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/timer.h>
14
15 #include <asm/sched_clock.h>
16
17 struct clock_data {
18 u64 epoch_ns;
19 u32 epoch_cyc;
20 u32 epoch_cyc_copy;
21 u32 mult;
22 u32 shift;
23 };
24
25 static void sched_clock_poll(unsigned long wrap_ticks);
26 static DEFINE_TIMER(sched_clock_timer, sched_clock_poll, 0, 0);
27
28 static struct clock_data cd = {
29 .mult = NSEC_PER_SEC / HZ,
30 };
31
32 static u32 __read_mostly sched_clock_mask = 0xffffffff;
33
jiffy_sched_clock_read(void)34 static u32 notrace jiffy_sched_clock_read(void)
35 {
36 return (u32)(jiffies - INITIAL_JIFFIES);
37 }
38
39 static u32 __read_mostly (*read_sched_clock)(void) = jiffy_sched_clock_read;
40
cyc_to_ns(u64 cyc,u32 mult,u32 shift)41 static inline u64 cyc_to_ns(u64 cyc, u32 mult, u32 shift)
42 {
43 return (cyc * mult) >> shift;
44 }
45
cyc_to_sched_clock(u32 cyc,u32 mask)46 static unsigned long long cyc_to_sched_clock(u32 cyc, u32 mask)
47 {
48 u64 epoch_ns;
49 u32 epoch_cyc;
50
51 /*
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.
57 */
58 do {
59 epoch_cyc = cd.epoch_cyc;
60 smp_rmb();
61 epoch_ns = cd.epoch_ns;
62 smp_rmb();
63 } while (epoch_cyc != cd.epoch_cyc_copy);
64
65 return epoch_ns + cyc_to_ns((cyc - epoch_cyc) & mask, cd.mult, cd.shift);
66 }
67
68 /*
69 * Atomically update the sched_clock epoch.
70 */
update_sched_clock(void)71 static void notrace update_sched_clock(void)
72 {
73 unsigned long flags;
74 u32 cyc;
75 u64 ns;
76
77 cyc = read_sched_clock();
78 ns = cd.epoch_ns +
79 cyc_to_ns((cyc - cd.epoch_cyc) & sched_clock_mask,
80 cd.mult, cd.shift);
81 /*
82 * Write epoch_cyc and epoch_ns in a way that the update is
83 * detectable in cyc_to_fixed_sched_clock().
84 */
85 raw_local_irq_save(flags);
86 cd.epoch_cyc = cyc;
87 smp_wmb();
88 cd.epoch_ns = ns;
89 smp_wmb();
90 cd.epoch_cyc_copy = cyc;
91 raw_local_irq_restore(flags);
92 }
93
sched_clock_poll(unsigned long wrap_ticks)94 static void sched_clock_poll(unsigned long wrap_ticks)
95 {
96 mod_timer(&sched_clock_timer, round_jiffies(jiffies + wrap_ticks));
97 update_sched_clock();
98 }
99
setup_sched_clock(u32 (* read)(void),int bits,unsigned long rate)100 void __init setup_sched_clock(u32 (*read)(void), int bits, unsigned long rate)
101 {
102 unsigned long r, w;
103 u64 res, wrap;
104 char r_unit;
105
106 BUG_ON(bits > 32);
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;
111
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);
114
115 r = rate;
116 if (r >= 4000000) {
117 r /= 1000000;
118 r_unit = 'M';
119 } else if (r >= 1000) {
120 r /= 1000;
121 r_unit = 'k';
122 } else
123 r_unit = ' ';
124
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);
128 w = wrap;
129
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);
134
135 /*
136 * Start the timer to keep sched_clock() properly updated and
137 * sets the initial epoch.
138 */
139 sched_clock_timer.data = msecs_to_jiffies(w - (w / 10));
140 update_sched_clock();
141
142 /*
143 * Ensure that sched_clock() starts off at 0ns
144 */
145 cd.epoch_ns = 0;
146
147 pr_debug("Registered %pF as sched_clock source\n", read);
148 }
149
sched_clock(void)150 unsigned long long notrace sched_clock(void)
151 {
152 u32 cyc = read_sched_clock();
153 return cyc_to_sched_clock(cyc, sched_clock_mask);
154 }
155
sched_clock_postinit(void)156 void __init sched_clock_postinit(void)
157 {
158 /*
159 * If no sched_clock function has been provided at that point,
160 * make it the final one one.
161 */
162 if (read_sched_clock == jiffy_sched_clock_read)
163 setup_sched_clock(jiffy_sched_clock_read, 32, HZ);
164
165 sched_clock_poll(sched_clock_timer.data);
166 }
167