include/linux/timex.h

Source file repositories/reference/linux-study-clean/include/linux/timex.h

File Facts

System
Linux kernel
Corpus path
include/linux/timex.h
Extension
.h
Size
6567 bytes
Lines
165
Domain
Core OS
Bucket
Core Kernel Interface
Inferred role
Core OS: implementation source
Status
source implementation candidate

Why This File Exists

Core operating-system implementation surface: boot, tasks, memory, VFS, syscall-facing interfaces, synchronization, credentials, and isolation.

Dependency Surface

Detected Declarations

Annotated Snippet

#ifndef _LINUX_TIMEX_H
#define _LINUX_TIMEX_H

#include <uapi/linux/timex.h>

#define ADJ_ADJTIME		0x8000	/* switch between adjtime/adjtimex modes */
#define ADJ_OFFSET_SINGLESHOT	0x0001	/* old-fashioned adjtime */
#define ADJ_OFFSET_READONLY	0x2000	/* read-only adjtime */
#include <linux/compiler.h>
#include <linux/types.h>
#include <linux/param.h>

unsigned long random_get_entropy_fallback(void);

#include <asm/timex.h>

#ifndef random_get_entropy
/*
 * The random_get_entropy() function is used by the /dev/random driver
 * in order to extract entropy via the relative unpredictability of
 * when an interrupt takes places versus a high speed, fine-grained
 * timing source or cycle counter.  Since it will be occurred on every
 * single interrupt, it must have a very low cost/overhead.
 *
 * By default we use get_cycles() for this purpose, but individual
 * architectures may override this in their asm/timex.h header file.
 * If a given arch does not have get_cycles(), then we fallback to
 * using random_get_entropy_fallback().
 */
#ifdef get_cycles
#define random_get_entropy()	((unsigned long)get_cycles())
#else
#define random_get_entropy()	random_get_entropy_fallback()
#endif
#endif

/*
 * SHIFT_PLL is used as a dampening factor to define how much we
 * adjust the frequency correction for a given offset in PLL mode.
 * It also used in dampening the offset correction, to define how
 * much of the current value in time_offset we correct for each
 * second. Changing this value changes the stiffness of the ntp
 * adjustment code. A lower value makes it more flexible, reducing
 * NTP convergence time. A higher value makes it stiffer, increasing
 * convergence time, but making the clock more stable.
 *
 * In David Mills' nanokernel reference implementation SHIFT_PLL is 4.
 * However this seems to increase convergence time much too long.
 *
 * https://lists.ntp.org/pipermail/hackers/2008-January/003487.html
 *
 * In the above mailing list discussion, it seems the value of 4
 * was appropriate for other Unix systems with HZ=100, and that
 * SHIFT_PLL should be decreased as HZ increases. However, Linux's
 * clock steering implementation is HZ independent.
 *
 * Through experimentation, a SHIFT_PLL value of 2 was found to allow
 * for fast convergence (very similar to the NTPv3 code used prior to
 * v2.6.19), with good clock stability.
 *
 *
 * SHIFT_FLL is used as a dampening factor to define how much we
 * adjust the frequency correction for a given offset in FLL mode.
 * In David Mills' nanokernel reference implementation SHIFT_FLL is 2.
 *
 * MAXTC establishes the maximum time constant of the PLL.
 */
#define SHIFT_PLL	2	/* PLL frequency factor (shift) */
#define SHIFT_FLL	2	/* FLL frequency factor (shift) */
#define MAXTC		10	/* maximum time constant (shift) */

/*
 * SHIFT_USEC defines the scaling (shift) of the time_freq and
 * time_tolerance variables, which represent the current frequency
 * offset and maximum frequency tolerance.
 */
#define SHIFT_USEC 16		/* frequency offset scale (shift) */
#define PPM_SCALE ((s64)NSEC_PER_USEC << (NTP_SCALE_SHIFT - SHIFT_USEC))
#define PPM_SCALE_INV_SHIFT 19
#define PPM_SCALE_INV ((1LL << (PPM_SCALE_INV_SHIFT + NTP_SCALE_SHIFT)) / \
		       PPM_SCALE + 1)

#define MAXPHASE 500000000L	/* max phase error (ns) */
#define MAXFREQ 500000		/* max frequency error (ns/s) */
#define MAXFREQ_SCALED ((s64)MAXFREQ << NTP_SCALE_SHIFT)
#define MINSEC 256		/* min interval between updates (s) */
#define MAXSEC 2048		/* max interval between updates (s) */
#define NTP_PHASE_LIMIT ((MAXPHASE / NSEC_PER_USEC) << 5) /* beyond max. dispersion */

/* Required to safely shift negative values */

Annotation

Implementation Notes