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d78c9300c5
timeval_to_jiffies tried to round a timeval up to an integral number
of jiffies, but the logic for doing so was incorrect: intervals
corresponding to exactly N jiffies would become N+1. This manifested
itself particularly repeatedly stopping/starting an itimer:
setitimer(ITIMER_PROF, &val, NULL);
setitimer(ITIMER_PROF, NULL, &val);
would add a full tick to val, _even if it was exactly representable in
terms of jiffies_ (say, the result of a previous rounding.) Doing
this repeatedly would cause unbounded growth in val. So fix the math.
Here's what was wrong with the conversion: we essentially computed
(eliding seconds)
jiffies = usec * (NSEC_PER_USEC/TICK_NSEC)
by using scaling arithmetic, which took the best approximation of
NSEC_PER_USEC/TICK_NSEC with denominator of 2^USEC_JIFFIE_SC =
x/(2^USEC_JIFFIE_SC), and computed:
jiffies = (usec * x) >> USEC_JIFFIE_SC
and rounded this calculation up in the intermediate form (since we
can't necessarily exactly represent TICK_NSEC in usec.) But the
scaling arithmetic is a (very slight) *over*approximation of the true
value; that is, instead of dividing by (1 usec/ 1 jiffie), we
effectively divided by (1 usec/1 jiffie)-epsilon (rounding
down). This would normally be fine, but we want to round timeouts up,
and we did so by adding 2^USEC_JIFFIE_SC - 1 before the shift; this
would be fine if our division was exact, but dividing this by the
slightly smaller factor was equivalent to adding just _over_ 1 to the
final result (instead of just _under_ 1, as desired.)
In particular, with HZ=1000, we consistently computed that 10000 usec
was 11 jiffies; the same was true for any exact multiple of
TICK_NSEC.
We could possibly still round in the intermediate form, adding
something less than 2^USEC_JIFFIE_SC - 1, but easier still is to
convert usec->nsec, round in nanoseconds, and then convert using
time*spec*_to_jiffies. This adds one constant multiplication, and is
not observably slower in microbenchmarks on recent x86 hardware.
Tested: the following program:
int main() {
struct itimerval zero = {{0, 0}, {0, 0}};
/* Initially set to 10 ms. */
struct itimerval initial = zero;
initial.it_interval.tv_usec = 10000;
setitimer(ITIMER_PROF, &initial, NULL);
/* Save and restore several times. */
for (size_t i = 0; i < 10; ++i) {
struct itimerval prev;
setitimer(ITIMER_PROF, &zero, &prev);
/* on old kernels, this goes up by TICK_USEC every iteration */
printf("previous value: %ld %ld %ld %ld\n",
prev.it_interval.tv_sec, prev.it_interval.tv_usec,
prev.it_value.tv_sec, prev.it_value.tv_usec);
setitimer(ITIMER_PROF, &prev, NULL);
}
return 0;
}
Cc: stable@vger.kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Paul Turner <pjt@google.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Reviewed-by: Paul Turner <pjt@google.com>
Reported-by: Aaron Jacobs <jacobsa@google.com>
Signed-off-by: Andrew Hunter <ahh@google.com>
[jstultz: Tweaked to apply to 3.17-rc]
Signed-off-by: John Stultz <john.stultz@linaro.org>
315 lines
11 KiB
C
315 lines
11 KiB
C
#ifndef _LINUX_JIFFIES_H
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#define _LINUX_JIFFIES_H
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#include <linux/math64.h>
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#include <linux/kernel.h>
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#include <linux/types.h>
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#include <linux/time.h>
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#include <linux/timex.h>
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#include <asm/param.h> /* for HZ */
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/*
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* The following defines establish the engineering parameters of the PLL
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* model. The HZ variable establishes the timer interrupt frequency, 100 Hz
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* for the SunOS kernel, 256 Hz for the Ultrix kernel and 1024 Hz for the
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* OSF/1 kernel. The SHIFT_HZ define expresses the same value as the
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* nearest power of two in order to avoid hardware multiply operations.
