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104 lines
3.5 KiB
104 lines
3.5 KiB
/* SPDX-License-Identifier: GPL-2.0-only */ |
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/* |
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* Extend a 32-bit counter to 63 bits |
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* |
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* Author: Nicolas Pitre |
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* Created: December 3, 2006 |
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* Copyright: MontaVista Software, Inc. |
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*/ |
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#ifndef __LINUX_CNT32_TO_63_H__ |
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#define __LINUX_CNT32_TO_63_H__ |
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#include <linux/compiler.h> |
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#include <linux/types.h> |
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#include <asm/byteorder.h> |
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/* this is used only to give gcc a clue about good code generation */ |
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union cnt32_to_63 { |
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struct { |
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#if defined(__LITTLE_ENDIAN) |
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u32 lo, hi; |
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#elif defined(__BIG_ENDIAN) |
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u32 hi, lo; |
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#endif |
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}; |
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u64 val; |
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}; |
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/** |
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* cnt32_to_63 - Expand a 32-bit counter to a 63-bit counter |
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* @cnt_lo: The low part of the counter |
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* |
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* Many hardware clock counters are only 32 bits wide and therefore have |
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* a relatively short period making wrap-arounds rather frequent. This |
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* is a problem when implementing sched_clock() for example, where a 64-bit |
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* non-wrapping monotonic value is expected to be returned. |
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* |
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* To overcome that limitation, let's extend a 32-bit counter to 63 bits |
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* in a completely lock free fashion. Bits 0 to 31 of the clock are provided |
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* by the hardware while bits 32 to 62 are stored in memory. The top bit in |
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* memory is used to synchronize with the hardware clock half-period. When |
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* the top bit of both counters (hardware and in memory) differ then the |
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* memory is updated with a new value, incrementing it when the hardware |
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* counter wraps around. |
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* |
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* Because a word store in memory is atomic then the incremented value will |
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* always be in synch with the top bit indicating to any potential concurrent |
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* reader if the value in memory is up to date or not with regards to the |
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* needed increment. And any race in updating the value in memory is harmless |
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* as the same value would simply be stored more than once. |
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* |
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* The restrictions for the algorithm to work properly are: |
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* |
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* 1) this code must be called at least once per each half period of the |
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* 32-bit counter; |
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* |
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* 2) this code must not be preempted for a duration longer than the |
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* 32-bit counter half period minus the longest period between two |
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* calls to this code; |
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* |
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* Those requirements ensure proper update to the state bit in memory. |
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* This is usually not a problem in practice, but if it is then a kernel |
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* timer should be scheduled to manage for this code to be executed often |
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* enough. |
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* |
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* And finally: |
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* |
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* 3) the cnt_lo argument must be seen as a globally incrementing value, |
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* meaning that it should be a direct reference to the counter data which |
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* can be evaluated according to a specific ordering within the macro, |
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* and not the result of a previous evaluation stored in a variable. |
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* |
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* For example, this is wrong: |
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* |
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* u32 partial = get_hw_count(); |
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* u64 full = cnt32_to_63(partial); |
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* return full; |
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* |
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* This is fine: |
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* |
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* u64 full = cnt32_to_63(get_hw_count()); |
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* return full; |
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* |
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* Note that the top bit (bit 63) in the returned value should be considered |
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* as garbage. It is not cleared here because callers are likely to use a |
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* multiplier on the returned value which can get rid of the top bit |
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* implicitly by making the multiplier even, therefore saving on a runtime |
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* clear-bit instruction. Otherwise caller must remember to clear the top |
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* bit explicitly. |
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*/ |
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#define cnt32_to_63(cnt_lo) \ |
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({ \ |
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static u32 __m_cnt_hi; \ |
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union cnt32_to_63 __x; \ |
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__x.hi = __m_cnt_hi; \ |
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smp_rmb(); \ |
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__x.lo = (cnt_lo); \ |
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if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \ |
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__m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \ |
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__x.val; \ |
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}) |
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#endif
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