mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-12-17 17:36:50 +07:00
9ffc93f203
Remove all #inclusions of asm/system.h preparatory to splitting and killing it. Performed with the following command: perl -p -i -e 's!^#\s*include\s*<asm/system[.]h>.*\n!!' `grep -Irl '^#\s*include\s*<asm/system[.]h>' *` Signed-off-by: David Howells <dhowells@redhat.com>
108 lines
3.6 KiB
C
108 lines
3.6 KiB
C
/*
|
|
* Extend a 32-bit counter to 63 bits
|
|
*
|
|
* Author: Nicolas Pitre
|
|
* Created: December 3, 2006
|
|
* Copyright: MontaVista Software, Inc.
|
|
*
|
|
* This program is free software; you can redistribute it and/or modify
|
|
* it under the terms of the GNU General Public License version 2
|
|
* as published by the Free Software Foundation.
|
|
*/
|
|
|
|
#ifndef __LINUX_CNT32_TO_63_H__
|
|
#define __LINUX_CNT32_TO_63_H__
|
|
|
|
#include <linux/compiler.h>
|
|
#include <linux/types.h>
|
|
#include <asm/byteorder.h>
|
|
|
|
/* this is used only to give gcc a clue about good code generation */
|
|
union cnt32_to_63 {
|
|
struct {
|
|
#if defined(__LITTLE_ENDIAN)
|
|
u32 lo, hi;
|
|
#elif defined(__BIG_ENDIAN)
|
|
u32 hi, lo;
|
|
#endif
|
|
};
|
|
u64 val;
|
|
};
|
|
|
|
|
|
/**
|
|
* cnt32_to_63 - Expand a 32-bit counter to a 63-bit counter
|
|
* @cnt_lo: The low part of the counter
|
|
*
|
|
* Many hardware clock counters are only 32 bits wide and therefore have
|
|
* a relatively short period making wrap-arounds rather frequent. This
|
|
* is a problem when implementing sched_clock() for example, where a 64-bit
|
|
* non-wrapping monotonic value is expected to be returned.
|
|
*
|
|
* To overcome that limitation, let's extend a 32-bit counter to 63 bits
|
|
* in a completely lock free fashion. Bits 0 to 31 of the clock are provided
|
|
* by the hardware while bits 32 to 62 are stored in memory. The top bit in
|
|
* memory is used to synchronize with the hardware clock half-period. When
|
|
* the top bit of both counters (hardware and in memory) differ then the
|
|
* memory is updated with a new value, incrementing it when the hardware
|
|
* counter wraps around.
|
|
*
|
|
* Because a word store in memory is atomic then the incremented value will
|
|
* always be in synch with the top bit indicating to any potential concurrent
|
|
* reader if the value in memory is up to date or not with regards to the
|
|
* needed increment. And any race in updating the value in memory is harmless
|
|
* as the same value would simply be stored more than once.
|
|
*
|
|
* The restrictions for the algorithm to work properly are:
|
|
*
|
|
* 1) this code must be called at least once per each half period of the
|
|
* 32-bit counter;
|
|
*
|
|
* 2) this code must not be preempted for a duration longer than the
|
|
* 32-bit counter half period minus the longest period between two
|
|
* calls to this code;
|
|
*
|
|
* Those requirements ensure proper update to the state bit in memory.
|
|
* This is usually not a problem in practice, but if it is then a kernel
|
|
* timer should be scheduled to manage for this code to be executed often
|
|
* enough.
|
|
*
|
|
* And finally:
|
|
*
|
|
* 3) the cnt_lo argument must be seen as a globally incrementing value,
|
|
* meaning that it should be a direct reference to the counter data which
|
|
* can be evaluated according to a specific ordering within the macro,
|
|
* and not the result of a previous evaluation stored in a variable.
|
|
*
|
|
* For example, this is wrong:
|
|
*
|
|
* u32 partial = get_hw_count();
|
|
* u64 full = cnt32_to_63(partial);
|
|
* return full;
|
|
*
|
|
* This is fine:
|
|
*
|
|
* u64 full = cnt32_to_63(get_hw_count());
|
|
* return full;
|
|
*
|
|
* Note that the top bit (bit 63) in the returned value should be considered
|
|
* as garbage. It is not cleared here because callers are likely to use a
|
|
* multiplier on the returned value which can get rid of the top bit
|
|
* implicitly by making the multiplier even, therefore saving on a runtime
|
|
* clear-bit instruction. Otherwise caller must remember to clear the top
|
|
* bit explicitly.
|
|
*/
|
|
#define cnt32_to_63(cnt_lo) \
|
|
({ \
|
|
static u32 __m_cnt_hi; \
|
|
union cnt32_to_63 __x; \
|
|
__x.hi = __m_cnt_hi; \
|
|
smp_rmb(); \
|
|
__x.lo = (cnt_lo); \
|
|
if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \
|
|
__m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \
|
|
__x.val; \
|
|
})
|
|
|
|
#endif
|