cnt32_to_63.h

  1/* SPDX-License-Identifier: GPL-2.0-only */
  2/*
  3 *  Extend a 32-bit counter to 63 bits
  4 *
  5 *  Author:	Nicolas Pitre
  6 *  Created:	December 3, 2006
  7 *  Copyright:	MontaVista Software, Inc.
  8 */
  9
 10#ifndef __LINUX_CNT32_TO_63_H__
 11#define __LINUX_CNT32_TO_63_H__
 12
 13#include <linux/compiler.h>
 14#include <linux/types.h>
 15#include <asm/byteorder.h>
 16
 17/* this is used only to give gcc a clue about good code generation */
 18union cnt32_to_63 {
 19	struct {
 20#if defined(__LITTLE_ENDIAN)
 21		u32 lo, hi;
 22#elif defined(__BIG_ENDIAN)
 23		u32 hi, lo;
 24#endif
 25	};
 26	u64 val;
 27};
 28
 29
 30/**
 31 * cnt32_to_63 - Expand a 32-bit counter to a 63-bit counter
 32 * @cnt_lo: The low part of the counter
 33 *
 34 * Many hardware clock counters are only 32 bits wide and therefore have
 35 * a relatively short period making wrap-arounds rather frequent.  This
 36 * is a problem when implementing sched_clock() for example, where a 64-bit
 37 * non-wrapping monotonic value is expected to be returned.
 38 *
 39 * To overcome that limitation, let's extend a 32-bit counter to 63 bits
 40 * in a completely lock free fashion. Bits 0 to 31 of the clock are provided
 41 * by the hardware while bits 32 to 62 are stored in memory.  The top bit in
 42 * memory is used to synchronize with the hardware clock half-period.  When
 43 * the top bit of both counters (hardware and in memory) differ then the
 44 * memory is updated with a new value, incrementing it when the hardware
 45 * counter wraps around.
 46 *
 47 * Because a word store in memory is atomic then the incremented value will
 48 * always be in synch with the top bit indicating to any potential concurrent
 49 * reader if the value in memory is up to date or not with regards to the
 50 * needed increment.  And any race in updating the value in memory is harmless
 51 * as the same value would simply be stored more than once.
 52 *
 53 * The restrictions for the algorithm to work properly are:
 54 *
 55 * 1) this code must be called at least once per each half period of the
 56 *    32-bit counter;
 57 *
 58 * 2) this code must not be preempted for a duration longer than the
 59 *    32-bit counter half period minus the longest period between two
 60 *    calls to this code;
 61 *
 62 * Those requirements ensure proper update to the state bit in memory.
 63 * This is usually not a problem in practice, but if it is then a kernel
 64 * timer should be scheduled to manage for this code to be executed often
 65 * enough.
 66 *
 67 * And finally:
 68 *
 69 * 3) the cnt_lo argument must be seen as a globally incrementing value,
 70 *    meaning that it should be a direct reference to the counter data which
 71 *    can be evaluated according to a specific ordering within the macro,
 72 *    and not the result of a previous evaluation stored in a variable.
 73 *
 74 * For example, this is wrong:
 75 *
 76 *	u32 partial = get_hw_count();
 77 *	u64 full = cnt32_to_63(partial);
 78 *	return full;
 79 *
 80 * This is fine:
 81 *
 82 *	u64 full = cnt32_to_63(get_hw_count());
 83 *	return full;
 84 *
 85 * Note that the top bit (bit 63) in the returned value should be considered
 86 * as garbage.  It is not cleared here because callers are likely to use a
 87 * multiplier on the returned value which can get rid of the top bit
 88 * implicitly by making the multiplier even, therefore saving on a runtime
 89 * clear-bit instruction. Otherwise caller must remember to clear the top
 90 * bit explicitly.
 91 */
 92#define cnt32_to_63(cnt_lo) \
 93({ \
 94	static u32 __m_cnt_hi; \
 95	union cnt32_to_63 __x; \
 96	__x.hi = __m_cnt_hi; \
 97 	smp_rmb(); \
 98	__x.lo = (cnt_lo); \
 99	if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \
100		__m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \
101	__x.val; \
102})
103
104#endif