This commit was manufactured by cvs2svn to create branch 'vserver'.

[linux-2.6.git] / include / asm-arm / cnt32_to_63.h

diff --git a/include/asm-arm/cnt32_to_63.h b/include/asm-arm/cnt32_to_63.h
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+/*
+ *  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