1/*
  2 * sched_clock.c: support for extending counters to full 64-bit ns counter
  3 *
  4 * This program is free software; you can redistribute it and/or modify
  5 * it under the terms of the GNU General Public License version 2 as
  6 * published by the Free Software Foundation.
  7 */
  8#include <linux/clocksource.h>
  9#include <linux/init.h>
 10#include <linux/jiffies.h>
 11#include <linux/kernel.h>
 12#include <linux/sched.h>
 13#include <linux/syscore_ops.h>
 14#include <linux/timer.h>
 15
 16#include <asm/sched_clock.h>
 17
 18struct clock_data {
 19	u64 epoch_ns;
 20	u32 epoch_cyc;
 21	u32 epoch_cyc_copy;
 22	u32 mult;
 23	u32 shift;
 24};
 25
 26static void sched_clock_poll(unsigned long wrap_ticks);
 27static DEFINE_TIMER(sched_clock_timer, sched_clock_poll, 0, 0);
 28
 29static struct clock_data cd = {
 30	.mult	= NSEC_PER_SEC / HZ,
 31};
 32
 33static u32 __read_mostly sched_clock_mask = 0xffffffff;
 34
 35static u32 notrace jiffy_sched_clock_read(void)
 36{
 37	return (u32)(jiffies - INITIAL_JIFFIES);
 38}
 39
 40static u32 __read_mostly (*read_sched_clock)(void) = jiffy_sched_clock_read;
 41
 42static inline u64 cyc_to_ns(u64 cyc, u32 mult, u32 shift)
 43{
 44	return (cyc * mult) >> shift;
 45}
 46
 47static unsigned long long cyc_to_sched_clock(u32 cyc, u32 mask)
 48{
 49	u64 epoch_ns;
 50	u32 epoch_cyc;
 51
 52	/*
 53	 * Load the epoch_cyc and epoch_ns atomically.  We do this by
 54	 * ensuring that we always write epoch_cyc, epoch_ns and
 55	 * epoch_cyc_copy in strict order, and read them in strict order.
 56	 * If epoch_cyc and epoch_cyc_copy are not equal, then we're in
 57	 * the middle of an update, and we should repeat the load.
 58	 */
 59	do {
 60		epoch_cyc = cd.epoch_cyc;
 61		smp_rmb();
 62		epoch_ns = cd.epoch_ns;
 63		smp_rmb();
 64	} while (epoch_cyc != cd.epoch_cyc_copy);
 65
 66	return epoch_ns + cyc_to_ns((cyc - epoch_cyc) & mask, cd.mult, cd.shift);
 67}
 68
 69/*
 70 * Atomically update the sched_clock epoch.
 71 */
 72static void notrace update_sched_clock(void)
 73{
 74	unsigned long flags;
 75	u32 cyc;
 76	u64 ns;
 77
 78	cyc = read_sched_clock();
 79	ns = cd.epoch_ns +
 80		cyc_to_ns((cyc - cd.epoch_cyc) & sched_clock_mask,
 81			  cd.mult, cd.shift);
 82	/*
 83	 * Write epoch_cyc and epoch_ns in a way that the update is
 84	 * detectable in cyc_to_fixed_sched_clock().
 85	 */
 86	raw_local_irq_save(flags);
 87	cd.epoch_cyc = cyc;
 88	smp_wmb();
 89	cd.epoch_ns = ns;
 90	smp_wmb();
 91	cd.epoch_cyc_copy = cyc;
 92	raw_local_irq_restore(flags);
 93}
 94
 95static void sched_clock_poll(unsigned long wrap_ticks)
 96{
 97	mod_timer(&sched_clock_timer, round_jiffies(jiffies + wrap_ticks));
 98	update_sched_clock();
 99}
100
101void __init setup_sched_clock(u32 (*read)(void), int bits, unsigned long rate)
102{
103	unsigned long r, w;
104	u64 res, wrap;
105	char r_unit;
106
107	BUG_ON(bits > 32);
108	WARN_ON(!irqs_disabled());
109	WARN_ON(read_sched_clock != jiffy_sched_clock_read);
110	read_sched_clock = read;
111	sched_clock_mask = (1 << bits) - 1;
112
113	/* calculate the mult/shift to convert counter ticks to ns. */
114	clocks_calc_mult_shift(&cd.mult, &cd.shift, rate, NSEC_PER_SEC, 0);
115
116	r = rate;
117	if (r >= 4000000) {
118		r /= 1000000;
119		r_unit = 'M';
120	} else if (r >= 1000) {
121		r /= 1000;
122		r_unit = 'k';
123	} else
124		r_unit = ' ';
125
126	/* calculate how many ns until we wrap */
127	wrap = cyc_to_ns((1ULL << bits) - 1, cd.mult, cd.shift);
128	do_div(wrap, NSEC_PER_MSEC);
129	w = wrap;
130
131	/* calculate the ns resolution of this counter */
132	res = cyc_to_ns(1ULL, cd.mult, cd.shift);
133	pr_info("sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lums\n",
134		bits, r, r_unit, res, w);
135
136	/*
137	 * Start the timer to keep sched_clock() properly updated and
138	 * sets the initial epoch.
139	 */
140	sched_clock_timer.data = msecs_to_jiffies(w - (w / 10));
141	update_sched_clock();
142
143	/*
144	 * Ensure that sched_clock() starts off at 0ns
145	 */
146	cd.epoch_ns = 0;
147
148	pr_debug("Registered %pF as sched_clock source\n", read);
149}
150
151unsigned long long notrace sched_clock(void)
152{
153	u32 cyc = read_sched_clock();
154	return cyc_to_sched_clock(cyc, sched_clock_mask);
155}
156
157void __init sched_clock_postinit(void)
158{
159	/*
160	 * If no sched_clock function has been provided at that point,
161	 * make it the final one one.
162	 */
163	if (read_sched_clock == jiffy_sched_clock_read)
164		setup_sched_clock(jiffy_sched_clock_read, 32, HZ);
165
166	sched_clock_poll(sched_clock_timer.data);
167}
168
169static int sched_clock_suspend(void)
170{
171	sched_clock_poll(sched_clock_timer.data);
172	return 0;
173}
174
175static struct syscore_ops sched_clock_ops = {
176	.suspend = sched_clock_suspend,
177};
178
179static int __init sched_clock_syscore_init(void)
180{
181	register_syscore_ops(&sched_clock_ops);
182	return 0;
183}
184device_initcall(sched_clock_syscore_init);