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[PATCH v3 3/3] MIPS: r4k: Add a high resolution sched_clock()

To: Ralf Baechle <>
Subject: [PATCH v3 3/3] MIPS: r4k: Add a high resolution sched_clock()
From: Wu Zhangjin <>
Date: Sat, 10 Apr 2010 14:49:59 +0800
Cc: Wu Zhangjin <>, David Daney <>, Ralf Rösch <>,
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From: Wu Zhangjin <>

(v10 -> v11:

 o uses 32bit instead of 64bit for mult and shift for the new

 o choose a smaller scaling factor: 8 to ensure it overflows slower.
 With 8, if the clock frequency is 400 MHz, it will overflow after 12509
 hours(about 521 days) which is enough for generic debugging(i.e. Ftrace).

 o annotate the cnt32_to_63_keepwarm() with notrace.

 v9 -> v10:

 o use the new interface mips_cyc2ns() instead of the old
 o adds 32bit support via using a smaller shift to balance the overhead
 of 128bit arithmatic and the precision lost. please refer to the method
 used in X86 & ARM platforms, arch/x86/kernel/tsc.c,

 v8 -> v9:
 O Make it depends on 64BIT for the current mips_cyc2ns() only
 support 64bit currently.

 v7 -> v8:

 O Make it works with the exisiting clocksource_mips.mult,
 clocksource_mips.shift and copes with the 64bit calculation's overflow
 problem with the method introduced by David Daney in "MIPS: Octeon: Use
 non-overflowing arithmetic in sched_clock".

 To reduce the duplication, I have abstracted an inline
 mips_cyc2ns() function to arch/mips/include/asm/time.h from

 v6 -> v7:

 O Make it depends on !CPU_FREQ and CPU_HAS_FIXED_C0_COUNT

 This sched_clock() is only available with the processor has fixed cp0
 MIPS count register or even has dynamic cp0 MIPS count register but
 with CPU_FREQ disabled.

 NOTE: If your processor has fixed c0 count, please select
 CPU_HAS_FIXED_C0_COUNT for it and send a related patch to Ralf.

 v5 -> v6:

 o hard-codes the cycle2ns_scale_factor as 8 for 30(cs->shift) is too
 big. With 30, the return value of sched_clock() will also overflow quickly.
 o moves the sched_clock() back into csrc-r4k.c as David and Sergei
 o inits c0 count as zero for PRINTK_TIME=y.
 o drops the HR_SCHED_CLCOK option for the current sched_clock() is stable
 enough to replace the jiffies based one.

This patch adds a cnt32_to_63() and MIPS c0 count based sched_clock(),
which provides high resolution.

Without it, the Ftrace for MIPS will give useless timestamp information.

Because cnt32_to_63() needs to be called at least once per half period
to work properly, Differ from the old version, this v2 revision set up a
kernel timer to ensure the requirement of some MIPSs which have short c0
count period.

And also, we init the c0 count as ZERO(just as jiffies does) in
time_init() before plat_time_init(), without it, PRINTK_TIME=y will get
wrong timestamp information. (NOTE: some platforms have initiazlied c0
count as zero, but some not, this may introduce some duplication,
perhaps a new patch is needed to remove the initialized of c0 count in
the platforms later?)

This is originally from arch/arm/plat-orion/time.c

This revision works well for function graph tracer now, and also,
PRINTK_TIME=y will get normal timestamp informatin.

