aboutsummaryrefslogtreecommitdiffstats
path: root/include/linux/clocksource.h
blob: 5f4a7f72f3ee4adc4729dfdedd71f227091d70a0 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
/*  linux/include/linux/clocksource.h
 *
 *  This file contains the structure definitions for clocksources.
 *
 *  If you are not a clocksource, or timekeeping code, you should
 *  not be including this file!
 */
#ifndef _LINUX_CLOCKSOURCE_H
#define _LINUX_CLOCKSOURCE_H

#include <linux/types.h>
#include <linux/timex.h>
#include <linux/time.h>
#include <linux/list.h>
#include <asm/div64.h>
#include <asm/io.h>

/* clocksource cycle base type */
typedef u64 cycle_t;

/**
 * struct clocksource - hardware abstraction for a free running counter
 *	Provides mostly state-free accessors to the underlying hardware.
 *
 * @name:		ptr to clocksource name
 * @list:		list head for registration
 * @rating:		rating value for selection (higher is better)
 *			To avoid rating inflation the following
 *			list should give you a guide as to how
 *			to assign your clocksource a rating
 *			1-99: Unfit for real use
 *				Only available for bootup and testing purposes.
 *			100-199: Base level usability.
 *				Functional for real use, but not desired.
 *			200-299: Good.
 *				A correct and usable clocksource.
 *			300-399: Desired.
 *				A reasonably fast and accurate clocksource.
 *			400-499: Perfect
 *				The ideal clocksource. A must-use where
 *				available.
 * @read:		returns a cycle value
 * @mask:		bitmask for two's complement
 *			subtraction of non 64 bit counters
 * @mult:		cycle to nanosecond multiplier
 * @shift:		cycle to nanosecond divisor (power of two)
 * @update_callback:	called when safe to alter clocksource values
 * @is_continuous:	defines if clocksource is free-running.
 * @interval_cycles:	Used internally by timekeeping core, please ignore.
 * @interval_snsecs:	Used internally by timekeeping core, please ignore.
 */
struct clocksource {
	char *name;
	struct list_head list;
	int rating;
	cycle_t (*read)(void);
	cycle_t mask;
	u32 mult;
	u32 shift;
	int (*update_callback)(void);
	int is_continuous;

	/* timekeeping specific data, ignore */
	cycle_t interval_cycles;
	u64 interval_snsecs;
};


/**
 * clocksource_khz2mult - calculates mult from khz and shift
 * @khz:		Clocksource frequency in KHz
 * @shift_constant:	Clocksource shift factor
 *
 * Helper functions that converts a khz counter frequency to a timsource
 * multiplier, given the clocksource shift value
 */
static inline u32 clocksource_khz2mult(u32 khz, u32 shift_constant)
{
	/*  khz = cyc/(Million ns)
	 *  mult/2^shift  = ns/cyc
	 *  mult = ns/cyc * 2^shift
	 *  mult = 1Million/khz * 2^shift
	 *  mult = 1000000 * 2^shift / khz
	 *  mult = (1000000<<shift) / khz
	 */
	u64 tmp = ((u64)1000000) << shift_constant;

	tmp += khz/2; /* round for do_div */
	do_div(tmp, khz);

	return (u32)tmp;
}

/**
 * clocksource_hz2mult - calculates mult from hz and shift
 * @hz:			Clocksource frequency in Hz
 * @shift_constant:	Clocksource shift factor
 *
 * Helper functions that converts a hz counter
 * frequency to a timsource multiplier, given the
 * clocksource shift value
 */
static inline u32 clocksource_hz2mult(u32 hz, u32 shift_constant)
{
	/*  hz = cyc/(Billion ns)
	 *  mult/2^shift  = ns/cyc
	 *  mult = ns/cyc * 2^shift
	 *  mult = 1Billion/hz * 2^shift
	 *  mult = 1000000000 * 2^shift / hz
	 *  mult = (1000000000<<shift) / hz
	 */
	u64 tmp = ((u64)1000000000) << shift_constant;

	tmp += hz/2; /* round for do_div */
	do_div(tmp, hz);

	return (u32)tmp;
}

/**
 * clocksource_read: - Access the clocksource's current cycle value
 * @cs:		pointer to clocksource being read
 *
 * Uses the clocksource to return the current cycle_t value
 */
static inline cycle_t clocksource_read(struct clocksource *cs)
{
	return cs->read();
}

/**
 * cyc2ns - converts clocksource cycles to nanoseconds
 * @cs:		Pointer to clocksource
 * @cycles:	Cycles
 *
 * Uses the clocksource and ntp ajdustment to convert cycle_ts to nanoseconds.
 *
 * XXX - This could use some mult_lxl_ll() asm optimization
 */
static inline s64 cyc2ns(struct clocksource *cs, cycle_t cycles)
{
	u64 ret = (u64)cycles;
	ret = (ret * cs->mult) >> cs->shift;
	return ret;
}

