include/linux/clocksource.h

   1 /*  linux/include/linux/clocksource.h
   2  *
   3  *  This file contains the structure definitions for clocksources.
   4  *
   5  *  If you are not a clocksource, or timekeeping code, you should
   6  *  not be including this file!
   7  */
   8 #ifndef _LINUX_CLOCKSOURCE_H
   9 #define _LINUX_CLOCKSOURCE_H
  10
  11 #include <linux/types.h>
  12 #include <linux/timex.h>
  13 #include <linux/time.h>
  14 #include <linux/list.h>
  15 #include <asm/div64.h>
  16 #include <asm/io.h>
  17
  18 /* clocksource cycle base type */
  19 typedef u64 cycle_t;
  20
  21 /**
  22  * struct clocksource - hardware abstraction for a free running counter
  23  *      Provides mostly state-free accessors to the underlying hardware.
  24  *
  25  * @name:               ptr to clocksource name
  26  * @list:               list head for registration
  27  * @rating:             rating value for selection (higher is better)
  28  *                      To avoid rating inflation the following
  29  *                      list should give you a guide as to how
  30  *                      to assign your clocksource a rating
  31  *                      1-99: Unfit for real use
  32  *                              Only available for bootup and testing purposes.
  33  *                      100-199: Base level usability.
  34  *                              Functional for real use, but not desired.
  35  *                      200-299: Good.
  36  *                              A correct and usable clocksource.
  37  *                      300-399: Desired.
  38  *                              A reasonably fast and accurate clocksource.
  39  *                      400-499: Perfect
  40  *                              The ideal clocksource. A must-use where
  41  *                              available.
  42  * @read:               returns a cycle value
  43  * @mask:               bitmask for two's complement
  44  *                      subtraction of non 64 bit counters
  45  * @mult:               cycle to nanosecond multiplier
  46  * @shift:              cycle to nanosecond divisor (power of two)
  47  * @update_callback:    called when safe to alter clocksource values
  48  * @is_continuous:      defines if clocksource is free-running.
  49  * @interval_cycles:    Used internally by timekeeping core, please ignore.
  50  * @interval_snsecs:    Used internally by timekeeping core, please ignore.
  51  */
  52 struct clocksource {
  53         char *name;
  54         struct list_head list;
  55         int rating;
  56         cycle_t (*read)(void);
  57         cycle_t mask;
  58         u32 mult;
  59         u32 shift;
  60         int (*update_callback)(void);
  61         int is_continuous;
  62
  63         /* timekeeping specific data, ignore */
  64         cycle_t interval_cycles;
  65         u64 interval_snsecs;
  66 };
  67
  68 /* simplify initialization of mask field */
  69 #define CLOCKSOURCE_MASK(bits) (cycle_t)(bits<64 ? ((1ULL<<bits)-1) : -1)
  70
  71 /**
  72  * clocksource_khz2mult - calculates mult from khz and shift
  73  * @khz:                Clocksource frequency in KHz
  74  * @shift_constant:     Clocksource shift factor
  75  *
  76  * Helper functions that converts a khz counter frequency to a timsource
  77  * multiplier, given the clocksource shift value
  78  */
  79 static inline u32 clocksource_khz2mult(u32 khz, u32 shift_constant)
  80 {
  81         /*  khz = cyc/(Million ns)
  82          *  mult/2^shift  = ns/cyc
  83          *  mult = ns/cyc * 2^shift
  84          *  mult = 1Million/khz * 2^shift
  85          *  mult = 1000000 * 2^shift / khz
  86          *  mult = (1000000<<shift) / khz
  87          */
  88         u64 tmp = ((u64)1000000) << shift_constant;
  89
  90         tmp += khz/2; /* round for do_div */
  91         do_div(tmp, khz);
  92
  93         return (u32)tmp;
  94 }
  95
  96 /**
  97  * clocksource_hz2mult - calculates mult from hz and shift
  98  * @hz:                 Clocksource frequency in Hz
  99  * @shift_constant:     Clocksource shift factor
 100  *
 101  * Helper functions that converts a hz counter
 102  * frequency to a timsource multiplier, given the
 103  * clocksource shift value
 104  */
 105 static inline u32 clocksource_hz2mult(u32 hz, u32 shift_constant)
 106 {
 107         /*  hz = cyc/(Billion ns)
 108          *  mult/2^shift  = ns/cyc
 109          *  mult = ns/cyc * 2^shift
 110          *  mult = 1Billion/hz * 2^shift
 111          *  mult = 1000000000 * 2^shift / hz
 112          *  mult = (1000000000<<shift) / hz
 113          */
 114         u64 tmp = ((u64)1000000000) << shift_constant;
 115
 116         tmp += hz/2; /* round for do_div */
 117         do_div(tmp, hz);
 118
 119         return (u32)tmp;
 120 }
 121
 122 /**
 123  * clocksource_read: - Access the clocksource's current cycle value
 124  * @cs:         pointer to clocksource being read
 125  *
 126  * Uses the clocksource to return the current cycle_t value
 127  */
 128 static inline cycle_t clocksource_read(struct clocksource *cs)
 129 {
 130         return cs->read();
 131 }
 132
 133 /**
 134  * cyc2ns - converts clocksource cycles to nanoseconds
 135  * @cs:         Pointer to clocksource
 136  * @cycles:     Cycles
 137  *
 138  * Uses the clocksource and ntp ajdustment to convert cycle_ts to nanoseconds.
