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[PATCH] MIPS: Octeon: Use non-overflowing arithmetic in sched_clock

To: linux-mips@linux-mips.org, ralf@linux-mips.org
Subject: [PATCH] MIPS: Octeon: Use non-overflowing arithmetic in sched_clock
From: David Daney <ddaney@caviumnetworks.com>
Date: Fri, 8 Jan 2010 14:47:36 -0800
Cc: David Daney <ddaney@caviumnetworks.com>
Sender: linux-mips-bounce@linux-mips.org
With typical mult and shift values, the calculation for Octeon's
sched_clock overflows when using 64-bit arithmetic.  Use 128-bit
calculations instead.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
---
 arch/mips/cavium-octeon/csrc-octeon.c |   31 ++++++++++++++++++++++++++++---
 1 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/arch/mips/cavium-octeon/csrc-octeon.c 
b/arch/mips/cavium-octeon/csrc-octeon.c
index 96df821..0bf4bbe 100644
--- a/arch/mips/cavium-octeon/csrc-octeon.c
+++ b/arch/mips/cavium-octeon/csrc-octeon.c
@@ -52,9 +52,34 @@ static struct clocksource clocksource_mips = {
 
 unsigned long long notrace sched_clock(void)
 {
-       return clocksource_cyc2ns(read_c0_cvmcount(),
-                                 clocksource_mips.mult,
-                                 clocksource_mips.shift);
+       /* 64-bit arithmatic can overflow, so use 128-bit.  */
+#if (__GNUC__ < 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ <= 3))
+       u64 t1, t2, t3;
+       unsigned long long rv;
+       u64 mult = clocksource_mips.mult;
+       u64 shift = clocksource_mips.shift;
+       u64 cnt = read_c0_cvmcount();
+
+       asm (
+               "dmultu\t%[cnt],%[mult]\n\t"
+               "nor\t%[t1],$0,%[shift]\n\t"
+               "mfhi\t%[t2]\n\t"
+               "mflo\t%[t3]\n\t"
+               "dsll\t%[t2],%[t2],1\n\t"
+               "dsrlv\t%[rv],%[t3],%[shift]\n\t"
+               "dsllv\t%[t1],%[t2],%[t1]\n\t"
+               "or\t%[rv],%[t1],%[rv]\n\t"
+               : [rv] "=&r" (rv), [t1] "=&r" (t1), [t2] "=&r" (t2), [t3] "=&r" 
(t3)
+               : [cnt] "r" (cnt), [mult] "r" (mult), [shift] "r" (shift)
+               : "hi", "lo");
+       return rv;
+#else
+       /* GCC > 4.3 do it the easy way.  */
+       unsigned int __attribute__((mode(TI))) t;
+       t = read_c0_cvmcount();
+       t = t * clocksource_mips.mult;
+       return (unsigned long long)(t >> clocksource_mips.shift);
+#endif
 }
 
 void __init plat_time_init(void)
-- 
1.6.0.6


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