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1366992e16
The addition of random_get_entropy_fallback() provides access to whichever time source has the highest frequency, which is useful for gathering entropy on platforms without available cycle counters. It's not necessarily as good as being able to quickly access a cycle counter that the CPU has, but it's still something, even when it falls back to being jiffies-based. In the event that a given arch does not define get_cycles(), falling back to the get_cycles() default implementation that returns 0 is really not the best we can do. Instead, at least calling random_get_entropy_fallback() would be preferable, because that always needs to return _something_, even falling back to jiffies eventually. It's not as though random_get_entropy_fallback() is super high precision or guaranteed to be entropic, but basically anything that's not zero all the time is better than returning zero all the time. Finally, since random_get_entropy_fallback() is used during extremely early boot when randomizing freelists in mm_init(), it can be called before timekeeping has been initialized. In that case there really is nothing we can do; jiffies hasn't even started ticking yet. So just give up and return 0. Suggested-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> Reviewed-by: Thomas Gleixner <tglx@linutronix.de> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Theodore Ts'o <tytso@mit.edu>
172 lines
6.7 KiB
C
172 lines
6.7 KiB
C
/*****************************************************************************
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* *
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* Copyright (c) David L. Mills 1993 *
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* *
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* Permission to use, copy, modify, and distribute this software and its *
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* documentation for any purpose and without fee is hereby granted, provided *
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* that the above copyright notice appears in all copies and that both the *
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* copyright notice and this permission notice appear in supporting *
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* documentation, and that the name University of Delaware not be used in *
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* advertising or publicity pertaining to distribution of the software *
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* without specific, written prior permission. The University of Delaware *
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* makes no representations about the suitability this software for any *
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* purpose. It is provided "as is" without express or implied warranty. *
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* *
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*****************************************************************************/
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/*
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* Modification history timex.h
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*
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* 29 Dec 97 Russell King
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* Moved CLOCK_TICK_RATE, CLOCK_TICK_FACTOR and FINETUNE to asm/timex.h
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* for ARM machines
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*
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* 9 Jan 97 Adrian Sun
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* Shifted LATCH define to allow access to alpha machines.
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*
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* 26 Sep 94 David L. Mills
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* Added defines for hybrid phase/frequency-lock loop.
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*
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* 19 Mar 94 David L. Mills
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* Moved defines from kernel routines to header file and added new
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* defines for PPS phase-lock loop.
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*
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* 20 Feb 94 David L. Mills
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* Revised status codes and structures for external clock and PPS
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* signal discipline.
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*
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* 28 Nov 93 David L. Mills
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* Adjusted parameters to improve stability and increase poll
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* interval.
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*
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* 17 Sep 93 David L. Mills
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* Created file $NTP/include/sys/timex.h
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* 07 Oct 93 Torsten Duwe
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* Derived linux/timex.h
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* 1995-08-13 Torsten Duwe
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* kernel PLL updated to 1994-12-13 specs (rfc-1589)
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* 1997-08-30 Ulrich Windl
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* Added new constant NTP_PHASE_LIMIT
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* 2004-08-12 Christoph Lameter
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* Reworked time interpolation logic
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*/
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#ifndef _LINUX_TIMEX_H
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#define _LINUX_TIMEX_H
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#include <uapi/linux/timex.h>
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#define ADJ_ADJTIME 0x8000 /* switch between adjtime/adjtimex modes */
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#define ADJ_OFFSET_SINGLESHOT 0x0001 /* old-fashioned adjtime */
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#define ADJ_OFFSET_READONLY 0x2000 /* read-only adjtime */
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#include <linux/compiler.h>
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#include <linux/types.h>
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#include <linux/param.h>
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unsigned long random_get_entropy_fallback(void);
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#include <asm/timex.h>
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#ifndef random_get_entropy
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/*
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* The random_get_entropy() function is used by the /dev/random driver
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* in order to extract entropy via the relative unpredictability of
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* when an interrupt takes places versus a high speed, fine-grained
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* timing source or cycle counter. Since it will be occurred on every
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* single interrupt, it must have a very low cost/overhead.
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*
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* By default we use get_cycles() for this purpose, but individual
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* architectures may override this in their asm/timex.h header file.
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* If a given arch does not have get_cycles(), then we fallback to
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* using random_get_entropy_fallback().
