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@@ -0,0 +1,80 @@
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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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+ * This program is free software; you can redistribute it and/or modify
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+ * it under the terms of the GNU General Public License version 2
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+ * as published by the Free Software Foundation.
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+ */
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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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+
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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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+
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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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+
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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 only restriction for the algorithm to work properly is that this
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+ * code must be executed at least once per each half period of the 32-bit
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+ * counter to properly update the state bit in memory. This is usually not a
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+ * problem in practice, but if it is then a kernel timer could be scheduled
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+ * to manage for this code to be executed often enough.
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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 volatile 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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+ __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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+
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+#endif
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David Howells