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| -rw-r--r-- | include/asm-arm/cnt32_to_63.h | 78 |
1 files changed, 78 insertions, 0 deletions
diff --git a/include/asm-arm/cnt32_to_63.h b/include/asm-arm/cnt32_to_63.h new file mode 100644 index 0000000..480c873 --- /dev/null +++ b/include/asm-arm/cnt32_to_63.h @@ -0,0 +1,78 @@ +/* + * 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 |