diff options
Diffstat (limited to 'kernel/time/jiffies.c')
-rw-r--r-- | kernel/time/jiffies.c | 73 |
1 files changed, 73 insertions, 0 deletions
diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c new file mode 100644 index 0000000..126bb30 --- /dev/null +++ b/kernel/time/jiffies.c @@ -0,0 +1,73 @@ +/*********************************************************************** +* linux/kernel/time/jiffies.c +* +* This file contains the jiffies based clocksource. +* +* Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com) +* +* This program is free software; you can redistribute it and/or modify +* it under the terms of the GNU General Public License as published by +* the Free Software Foundation; either version 2 of the License, or +* (at your option) any later version. +* +* This program is distributed in the hope that it will be useful, +* but WITHOUT ANY WARRANTY; without even the implied warranty of +* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the +* GNU General Public License for more details. +* +* You should have received a copy of the GNU General Public License +* along with this program; if not, write to the Free Software +* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. +* +************************************************************************/ +#include <linux/clocksource.h> +#include <linux/jiffies.h> +#include <linux/init.h> + +/* The Jiffies based clocksource is the lowest common + * denominator clock source which should function on + * all systems. It has the same coarse resolution as + * the timer interrupt frequency HZ and it suffers + * inaccuracies caused by missed or lost timer + * interrupts and the inability for the timer + * interrupt hardware to accuratly tick at the + * requested HZ value. It is also not reccomended + * for "tick-less" systems. + */ +#define NSEC_PER_JIFFY ((u32)((((u64)NSEC_PER_SEC)<<8)/ACTHZ)) + +/* Since jiffies uses a simple NSEC_PER_JIFFY multiplier + * conversion, the .shift value could be zero. However + * this would make NTP adjustments impossible as they are + * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to + * shift both the nominator and denominator the same + * amount, and give ntp adjustments in units of 1/2^8 + * + * The value 8 is somewhat carefully chosen, as anything + * larger can result in overflows. NSEC_PER_JIFFY grows as + * HZ shrinks, so values greater then 8 overflow 32bits when + * HZ=100. + */ +#define JIFFIES_SHIFT 8 + +static cycle_t jiffies_read(void) +{ + return (cycle_t) jiffies; +} + +struct clocksource clocksource_jiffies = { + .name = "jiffies", + .rating = 0, /* lowest rating*/ + .read = jiffies_read, + .mask = 0xffffffff, /*32bits*/ + .mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */ + .shift = JIFFIES_SHIFT, + .is_continuous = 0, /* tick based, not free running */ +}; + +static int __init init_jiffies_clocksource(void) +{ + return clocksource_register(&clocksource_jiffies); +} + +module_init(init_jiffies_clocksource); |