diff options
Diffstat (limited to 'arch/i386/kernel/i8253.c')
-rw-r--r-- | arch/i386/kernel/i8253.c | 53 |
1 files changed, 53 insertions, 0 deletions
diff --git a/arch/i386/kernel/i8253.c b/arch/i386/kernel/i8253.c index 29cb2eb3436..a276bceade6 100644 --- a/arch/i386/kernel/i8253.c +++ b/arch/i386/kernel/i8253.c @@ -2,6 +2,7 @@ * i8253.c 8253/PIT functions * */ +#include <linux/clocksource.h> #include <linux/spinlock.h> #include <linux/jiffies.h> #include <linux/sysdev.h> @@ -30,3 +31,55 @@ void setup_pit_timer(void) outb(LATCH >> 8 , PIT_CH0); /* MSB */ spin_unlock_irqrestore(&i8253_lock, flags); } + +/* + * Since the PIT overflows every tick, its not very useful + * to just read by itself. So use jiffies to emulate a free + * running counter: + */ +static cycle_t pit_read(void) +{ + unsigned long flags; + int count; + u64 jifs; + + spin_lock_irqsave(&i8253_lock, flags); + outb_p(0x00, PIT_MODE); /* latch the count ASAP */ + count = inb_p(PIT_CH0); /* read the latched count */ + count |= inb_p(PIT_CH0) << 8; + + /* VIA686a test code... reset the latch if count > max + 1 */ + if (count > LATCH) { + outb_p(0x34, PIT_MODE); + outb_p(LATCH & 0xff, PIT_CH0); + outb(LATCH >> 8, PIT_CH0); + count = LATCH - 1; + } + spin_unlock_irqrestore(&i8253_lock, flags); + + jifs = jiffies_64; + + jifs -= INITIAL_JIFFIES; + count = (LATCH-1) - count; + + return (cycle_t)(jifs * LATCH) + count; +} + +static struct clocksource clocksource_pit = { + .name = "pit", + .rating = 110, + .read = pit_read, + .mask = (cycle_t)-1, + .mult = 0, + .shift = 20, +}; + +static int __init init_pit_clocksource(void) +{ + if (num_possible_cpus() > 4) /* PIT does not scale! */ + return 0; + + clocksource_pit.mult = clocksource_hz2mult(CLOCK_TICK_RATE, 20); + return register_clocksource(&clocksource_pit); +} +module_init(init_pit_clocksource); |