Signed-off-by: Wu Zhangjin <>
 arch/mips/Kconfig           |   12 +++++++
 arch/mips/kernel/csrc-r4k.c |   76 +++++++++++++++++++++++++++++++++++++++++++
 arch/mips/kernel/time.c     |    5 +++
 3 files changed, 93 insertions(+), 0 deletions(-)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index f2ead53..b302838 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -1962,6 +1962,18 @@ config NR_CPUS
 source "kernel/time/Kconfig"
+# High Resolution sched_clock() support
+       bool
+       bool
+       depends on CPU_HAS_FIXED_C0_COUNT || !CPU_FREQ
+       default y
 # Timer Interrupt Frequency Configuration
diff --git a/arch/mips/kernel/csrc-r4k.c b/arch/mips/kernel/csrc-r4k.c
index e95a3cd..92870cb 100644
--- a/arch/mips/kernel/csrc-r4k.c
+++ b/arch/mips/kernel/csrc-r4k.c
@@ -6,7 +6,9 @@
  * Copyright (C) 2007 by Ralf Baechle
 #include <linux/clocksource.h>
+#include <linux/cnt32_to_63.h>
 #include <linux/init.h>
+#include <linux/timer.h>
 #include <asm/time.h>
@@ -22,6 +24,78 @@ static struct clocksource clocksource_mips = {
        .flags          = CLOCK_SOURCE_IS_CONTINUOUS,
+ * MIPS sched_clock implementation.
+ *
+ * Because the hardware timer period is quite short and because cnt32_to_63()
+ * needs to be called at least once per half period to work properly, a kernel
+ * timer is set up to ensure this requirement is always met.
+ *
+ * Please refer to include/linux/cnt32_to_63.h, arch/arm/plat-orion/time.c and
+ * arch/mips/include/asm/time.h (mips_cyc2ns)
+ */
+#define CYC2NS_SHIFT 8
+static u32 mult __read_mostly;
+static u32 shift __read_mostly;
+unsigned long long notrace sched_clock(void)
+       u64 cyc = cnt32_to_63(read_c0_count());
+#ifdef CONFIG_64BIT
+       /* For we have used 128bit arithmatic to cope with the overflow
+        * problem, the method to clear the top bit with an event value doesn't
+        * work now, therefore, clear it at run-time is needed.
+        */
+       if (cyc & 0x8000000000000000)
+               cyc &= 0x7fffffffffffffff;
+       return mips_cyc2ns(cyc, mult, shift);
+static struct timer_list cnt32_to_63_keepwarm_timer;
+static void notrace cnt32_to_63_keepwarm(unsigned long data)
+       mod_timer(&cnt32_to_63_keepwarm_timer, round_jiffies(jiffies + data));
+       sched_clock();
+static inline void setup_hres_sched_clock(unsigned long clock)
+       unsigned long data;
+#ifdef CONFIG_32BIT
+       unsigned long long v;
+       v = NSEC_PER_SEC;
+       v <<= CYC2NS_SHIFT;
+       v += clock/2;
+       do_div(v, clock);
+       mult = v;
+       shift = CYC2NS_SHIFT;
+       /*
+        * We want an even value to automatically clear the top bit
+        * returned by cnt32_to_63() without an additional run time
+        * instruction. So if the LSB is 1 then round it up.
+        */
+       if (mult & 1)
+               mult++;
+       mult = clocksource_mips.mult;
+       shift = clocksource_mips.shift;
+       data = 0x80000000UL / clock * HZ;
+       setup_timer(&cnt32_to_63_keepwarm_timer, cnt32_to_63_keepwarm, data);
+       mod_timer(&cnt32_to_63_keepwarm_timer, round_jiffies(jiffies + data));
 int __init init_r4k_clocksource(void)
        if (!cpu_has_counter || !mips_hpt_frequency)
@@ -32,6 +106,8 @@ int __init init_r4k_clocksource(void)
        clocksource_set_clock(&clocksource_mips, mips_hpt_frequency);
+       setup_hres_sched_clock(mips_hpt_frequency);
        return 0;
diff --git a/arch/mips/kernel/time.c b/arch/mips/kernel/time.c
index fb74974..86cf18a 100644
--- a/arch/mips/kernel/time.c
+++ b/arch/mips/kernel/time.c
@@ -119,6 +119,11 @@ static __init int cpu_has_mfc0_count_bug(void)
 void __init time_init(void)
+       if (!mips_clockevent_init() || !cpu_has_mfc0_count_bug())
+               write_c0_count(0);
        if (!mips_clockevent_init() || !cpu_has_mfc0_count_bug())

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