/* * include/asm/cnt32_to_63.h -- 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 __INCLUDE_CNT32_TO_63_H__ #define __INCLUDE_CNT32_TO_63_H__ #include <linux/compiler.h> #include <asm/types.h> #include <asm/byteorder.h> /* * Prototype: u64 cnt32_to_63(u32 cnt) * 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 only restriction for the algorithm to work properly is that this * code must be executed at least once per each half period of the 32-bit * counter to properly update the state bit in memory. This is usually not a * problem in practice, but if it is then a kernel timer could 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. */ /* this is used only to give gcc a clue about good code generation */ typedef union { struct { #if defined(__LITTLE_ENDIAN) u32 lo, hi; #elif defined(__BIG_ENDIAN) u32 hi, lo; #endif }; u64 val; } cnt32_to_63_t; #define cnt32_to_63(cnt_lo) \ ({ \ static volatile u32 __m_cnt_hi = 0; \ cnt32_to_63_t __x; \ __x.hi = __m_cnt_hi; \ __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