/**
 * clocksource_calculate_interval - Calculates a clocksource interval struct
 *
 * @c:		Pointer to clocksource.
 * @length_nsec: Desired interval length in nanoseconds.
 *
 * Calculates a fixed cycle/nsec interval for a given clocksource/adjustment
 * pair and interval request.
 *
 * Unless you're the timekeeping code, you should not be using this!
 */
static inline void clocksource_calculate_interval(struct clocksource *c,
						unsigned long length_nsec)
{
	u64 tmp;

	/* XXX - All of this could use a whole lot of optimization */
	tmp = length_nsec;
	tmp <<= c->shift;
	tmp += c->mult/2;
	do_div(tmp, c->mult);

	c->interval_cycles = (cycle_t)tmp;
	if(c->interval_cycles == 0)
		c->interval_cycles = 1;

	c->interval_snsecs = (u64)c->interval_cycles * c->mult;
}


/**
 * error_aproximation - calculates an error adjustment for a given error
 *
 * @error:	Error value (unsigned)
 * @unit:	Adjustment unit
 *
 * For a given error value, this function takes the adjustment unit
 * and uses binary approximation to return a power of two adjustment value.
 *
 * This function is only for use by the the make_ntp_adj() function
 * and you must hold a write on the xtime_lock when calling.
 */
static inline int error_aproximation(u64 error, u64 unit)
{
	static int saved_adj = 0;
	u64 adjusted_unit = unit << saved_adj;

	if (error > (adjusted_unit * 2)) {
		/* large error, so increment the adjustment factor */
		saved_adj++;
	} else if (error > adjusted_unit) {
		/* just right, don't touch it */
	} else if (saved_adj) {
		/* small error, so drop the adjustment factor */
		saved_adj--;
		return 0;
	}

	return saved_adj;
}


/**
 * make_ntp_adj - Adjusts the specified clocksource for a given error
 *
 * @clock:		Pointer to clock to be adjusted
 * @cycles_delta:	Current unacounted cycle delta
 * @error:		Pointer to current error value
 *
 * Returns clock shifted nanosecond adjustment to be applied against
 * the accumulated time value (ie: xtime).
 *
 * If the error value is large enough, this function calulates the
 * (power of two) adjustment value, and adjusts the clock's mult and
 * interval_snsecs values accordingly.
 *
 * However, since there may be some unaccumulated cycles, to avoid
 * time inconsistencies we must adjust the accumulation value
 * accordingly.
 *
 * This is not very intuitive, so the following proof should help:
 * The basic timeofday algorithm:  base + cycle * mult
 * Thus:
 *    new_base + cycle * new_mult = old_base + cycle * old_mult
 *    new_base = old_base + cycle * old_mult - cycle * new_mult
 *    new_base = old_base + cycle * (old_mult - new_mult)
 *    new_base - old_base = cycle * (old_mult - new_mult)
 *    base_delta = cycle * (old_mult - new_mult)
 *    base_delta = cycle * (mult_delta)
 *
 * Where mult_delta is the adjustment value made to mult
 *
 */
static inline s64 make_ntp_adj(struct clocksource *clock,
				cycles_t cycles_delta, s64* error)
{
	s64 ret = 0;
	if (*error  > ((s64)clock->interval_cycles+1)/2) {
		/* calculate adjustment value */
		int adjustment = error_aproximation(*error,
						clock->interval_cycles);
		/* adjust clock */
		clock->mult += 1 << adjustment;
		clock->interval_snsecs += clock->interval_cycles << adjustment;

		/* adjust the base and error for the adjustment */
		ret =  -(cycles_delta << adjustment);
		*error -= clock->interval_cycles << adjustment;
		/* XXX adj error for cycle_delta offset? */
	} else if ((-(*error))  > ((s64)clock->interval_cycles+1)/2) {
		/* calculate adjustment value */
		int adjustment = error_aproximation(-(*error),
						clock->interval_cycles);
		/* adjust clock */
		clock->mult -= 1 << adjustment;
		clock->interval_snsecs -= clock->interval_cycles << adjustment;

		/* adjust the base and error for the adjustment */
		ret =  cycles_delta << adjustment;
		*error += clock->interval_cycles << adjustment;
		/* XXX adj error for cycle_delta offset? */
	}
	return ret;
}


/* used to install a new clocksource */
int clocksource_register(struct clocksource*);
void clocksource_reselect(void);
struct clocksource* clocksource_get_next(void);

#endif /* _LINUX_CLOCKSOURCE_H */