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*/
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#if HZ >= 12 && HZ < 24
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# define SHIFT_HZ 4
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#elif HZ >= 24 && HZ < 48
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# define SHIFT_HZ 5
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#elif HZ >= 48 && HZ < 96
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# define SHIFT_HZ 6
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#elif HZ >= 96 && HZ < 192
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# define SHIFT_HZ 7
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#elif HZ >= 192 && HZ < 384
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# define SHIFT_HZ 8
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#elif HZ >= 384 && HZ < 768
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# define SHIFT_HZ 9
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#elif HZ >= 768 && HZ < 1536
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# define SHIFT_HZ 10
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#elif HZ >= 1536 && HZ < 3072
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# define SHIFT_HZ 11
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#elif HZ >= 3072 && HZ < 6144
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# define SHIFT_HZ 12
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#elif HZ >= 6144 && HZ < 12288
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# define SHIFT_HZ 13
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#else
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# error Invalid value of HZ.
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#endif
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/* Suppose we want to divide two numbers NOM and DEN: NOM/DEN, then we can
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* improve accuracy by shifting LSH bits, hence calculating:
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* (NOM << LSH) / DEN
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* This however means trouble for large NOM, because (NOM << LSH) may no
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* longer fit in 32 bits. The following way of calculating this gives us
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* some slack, under the following conditions:
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* - (NOM / DEN) fits in (32 - LSH) bits.
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* - (NOM % DEN) fits in (32 - LSH) bits.
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*/
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#define SH_DIV(NOM,DEN,LSH) ( (((NOM) / (DEN)) << (LSH)) \
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+ ((((NOM) % (DEN)) << (LSH)) + (DEN) / 2) / (DEN))
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/* LATCH is used in the interval timer and ftape setup. */
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#define LATCH ((CLOCK_TICK_RATE + HZ/2) / HZ) /* For divider */
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extern int register_refined_jiffies(long clock_tick_rate);
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/* TICK_NSEC is the time between ticks in nsec assuming SHIFTED_HZ */
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#define TICK_NSEC ((NSEC_PER_SEC+HZ/2)/HZ)
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/* TICK_USEC is the time between ticks in usec assuming fake USER_HZ */
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#define TICK_USEC ((1000000UL + USER_HZ/2) / USER_HZ)
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/* some arch's have a small-data section that can be accessed register-relative
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* but that can only take up to, say, 4-byte variables. jiffies being part of
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* an 8-byte variable may not be correctly accessed unless we force the issue
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*/
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#define __jiffy_data __attribute__((section(".data")))
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/*
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* The 64-bit value is not atomic - you MUST NOT read it
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* without sampling the sequence number in jiffies_lock.
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* get_jiffies_64() will do this for you as appropriate.
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*/
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extern u64 __jiffy_data jiffies_64;
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extern unsigned long volatile __jiffy_data jiffies;
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#if (BITS_PER_LONG < 64)
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u64 get_jiffies_64(void);
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#else
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static inline u64 get_jiffies_64(void)
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{
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return (u64)jiffies;
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}
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#endif
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/*
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* These inlines deal with timer wrapping correctly. You are
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* strongly encouraged to use them
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* 1. Because people otherwise forget
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* 2. Because if the timer wrap changes in future you won't have to
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* alter your driver code.
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*
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* time_after(a,b) returns true if the time a is after time b.
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*
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* Do this with "<0" and ">=0" to only test the sign of the result. A
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* good compiler would generate better code (and a really good compiler
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* wouldn't care). Gcc is currently neither.
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*/
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#define time_after(a,b) \
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(typecheck(unsigned long, a) && \
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typecheck(unsigned long, b) && \
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((long)((b) - (a)) < 0))
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#define time_before(a,b) time_after(b,a)
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#define time_after_eq(a,b) \
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(typecheck(unsigned long, a) && \
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typecheck(unsigned long, b) && \
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((long)((a) - (b)) >= 0))
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#define time_before_eq(a,b) time_after_eq(b,a)
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/*
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* Calculate whether a is in the range of [b, c].
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*/
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#define time_in_range(a,b,c) \
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(time_after_eq(a,b) && \
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time_before_eq(a,c))
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/*
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* Calculate whether a is in the range of [b, c).
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*/
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#define time_in_range_open(a,b,c) \
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(time_after_eq(a,b) && \
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time_before(a,c))
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/* Same as above, but does so with platform independent 64bit types.
