mirror of
https://github.com/edk2-porting/linux-next.git
synced 2024-12-23 12:43:55 +08:00
e5d4d1756b
seqlock consists of a sequence counter and a spinlock_t which is used to serialize the writers. spinlock_t is substituted by a "sleeping" spinlock on PREEMPT_RT enabled kernels which breaks the usage in the timekeeping code as the writers are executed in hard interrupt and therefore non-preemptible context even on PREEMPT_RT. The spinlock in seqlock cannot be unconditionally replaced by a raw_spinlock_t as many seqlock users have nesting spinlock sections or other code which is not suitable to run in truly atomic context on RT. Instead of providing a raw_seqlock API for a single use case, open code the seqlock for the jiffies use case and implement it with a raw_spinlock_t and a sequence counter. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lkml.kernel.org/r/20200321113242.120587764@linutronix.de
122 lines
3.1 KiB
C
122 lines
3.1 KiB
C
// SPDX-License-Identifier: GPL-2.0+
|
|
/*
|
|
* This file contains the jiffies based clocksource.
|
|
*
|
|
* Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com)
|
|
*/
|
|
#include <linux/clocksource.h>
|
|
#include <linux/jiffies.h>
|
|
#include <linux/module.h>
|
|
#include <linux/init.h>
|
|
|
|
#include "timekeeping.h"
|
|
|
|
|
|
/* Since jiffies uses a simple TICK_NSEC multiplier
|
|
* conversion, the .shift value could be zero. However
|
|
* this would make NTP adjustments impossible as they are
|
|
* in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
|
|
* shift both the nominator and denominator the same
|
|
* amount, and give ntp adjustments in units of 1/2^8
|
|
*
|
|
* The value 8 is somewhat carefully chosen, as anything
|
|
* larger can result in overflows. TICK_NSEC grows as HZ
|
|
* shrinks, so values greater than 8 overflow 32bits when
|
|
* HZ=100.
|
|
*/
|
|
#if HZ < 34
|
|
#define JIFFIES_SHIFT 6
|
|
#elif HZ < 67
|
|
#define JIFFIES_SHIFT 7
|
|
#else
|
|
#define JIFFIES_SHIFT 8
|
|
#endif
|
|
|
|
static u64 jiffies_read(struct clocksource *cs)
|
|
{
|
|
return (u64) jiffies;
|
|
}
|
|
|
|
/*
|
|
* The Jiffies based clocksource is the lowest common
|
|
* denominator clock source which should function on
|
|
* all systems. It has the same coarse resolution as
|
|
* the timer interrupt frequency HZ and it suffers
|
|
* inaccuracies caused by missed or lost timer
|
|
* interrupts and the inability for the timer
|
|
* interrupt hardware to accuratly tick at the
|
|
* requested HZ value. It is also not recommended
|
|
* for "tick-less" systems.
|
|
*/
|
|
static struct clocksource clocksource_jiffies = {
|
|
.name = "jiffies",
|
|
.rating = 1, /* lowest valid rating*/
|
|
.read = jiffies_read,
|
|
.mask = CLOCKSOURCE_MASK(32),
|
|
.mult = TICK_NSEC << JIFFIES_SHIFT, /* details above */
|
|
.shift = JIFFIES_SHIFT,
|
|
.max_cycles = 10,
|
|
};
|
|
|
|
__cacheline_aligned_in_smp DEFINE_RAW_SPINLOCK(jiffies_lock);
|
|
__cacheline_aligned_in_smp seqcount_t jiffies_seq;
|
|
|
|
#if (BITS_PER_LONG < 64)
|
|
u64 get_jiffies_64(void)
|
|
{
|
|
unsigned int seq;
|
|
u64 ret;
|
|
|
|
do {
|
|
seq = read_seqcount_begin(&jiffies_seq);
|
|
ret = jiffies_64;
|
|
} while (read_seqcount_retry(&jiffies_seq, seq));
|
|
return ret;
|
|
}
|
|
EXPORT_SYMBOL(get_jiffies_64);
|
|
#endif
|
|
|
|
EXPORT_SYMBOL(jiffies);
|
|
|
|
static int __init init_jiffies_clocksource(void)
|
|
{
|
|
return __clocksource_register(&clocksource_jiffies);
|
|
}
|
|
|
|
core_initcall(init_jiffies_clocksource);
|
|
|
|
struct clocksource * __init __weak clocksource_default_clock(void)
|
|
{
|
|
return &clocksource_jiffies;
|
|
}
|
|
|
|
static struct clocksource refined_jiffies;
|
|
|
|
int register_refined_jiffies(long cycles_per_second)
|
|
{
|
|
u64 nsec_per_tick, shift_hz;
|
|
long cycles_per_tick;
|
|
|
|
|
|
|
|
refined_jiffies = clocksource_jiffies;
|
|
refined_jiffies.name = "refined-jiffies";
|
|
refined_jiffies.rating++;
|
|
|
|
/* Calc cycles per tick */
|
|
cycles_per_tick = (cycles_per_second + HZ/2)/HZ;
|
|
/* shift_hz stores hz<<8 for extra accuracy */
|
|
shift_hz = (u64)cycles_per_second << 8;
|
|
shift_hz += cycles_per_tick/2;
|
|
do_div(shift_hz, cycles_per_tick);
|
|
/* Calculate nsec_per_tick using shift_hz */
|
|
nsec_per_tick = (u64)NSEC_PER_SEC << 8;
|
|
nsec_per_tick += (u32)shift_hz/2;
|
|
do_div(nsec_per_tick, (u32)shift_hz);
|
|
|
|
refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;
|
|
|
|
__clocksource_register(&refined_jiffies);
|
|
return 0;
|
|
}
|