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ARM: 6488/1: nomadik: prevent sched_clock() wraparound

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The current implementation of sched_clock() for the Nomadik
family is based on the clock source that will wrap around without
any compensation. Currently on the Ux500 after 1030 seconds.

Utilize cnt32_to_63 to expand the sched_clock() counter to 63
bits and introduce a keepwarm() timer to assure that sched clock
and this cnt32_to_63 is called atleast once every half period.

When I print out the actual wrap-around time, and using
a year (3600*24*365 seconds) as minumum wrap limit I get an
actual wrap-around of:
sched_clock: using 55 bits @ 8333125 Hz wrap in 416 days

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
This commit is contained in:
Linus Walleij 2010-11-19 10:16:05 +01:00 committed by Russell King
parent 963fec4e0f
commit 8fbb97a2bc
1 changed files with 80 additions and 9 deletions
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89
arch/arm/plat-nomadik/timer.c
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@ -3,6 +3,7 @@
*
* Copyright (C) 2008 STMicroelectronics
* Copyright (C) 2010 Alessandro Rubini
* Copyright (C) 2010 Linus Walleij for ST-Ericsson
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2, as
@ -16,11 +17,13 @@
#include <linux/clk.h>
#include <linux/jiffies.h>
#include <linux/err.h>
#include <linux/cnt32_to_63.h>
#include <linux/timer.h>
#include <asm/mach/time.h>
#include <plat/mtu.h>
void __iomem *mtu_base; /* ssigned by machine code */
void __iomem *mtu_base; /* Assigned by machine code */
/*
* Kernel assumes that sched_clock can be called early
@ -48,16 +51,82 @@ static struct clocksource nmdk_clksrc = {
/*
* Override the global weak sched_clock symbol with this
* local implementation which uses the clocksource to get some
* better resolution when scheduling the kernel. We accept that
* this wraps around for now, since it is just a relative time
* stamp. (Inspired by OMAP implementation.)
* better resolution when scheduling the kernel.
*
* Because the hardware timer period may be quite short
* (32.3 secs on the 133 MHz MTU timer selection on ux500)
* and because cnt32_to_63() needs to be called at least once per
* half period to work properly, a kernel keepwarm() timer is set up
* to ensure this requirement is always met.
*
* Also the sched_clock timer will wrap around at some point,
* here we set it to run continously for a year.
*/
#define SCHED_CLOCK_MIN_WRAP 3600*24*365
static struct timer_list cnt32_to_63_keepwarm_timer;
static u32 sched_mult;
static u32 sched_shift;
unsigned long long notrace sched_clock(void)
{
return clocksource_cyc2ns(nmdk_clksrc.read(
&nmdk_clksrc),
nmdk_clksrc.mult,
nmdk_clksrc.shift);
u64 cycles;
if (unlikely(!mtu_base))
return 0;
cycles = cnt32_to_63(-readl(mtu_base + MTU_VAL(0)));
/*
* sched_mult is guaranteed to be even so will
* shift out bit 63
*/
return (cycles * sched_mult) >> sched_shift;
}
/* Just kick sched_clock every so often */
static void cnt32_to_63_keepwarm(unsigned long data)
{
mod_timer(&cnt32_to_63_keepwarm_timer, round_jiffies(jiffies + data));
(void) sched_clock();
}
/*
* Set up a timer to keep sched_clock():s 32_to_63 algorithm warm
* once in half a 32bit timer wrap interval.
*/
static void __init nmdk_sched_clock_init(unsigned long rate)
{
u32 v;
unsigned long delta;
u64 days;
/* Find the apropriate mult and shift factors */
clocks_calc_mult_shift(&sched_mult, &sched_shift,
rate, NSEC_PER_SEC, SCHED_CLOCK_MIN_WRAP);
/* We need to multiply by an even number to get rid of bit 63 */
if (sched_mult & 1)
sched_mult++;
/* Let's see what we get, take max counter and scale it */
days = (0xFFFFFFFFFFFFFFFFLLU * sched_mult) >> sched_shift;
do_div(days, NSEC_PER_SEC);
do_div(days, (3600*24));
pr_info("sched_clock: using %d bits @ %lu Hz wrap in %lu days\n",
(64 - sched_shift), rate, (unsigned long) days);
/*
* Program a timer to kick us at half 32bit wraparound
* Formula: seconds per wrap = (2^32) / f
*/
v = 0xFFFFFFFFUL / rate;
/* We want half of the wrap time to keep cnt32_to_63 warm */
v /= 2;
pr_debug("sched_clock: prescaled timer rate: %lu Hz, "
"initialize keepwarm timer every %d seconds\n", rate, v);
/* Convert seconds to jiffies */
delta = msecs_to_jiffies(v*1000);
setup_timer(&cnt32_to_63_keepwarm_timer, cnt32_to_63_keepwarm, delta);
mod_timer(&cnt32_to_63_keepwarm_timer, round_jiffies(jiffies + delta));
}
/* Clockevent device: use one-shot mode */
@ -161,13 +230,15 @@ void __init nmdk_timer_init(void)
writel(0, mtu_base + MTU_BGLR(0));
writel(cr | MTU_CRn_ENA, mtu_base + MTU_CR(0));
/* Now the scheduling clock is ready */
/* Now the clock source is ready */
nmdk_clksrc.read = nmdk_read_timer;
if (clocksource_register(&nmdk_clksrc))
pr_err("timer: failed to initialize clock source %s\n",
nmdk_clksrc.name);
nmdk_sched_clock_init(rate);
/* Timer 1 is used for events */
clockevents_calc_mult_shift(&nmdk_clkevt, rate, MTU_MIN_RANGE);