefan Roese]

We noticed that recent kernels didn't boot on our 1GHz Canyonlands 460EX
boards anymore. As it seems, patch 8d165db1 [powerpc: Improve
decrementer accuracy] introduced this problem. The routine div_sc()
overflows with shift = 32 resulting in this incorrect setup:

time_init: decrementer frequency = 1000.000012 MHz
time_init: processor frequency   = 1000.000012 MHz
clocksource: timebase mult[400000] shift[22] registered
clockevent: decrementer mult[33] shift[32] cpu[0]

This patch now introduces a local div_dc64() version of this function
so that this overflow doesn't happen anymore.

Signed-off-by: Stefan Roese <sr at denx.de>
Cc: Benjamin Herrenschmidt <benh at kernel.crashing.org>
Cc: Anton Blanchard <anton at samba.org>
Cc: Detlev Zundel <dzu at denx.de>
---
Ben, Anton, how should we handle this? Is this patch acceptable? Or how
should this be solved?

 arch/powerpc/kernel/time.c |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 92dc844..3ad729f 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -882,12 +882,21 @@ static void decrementer_set_mode(enum clock_event_mode mode,
 		decrementer_set_next_event(DECREMENTER_MAX, dev);
 }
 
+static inline uint64_t div_sc64(unsigned long ticks, unsigned long nsec,
+				int shift)
+{
+	uint64_t tmp = ((uint64_t)ticks) << shift;
+
+	do_div(tmp, nsec);
+	return tmp;
+}
+
 static void __init setup_clockevent_multiplier(unsigned long hz)
 {
 	u64 mult, shift = 32;
 
 	while (1) {
-		mult = div_sc(hz, NSEC_PER_SEC, shift);
+		mult = div_sc64(hz, NSEC_PER_SEC, shift);
 		if (mult && (mult >> 32UL) == 0UL)
 			break;
 
-- 
1.6.4.4