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Re: [PATCH v3] MIPS: Add a high resolution sched_clock() via cnt32_to_63

To: Wu Zhangjin <wuzhangjin@gmail.com>
Subject: Re: [PATCH v3] MIPS: Add a high resolution sched_clock() via cnt32_to_63().
From: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Date: Sun, 22 Nov 2009 22:03:00 +0300
Cc: Ralf Baechle <ralf@linux-mips.org>, Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>, linux-mips@linux-mips.org, Michal Simek <monstr@monstr.eu>
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Hello.

Wu Zhangjin wrote:

From: Wu Zhangjin <wuzhangjin@gmail.com>

(This v3 revision incorporates with the feedbacks from Ingo, Ralf and Sergei.)

This patch adds a cnt32_to_63() and MIPS c0 count based sched_clock(),
which provides high resolution. and also, one new kernel option
(HR_SCHED_CLOCK) is added to enable/disable this sched_clock().

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.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
---
 arch/mips/Kconfig           |   18 ++++++++++++++
 arch/mips/kernel/csrc-r4k.c |   55 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 73 insertions(+), 0 deletions(-)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index b342197..5ea55f5 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -1952,6 +1952,24 @@ config NR_CPUS
 source "kernel/time/Kconfig"
#
+# High Resolution sched_clock() Configuration
+#
+
+config HR_SCHED_CLOCK
+       bool "High Resolution sched_clock()"
+       depends on CSRC_R4K
+       default n
+       help
+         This option enables the MIPS c0 count based high resolution
+         sched_clock().
+
+         If you need a ns precision timestamp, you are recommended to enable
+         this option. For example, If you are using the Ftrace subsystem to do
+         real time tracing, this option is needed.
+
+         If unsure, disable it.
+
+#
 # Timer Interrupt Frequency Configuration
 #
diff --git a/arch/mips/kernel/csrc-r4k.c b/arch/mips/kernel/csrc-r4k.c
index e95a3cd..e287f0d 100644
--- a/arch/mips/kernel/csrc-r4k.c
+++ b/arch/mips/kernel/csrc-r4k.c
@@ -10,6 +10,57 @@
#include <asm/time.h> +#ifdef CONFIG_HR_SCHED_CLOCK
+#include <linux/cnt32_to_63.h>
+#include <linux/timer.h>
+
+/*
+ * MIPS sched_clock implementation.
+ *
+ * because cnt32_to_63() needs to be called at least once per half period to
+ * work properly, and some of the MIPS frequency is high, perhaps a kernel
+ * timer is needed to be set up to ensure this requirement is always met.
+ * Please refer to arch/arm/plat-orion/time.c and include/linux/cnt32_to_63.h
+ */
+static unsigned long __read_mostly tclk2ns_scale;
+static unsigned long __read_mostly tclk2ns_scale_factor;
+
+unsigned long long notrace sched_clock(void)
+{
+       unsigned long long v = cnt32_to_63(read_c0_count());
+       return (v * tclk2ns_scale) >> tclk2ns_scale_factor;
+}
+
+static struct timer_list cnt32_to_63_keepwarm_timer;
+
+static void cnt32_to_63_keepwarm(unsigned long data)
+{
+       mod_timer(&cnt32_to_63_keepwarm_timer, round_jiffies(jiffies + data));
+       (void) sched_clock();
+}
+
+static void __init setup_sched_clock(struct clocksource *cs, unsigned long 
tclk)
+{
+       unsigned long long v;
+       unsigned long data;
+
+       v = cs->mult;
+       /*
+        * 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 (v & 1)
+               v++;
+       tclk2ns_scale = v;
+       tclk2ns_scale_factor = cs->shift;
+
+       data = 0x80000000 / tclk * HZ;
+       setup_timer(&cnt32_to_63_keepwarm_timer, cnt32_to_63_keepwarm, data);
+       mod_timer(&cnt32_to_63_keepwarm_timer, round_jiffies(jiffies + data));
+}
+#endif /* !CONFIG_HR_SCHED_CLOCK */
+
 static cycle_t c0_hpt_read(struct clocksource *cs)
 {
        return read_c0_count();
@@ -32,6 +83,10 @@ int __init init_r4k_clocksource(void)
clocksource_set_clock(&clocksource_mips, mips_hpt_frequency); +#ifdef CONFIG_HR_SCHED_CLOCK
+       setup_sched_clock(&clocksource_mips, mips_hpt_frequency);
+#endif
+

According to CodingStyle #ifdef's in the code are frowned upon. I suggest that you add an empty setup_sched_clock() definition under #else above and have the call here without #ifdef. That's the preferred way of doing such things...

WBR, Sergei



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