diff options
| -rw-r--r-- | arch/arm/mach-pxa/time.c | 2 | ||||
| -rw-r--r-- | arch/arm/mach-sa1100/generic.c | 2 | ||||
| -rw-r--r-- | arch/arm/mach-versatile/core.c | 2 | ||||
| -rw-r--r-- | include/linux/cnt32_to_63.h | 80 |
4 files changed, 83 insertions, 3 deletions
diff --git a/arch/arm/mach-pxa/time.c b/arch/arm/mach-pxa/time.c index 67e18509d7b..b0d6b32654c 100644 --- a/arch/arm/mach-pxa/time.c +++ b/arch/arm/mach-pxa/time.c @@ -17,9 +17,9 @@ #include <linux/interrupt.h> #include <linux/clockchips.h> #include <linux/sched.h> +#include <linux/cnt32_to_63.h> #include <asm/div64.h> -#include <asm/cnt32_to_63.h> #include <asm/mach/irq.h> #include <asm/mach/time.h> #include <mach/pxa-regs.h> diff --git a/arch/arm/mach-sa1100/generic.c b/arch/arm/mach-sa1100/generic.c index 1362994c78a..b422526f6d8 100644 --- a/arch/arm/mach-sa1100/generic.c +++ b/arch/arm/mach-sa1100/generic.c @@ -18,9 +18,9 @@ #include <linux/ioport.h> #include <linux/sched.h> /* just for sched_clock() - funny that */ #include <linux/platform_device.h> +#include <linux/cnt32_to_63.h> #include <asm/div64.h> -#include <asm/cnt32_to_63.h> #include <mach/hardware.h> #include <asm/system.h> #include <asm/pgtable.h> diff --git a/arch/arm/mach-versatile/core.c b/arch/arm/mach-versatile/core.c index d75e795c893..b638f10411e 100644 --- a/arch/arm/mach-versatile/core.c +++ b/arch/arm/mach-versatile/core.c @@ -28,8 +28,8 @@ #include <linux/amba/clcd.h> #include <linux/clocksource.h> #include <linux/clockchips.h> +#include <linux/cnt32_to_63.h> -#include <asm/cnt32_to_63.h> #include <asm/system.h> #include <mach/hardware.h> #include <asm/io.h> diff --git a/include/linux/cnt32_to_63.h b/include/linux/cnt32_to_63.h new file mode 100644 index 00000000000..8c0f9505b48 --- /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 |