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*/
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#ifdef get_cycles
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#define random_get_entropy() ((unsigned long)get_cycles())
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#else
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#define random_get_entropy() random_get_entropy_fallback()
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#endif
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#endif
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/*
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* SHIFT_PLL is used as a dampening factor to define how much we
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* adjust the frequency correction for a given offset in PLL mode.
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* It also used in dampening the offset correction, to define how
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* much of the current value in time_offset we correct for each
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* second. Changing this value changes the stiffness of the ntp
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* adjustment code. A lower value makes it more flexible, reducing
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* NTP convergence time. A higher value makes it stiffer, increasing
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* convergence time, but making the clock more stable.
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*
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* In David Mills' nanokernel reference implementation SHIFT_PLL is 4.
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* However this seems to increase convergence time much too long.
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*
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* https://lists.ntp.org/pipermail/hackers/2008-January/003487.html
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*
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* In the above mailing list discussion, it seems the value of 4
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* was appropriate for other Unix systems with HZ=100, and that
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* SHIFT_PLL should be decreased as HZ increases. However, Linux's
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* clock steering implementation is HZ independent.
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*
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* Through experimentation, a SHIFT_PLL value of 2 was found to allow
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* for fast convergence (very similar to the NTPv3 code used prior to
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* v2.6.19), with good clock stability.
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*
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*
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* SHIFT_FLL is used as a dampening factor to define how much we
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* adjust the frequency correction for a given offset in FLL mode.
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* In David Mills' nanokernel reference implementation SHIFT_FLL is 2.
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*
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* MAXTC establishes the maximum time constant of the PLL.
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*/
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#define SHIFT_PLL 2 /* PLL frequency factor (shift) */
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#define SHIFT_FLL 2 /* FLL frequency factor (shift) */
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#define MAXTC 10 /* maximum time constant (shift) */
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/*
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* SHIFT_USEC defines the scaling (shift) of the time_freq and
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* time_tolerance variables, which represent the current frequency
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* offset and maximum frequency tolerance.
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*/
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#define SHIFT_USEC 16 /* frequency offset scale (shift) */
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#define PPM_SCALE ((s64)NSEC_PER_USEC << (NTP_SCALE_SHIFT - SHIFT_USEC))
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#define PPM_SCALE_INV_SHIFT 19
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#define PPM_SCALE_INV ((1LL << (PPM_SCALE_INV_SHIFT + NTP_SCALE_SHIFT)) / \
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PPM_SCALE + 1)
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#define MAXPHASE 500000000L /* max phase error (ns) */
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#define MAXFREQ 500000 /* max frequency error (ns/s) */
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#define MAXFREQ_SCALED ((s64)MAXFREQ << NTP_SCALE_SHIFT)
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#define MINSEC 256 /* min interval between updates (s) */
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#define MAXSEC 2048 /* max interval between updates (s) */
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#define NTP_PHASE_LIMIT ((MAXPHASE / NSEC_PER_USEC) << 5) /* beyond max. dispersion */
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/*
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* kernel variables
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* Note: maximum error = NTP sync distance = dispersion + delay / 2;
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* estimated error = NTP dispersion.
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*/
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extern unsigned long tick_usec; /* USER_HZ period (usec) */
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extern unsigned long tick_nsec; /* SHIFTED_HZ period (nsec) */
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/* Required to safely shift negative values */
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#define shift_right(x, s) ({ \
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__typeof__(x) __x = (x); \
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__typeof__(s) __s = (s); \
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__x < 0 ? -(-__x >> __s) : __x >> __s; \
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})
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#define NTP_SCALE_SHIFT 32
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#define NTP_INTERVAL_FREQ (HZ)
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#define NTP_INTERVAL_LENGTH (NSEC_PER_SEC/NTP_INTERVAL_FREQ)
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extern int do_adjtimex(struct __kernel_timex *);
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extern int do_clock_adjtime(const clockid_t which_clock, struct __kernel_timex * ktx);
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extern void hardpps(const struct timespec64 *, const struct timespec64 *);
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int read_current_timer(unsigned long *timer_val);
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/* The clock frequency of the i8253/i8254 PIT */
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#define PIT_TICK_RATE 1193182ul
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#endif /* LINUX_TIMEX_H */
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