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* These must be used when utilizing jiffies_64 (i.e. return value of
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* get_jiffies_64() */
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#define time_after64(a,b) \
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(typecheck(__u64, a) && \
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typecheck(__u64, b) && \
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((__s64)((b) - (a)) < 0))
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#define time_before64(a,b) time_after64(b,a)
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#define time_after_eq64(a,b) \
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(typecheck(__u64, a) && \
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typecheck(__u64, b) && \
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((__s64)((a) - (b)) >= 0))
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#define time_before_eq64(a,b) time_after_eq64(b,a)
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#define time_in_range64(a, b, c) \
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(time_after_eq64(a, b) && \
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time_before_eq64(a, c))
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/*
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* These four macros compare jiffies and 'a' for convenience.
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*/
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/* time_is_before_jiffies(a) return true if a is before jiffies */
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#define time_is_before_jiffies(a) time_after(jiffies, a)
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/* time_is_after_jiffies(a) return true if a is after jiffies */
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#define time_is_after_jiffies(a) time_before(jiffies, a)
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/* time_is_before_eq_jiffies(a) return true if a is before or equal to jiffies*/
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#define time_is_before_eq_jiffies(a) time_after_eq(jiffies, a)
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/* time_is_after_eq_jiffies(a) return true if a is after or equal to jiffies*/
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#define time_is_after_eq_jiffies(a) time_before_eq(jiffies, a)
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/*
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* Have the 32 bit jiffies value wrap 5 minutes after boot
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* so jiffies wrap bugs show up earlier.
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*/
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#define INITIAL_JIFFIES ((unsigned long)(unsigned int) (-300*HZ))
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/*
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* Change timeval to jiffies, trying to avoid the
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* most obvious overflows..
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*
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* And some not so obvious.
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*
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* Note that we don't want to return LONG_MAX, because
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* for various timeout reasons we often end up having
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* to wait "jiffies+1" in order to guarantee that we wait
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* at _least_ "jiffies" - so "jiffies+1" had better still
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* be positive.
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*/
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#define MAX_JIFFY_OFFSET ((LONG_MAX >> 1)-1)
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extern unsigned long preset_lpj;
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/*
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* We want to do realistic conversions of time so we need to use the same
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* values the update wall clock code uses as the jiffies size. This value
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* is: TICK_NSEC (which is defined in timex.h). This
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* is a constant and is in nanoseconds. We will use scaled math
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* with a set of scales defined here as SEC_JIFFIE_SC, USEC_JIFFIE_SC and
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* NSEC_JIFFIE_SC. Note that these defines contain nothing but
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* constants and so are computed at compile time. SHIFT_HZ (computed in
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* timex.h) adjusts the scaling for different HZ values.
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* Scaled math??? What is that?
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*
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* Scaled math is a way to do integer math on values that would,
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* otherwise, either overflow, underflow, or cause undesired div
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* instructions to appear in the execution path. In short, we "scale"
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* up the operands so they take more bits (more precision, less
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* underflow), do the desired operation and then "scale" the result back
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* by the same amount. If we do the scaling by shifting we avoid the
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* costly mpy and the dastardly div instructions.
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* Suppose, for example, we want to convert from seconds to jiffies
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* where jiffies is defined in nanoseconds as NSEC_PER_JIFFIE. The
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* simple math is: jiff = (sec * NSEC_PER_SEC) / NSEC_PER_JIFFIE; We
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* observe that (NSEC_PER_SEC / NSEC_PER_JIFFIE) is a constant which we
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* might calculate at compile time, however, the result will only have
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* about 3-4 bits of precision (less for smaller values of HZ).
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*
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* So, we scale as follows:
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* jiff = (sec) * (NSEC_PER_SEC / NSEC_PER_JIFFIE);
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* jiff = ((sec) * ((NSEC_PER_SEC * SCALE)/ NSEC_PER_JIFFIE)) / SCALE;
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* Then we make SCALE a power of two so:
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* jiff = ((sec) * ((NSEC_PER_SEC << SCALE)/ NSEC_PER_JIFFIE)) >> SCALE;
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* Now we define:
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* #define SEC_CONV = ((NSEC_PER_SEC << SCALE)/ NSEC_PER_JIFFIE))
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* jiff = (sec * SEC_CONV) >> SCALE;
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*
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* Often the math we use will expand beyond 32-bits so we tell C how to
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* do this and pass the 64-bit result of the mpy through the ">> SCALE"
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* which should take the result back to 32-bits. We want this expansion
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* to capture as much precision as possible. At the same time we don't
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* want to overflow so we pick the SCALE to avoid this. In this file,
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* that means using a different scale for each range of HZ values (as
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* defined in timex.h).
