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authorDavid Howells <dhowells@redhat.com>2008-09-24 20:48:26 +0400
committerLinus Torvalds <torvalds@linux-foundation.org>2008-09-25 03:38:17 +0400
commitb4f151ff899362fec952c45d166252c9912c041f (patch)
tree7c4453472c21c7d3368b19a504c6841606f36c60 /include/linux/cnt32_to_63.h
parent7a528159b93bc52c14aedff55d53e741227fc846 (diff)
downloadlinux-b4f151ff899362fec952c45d166252c9912c041f.tar.xz
MN10300: Move asm-arm/cnt32_to_63.h to include/linux/
Move asm-arm/cnt32_to_63.h to include/linux/ so that MN10300 can make use of it too. Signed-off-by: David Howells <dhowells@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'include/linux/cnt32_to_63.h')
-rw-r--r--include/linux/cnt32_to_63.h80
1 files changed, 80 insertions, 0 deletions
diff --git a/include/linux/cnt32_to_63.h b/include/linux/cnt32_to_63.h
new file mode 100644
index 000000000000..8c0f9505b48c
--- /dev/null
+++ b/include/linux/cnt32_to_63.h
@@ -0,0 +1,80 @@
+/*
+ * 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 __LINUX_CNT32_TO_63_H__
+#define __LINUX_CNT32_TO_63_H__
+
+#include <linux/compiler.h>
+#include <linux/types.h>
+#include <asm/byteorder.h>
+
+/* this is used only to give gcc a clue about good code generation */
+union cnt32_to_63 {
+ struct {
+#if defined(__LITTLE_ENDIAN)
+ u32 lo, hi;
+#elif defined(__BIG_ENDIAN)
+ u32 hi, lo;
+#endif
+ };
+ u64 val;
+};
+
+
+/**
+ * cnt32_to_63 - Expand a 32-bit counter to a 63-bit counter
+ * @cnt_lo: The low part of the counter
+ *
+ * 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.
+ */
+#define cnt32_to_63(cnt_lo) \
+({ \
+ static volatile u32 __m_cnt_hi; \
+ union cnt32_to_63 __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