On 04/07/2010 09:05 AM, Wu Zhangin wrote:
From: Wu Zhangjin<wuzhangjin@xxxxxxxxx>
Changes:
v1 -> v2:
o change the old mips_sched_clock() to mips_cyc2ns() and modify the
arguments to support 32bit.
o add 32bit support: use a smaller shift to avoid the quick overflow
of 64bit arithmatic and balance the overhead of the 128bit arithmatic
and the precision lost with the smaller shift.
----------------------
Because the high resolution sched_clock() for r4k has the same overflow
problem and solution mentioned in "MIPS: Octeon: Use non-overflowing
arithmetic in sched_clock".
"With typical mult and shift values, the calculation for Octeon's
sched_clock overflows when using 64-bit arithmetic. Use 128-bit
calculations instead."
To reduce the duplication, This patch abstracts the solution into an
inline funciton mips_cyc2ns() into arch/mips/include/asm/time.h from
arch/mips/cavium-octeon/csrc-octeon.c.
Two patches for Cavium and R4K will be sent out respectively to use this
common function.
Signed-off-by: Wu Zhangjin<wuzhangjin@xxxxxxxxx>
---
arch/mips/include/asm/time.h | 38 ++++++++++++++++++++++++++++++++++++++
1 files changed, 38 insertions(+), 0 deletions(-)
diff --git a/arch/mips/include/asm/time.h b/arch/mips/include/asm/time.h
index c7f1bfe..898f0e0 100644
--- a/arch/mips/include/asm/time.h
+++ b/arch/mips/include/asm/time.h
@@ -96,4 +96,42 @@ static inline void clockevent_set_clock(struct clock_event_device *cd,
clockevents_calc_mult_shift(cd, clock, 4);
}
+static inline unsigned long long mips_cyc2ns(u64 cyc, u64 mult, u64 shift)
+{
+#ifdef CONFIG_32BIT
+ /*
+ * To balance the overhead of 128bit-arithematic and the precision
+ * lost, we choose a smaller shift to avoid the quick overflow as the
+ * X86& ARM does. please refer to arch/x86/kernel/tsc.c and
+ * arch/arm/plat-orion/time.c
+ */
+ return (cyc * mult)>> shift;
Have you tested that on a 32-bit kernel? I think it may overflow for
many cases.
David Daney