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*
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* For those who want to know, gcc will give a 64-bit result from a "*"
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* operator if the result is a long long AND at least one of the
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* operands is cast to long long (usually just prior to the "*" so as
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* not to confuse it into thinking it really has a 64-bit operand,
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* which, buy the way, it can do, but it takes more code and at least 2
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* mpys).
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* We also need to be aware that one second in nanoseconds is only a
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* couple of bits away from overflowing a 32-bit word, so we MUST use
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* 64-bits to get the full range time in nanoseconds.
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*/
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/*
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* Here are the scales we will use. One for seconds, nanoseconds and
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* microseconds.
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*
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* Within the limits of cpp we do a rough cut at the SEC_JIFFIE_SC and
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* check if the sign bit is set. If not, we bump the shift count by 1.
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* (Gets an extra bit of precision where we can use it.)
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* We know it is set for HZ = 1024 and HZ = 100 not for 1000.
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* Haven't tested others.
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* Limits of cpp (for #if expressions) only long (no long long), but
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* then we only need the most signicant bit.
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*/
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#define SEC_JIFFIE_SC (31 - SHIFT_HZ)
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#if !((((NSEC_PER_SEC << 2) / TICK_NSEC) << (SEC_JIFFIE_SC - 2)) & 0x80000000)
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#undef SEC_JIFFIE_SC
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#define SEC_JIFFIE_SC (32 - SHIFT_HZ)
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#endif
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#define NSEC_JIFFIE_SC (SEC_JIFFIE_SC + 29)
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#define SEC_CONVERSION ((unsigned long)((((u64)NSEC_PER_SEC << SEC_JIFFIE_SC) +\
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TICK_NSEC -1) / (u64)TICK_NSEC))
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#define NSEC_CONVERSION ((unsigned long)((((u64)1 << NSEC_JIFFIE_SC) +\
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TICK_NSEC -1) / (u64)TICK_NSEC))
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/*
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* The maximum jiffie value is (MAX_INT >> 1). Here we translate that
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* into seconds. The 64-bit case will overflow if we are not careful,
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* so use the messy SH_DIV macro to do it. Still all constants.
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*/
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#if BITS_PER_LONG < 64
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# define MAX_SEC_IN_JIFFIES \
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(long)((u64)((u64)MAX_JIFFY_OFFSET * TICK_NSEC) / NSEC_PER_SEC)
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#else /* take care of overflow on 64 bits machines */
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# define MAX_SEC_IN_JIFFIES \
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(SH_DIV((MAX_JIFFY_OFFSET >> SEC_JIFFIE_SC) * TICK_NSEC, NSEC_PER_SEC, 1) - 1)
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#endif
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/*
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* Convert various time units to each other:
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*/
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extern unsigned int jiffies_to_msecs(const unsigned long j);
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extern unsigned int jiffies_to_usecs(const unsigned long j);
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static inline u64 jiffies_to_nsecs(const unsigned long j)
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{
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return (u64)jiffies_to_usecs(j) * NSEC_PER_USEC;
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}
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extern unsigned long msecs_to_jiffies(const unsigned int m);
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extern unsigned long usecs_to_jiffies(const unsigned int u);
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extern unsigned long timespec_to_jiffies(const struct timespec *value);
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extern void jiffies_to_timespec(const unsigned long jiffies,
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struct timespec *value);
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extern unsigned long timeval_to_jiffies(const struct timeval *value);
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extern void jiffies_to_timeval(const unsigned long jiffies,
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struct timeval *value);
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extern clock_t jiffies_to_clock_t(unsigned long x);
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static inline clock_t jiffies_delta_to_clock_t(long delta)
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{
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return jiffies_to_clock_t(max(0L, delta));
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}
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extern unsigned long clock_t_to_jiffies(unsigned long x);
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extern u64 jiffies_64_to_clock_t(u64 x);
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extern u64 nsec_to_clock_t(u64 x);
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extern u64 nsecs_to_jiffies64(u64 n);
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extern unsigned long nsecs_to_jiffies(u64 n);
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#define TIMESTAMP_SIZE 30
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#endif
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