/* * 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> #include <asm/system.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. * * 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