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Re: [PATCH v2 1/3] MIPS: add a common mips_cyc2ns()

To: Wu Zhangin <wuzhangjin@gmail.com>
Subject: Re: [PATCH v2 1/3] MIPS: add a common mips_cyc2ns()
From: David Daney <ddaney@caviumnetworks.com>
Date: Wed, 07 Apr 2010 09:50:52 -0700
Cc: Ralf Baechle <ralf@linux-mips.org>, linux-mips@linux-mips.org, Ralf Rösch <roesch.ralf@web.de>
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On 04/07/2010 09:05 AM, Wu Zhangin wrote:
From: Wu Zhangjin<wuzhangjin@gmail.com>

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@gmail.com>
---
  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

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