 139  *
 140  * XXX - This could use some mult_lxl_ll() asm optimization
 141  */
 142 static inline s64 cyc2ns(struct clocksource *cs, cycle_t cycles)
 143 {
 144         u64 ret = (u64)cycles;
 145         ret = (ret * cs->mult) >> cs->shift;
 146         return ret;
 147 }
 148
 149 /**
 150  * clocksource_calculate_interval - Calculates a clocksource interval struct
 151  *
 152  * @c:          Pointer to clocksource.
 153  * @length_nsec: Desired interval length in nanoseconds.
 154  *
 155  * Calculates a fixed cycle/nsec interval for a given clocksource/adjustment
 156  * pair and interval request.
 157  *
 158  * Unless you're the timekeeping code, you should not be using this!
 159  */
 160 static inline void clocksource_calculate_interval(struct clocksource *c,
 161                                                 unsigned long length_nsec)
 162 {
 163         u64 tmp;
 164
 165         /* XXX - All of this could use a whole lot of optimization */
 166         tmp = length_nsec;
 167         tmp <<= c->shift;
 168         tmp += c->mult/2;
 169         do_div(tmp, c->mult);
 170
 171         c->interval_cycles = (cycle_t)tmp;
 172         if(c->interval_cycles == 0)
 173                 c->interval_cycles = 1;
 174
 175         c->interval_snsecs = (u64)c->interval_cycles * c->mult;
 176 }
 177
 178
 179 /**
 180  * error_aproximation - calculates an error adjustment for a given error
 181  *
 182  * @error:      Error value (unsigned)
 183  * @unit:       Adjustment unit
 184  *
 185  * For a given error value, this function takes the adjustment unit
 186  * and uses binary approximation to return a power of two adjustment value.
 187  *
 188  * This function is only for use by the the make_ntp_adj() function
 189  * and you must hold a write on the xtime_lock when calling.
 190  */
 191 static inline int error_aproximation(u64 error, u64 unit)
 192 {
 193         static int saved_adj = 0;
 194         u64 adjusted_unit = unit << saved_adj;
 195
 196         if (error > (adjusted_unit * 2)) {
 197                 /* large error, so increment the adjustment factor */
 198                 saved_adj++;
 199         } else if (error > adjusted_unit) {
 200                 /* just right, don't touch it */
 201         } else if (saved_adj) {
 202                 /* small error, so drop the adjustment factor */
 203                 saved_adj--;
 204                 return 0;
 205         }
 206
 207         return saved_adj;
 208 }
 209
 210
 211 /**
 212  * make_ntp_adj - Adjusts the specified clocksource for a given error
 213  *
 214  * @clock:              Pointer to clock to be adjusted
 215  * @cycles_delta:       Current unacounted cycle delta
 216  * @error:              Pointer to current error value
 217  *
 218  * Returns clock shifted nanosecond adjustment to be applied against
 219  * the accumulated time value (ie: xtime).
 220  *
 221  * If the error value is large enough, this function calulates the
 222  * (power of two) adjustment value, and adjusts the clock's mult and
 223  * interval_snsecs values accordingly.
 224  *
 225  * However, since there may be some unaccumulated cycles, to avoid
 226  * time inconsistencies we must adjust the accumulation value
 227  * accordingly.
 228  *
 229  * This is not very intuitive, so the following proof should help:
 230  * The basic timeofday algorithm:  base + cycle * mult
 231  * Thus:
 232  *    new_base + cycle * new_mult = old_base + cycle * old_mult
 233  *    new_base = old_base + cycle * old_mult - cycle * new_mult
 234  *    new_base = old_base + cycle * (old_mult - new_mult)
 235  *    new_base - old_base = cycle * (old_mult - new_mult)
 236  *    base_delta = cycle * (old_mult - new_mult)
 237  *    base_delta = cycle * (mult_delta)
 238  *
 239  * Where mult_delta is the adjustment value made to mult
 240  *
 241  */
 242 static inline s64 make_ntp_adj(struct clocksource *clock,
 243                                 cycles_t cycles_delta, s64* error)
 244 {
 245         s64 ret = 0;
 246         if (*error  > ((s64)clock->interval_cycles+1)/2) {
 247                 /* calculate adjustment value */
 248                 int adjustment = error_aproximation(*error,
 249                                                 clock->interval_cycles);
 250                 /* adjust clock */
 251                 clock->mult += 1 << adjustment;
 252                 clock->interval_snsecs += clock->interval_cycles << adjustment;
 253
 254                 /* adjust the base and error for the adjustment */
 255                 ret =  -(cycles_delta << adjustment);
 256                 *error -= clock->interval_cycles << adjustment;
 257                 /* XXX adj error for cycle_delta offset? */
 258         } else if ((-(*error))  > ((s64)clock->interval_cycles+1)/2) {
 259                 /* calculate adjustment value */
 260                 int adjustment = error_aproximation(-(*error),
 261                                                 clock->interval_cycles);
 262                 /* adjust clock */
 263                 clock->mult -= 1 << adjustment;
 264                 clock->interval_snsecs -= clock->interval_cycles << adjustment;
 265
 266                 /* adjust the base and error for the adjustment */
 267                 ret =  cycles_delta << adjustment;
 268                 *error += clock->interval_cycles << adjustment;
 269                 /* XXX adj error for cycle_delta offset? */
 270         }
 271         return ret;
 272 }
 273
 274
 275 /* used to install a new clocksource */
 276 int clocksource_register(struct clocksource*);
 277 void clocksource_reselect(void);
 278 struct clocksource* clocksource_get_next(void);
 279
 280 #endif /* _LINUX_CLOCKSOURCE_H */