2 * sched_clock.c: support for extending counters to full 64-bit ns counter
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.
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/timer.h>
15 #include <asm/sched_clock.h>
25 static void sched_clock_poll(unsigned long wrap_ticks);
26 static DEFINE_TIMER(sched_clock_timer, sched_clock_poll, 0, 0);
28 static struct clock_data cd = {
29 .mult = NSEC_PER_SEC / HZ,
32 static u32 __read_mostly sched_clock_mask = 0xffffffff;
34 static u32 notrace jiffy_sched_clock_read(void)
36 return (u32)(jiffies - INITIAL_JIFFIES);
39 static u32 __read_mostly (*read_sched_clock)(void) = jiffy_sched_clock_read;
41 static inline u64 cyc_to_ns(u64 cyc, u32 mult, u32 shift)
43 return (cyc * mult) >> shift;
46 static unsigned long long cyc_to_sched_clock(u32 cyc, u32 mask)
52 * Load the epoch_cyc and epoch_ns atomically. We do this by
53 * ensuring that we always write epoch_cyc, epoch_ns and
54 * epoch_cyc_copy in strict order, and read them in strict order.
55 * If epoch_cyc and epoch_cyc_copy are not equal, then we're in
56 * the middle of an update, and we should repeat the load.
59 epoch_cyc = cd.epoch_cyc;
61 epoch_ns = cd.epoch_ns;
63 } while (epoch_cyc != cd.epoch_cyc_copy);
65 return epoch_ns + cyc_to_ns((cyc - epoch_cyc) & mask, cd.mult, cd.shift);
69 * Atomically update the sched_clock epoch.
71 static void notrace update_sched_clock(void)
77 cyc = read_sched_clock();
79 cyc_to_ns((cyc - cd.epoch_cyc) & sched_clock_mask,
82 * Write epoch_cyc and epoch_ns in a way that the update is
83 * detectable in cyc_to_fixed_sched_clock().
85 raw_local_irq_save(flags);
90 cd.epoch_cyc_copy = cyc;
91 raw_local_irq_restore(flags);
94 static void sched_clock_poll(unsigned long wrap_ticks)
96 mod_timer(&sched_clock_timer, round_jiffies(jiffies + wrap_ticks));
100 void __init setup_sched_clock(u32 (*read)(void), int bits, unsigned long rate)
107 WARN_ON(!irqs_disabled());
108 WARN_ON(read_sched_clock != jiffy_sched_clock_read);
109 read_sched_clock = read;
110 sched_clock_mask = (1 << bits) - 1;
112 /* calculate the mult/shift to convert counter ticks to ns. */
113 clocks_calc_mult_shift(&cd.mult, &cd.shift, rate, NSEC_PER_SEC, 0);
119 } else if (r >= 1000) {
125 /* calculate how many ns until we wrap */
126 wrap = cyc_to_ns((1ULL << bits) - 1, cd.mult, cd.shift);
127 do_div(wrap, NSEC_PER_MSEC);
130 /* calculate the ns resolution of this counter */
131 res = cyc_to_ns(1ULL, cd.mult, cd.shift);
132 pr_info("sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lums\n",
133 bits, r, r_unit, res, w);
136 * Start the timer to keep sched_clock() properly updated and
137 * sets the initial epoch.
139 sched_clock_timer.data = msecs_to_jiffies(w - (w / 10));
140 update_sched_clock();
143 * Ensure that sched_clock() starts off at 0ns
147 pr_debug("Registered %pF as sched_clock source\n", read);
150 unsigned long long notrace sched_clock(void)
152 u32 cyc = read_sched_clock();
153 return cyc_to_sched_clock(cyc, sched_clock_mask);
156 void __init sched_clock_postinit(void)
159 * If no sched_clock function has been provided at that point,
160 * make it the final one one.
162 if (read_sched_clock == jiffy_sched_clock_read)
163 setup_sched_clock(jiffy_sched_clock_read, 32, HZ);
165 sched_clock_poll(sched_clock_timer.data);