Commit 89c0b8e2 authored by Russell King's avatar Russell King

clocksource: add common i8253 PIT clocksource

This is based upon both arch/arm/mach-footbridge/isa-timer.c and
arch/x86/kernel/i8253.c.
Acked-by: default avatarJohn Stultz <john.stultz@linaro.org>
Acked-by: default avatarThomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
parent 8b061610
......@@ -119,4 +119,7 @@ source "drivers/platform/Kconfig"
source "drivers/clk/Kconfig"
source "drivers/hwspinlock/Kconfig"
source "drivers/clocksource/Kconfig"
endmenu
config CLKSRC_I8253
bool
......@@ -6,3 +6,4 @@ obj-$(CONFIG_CS5535_CLOCK_EVENT_SRC) += cs5535-clockevt.o
obj-$(CONFIG_SH_TIMER_CMT) += sh_cmt.o
obj-$(CONFIG_SH_TIMER_MTU2) += sh_mtu2.o
obj-$(CONFIG_SH_TIMER_TMU) += sh_tmu.o
obj-$(CONFIG_CLKSRC_I8253) += i8253.o
/*
* i8253 PIT clocksource
*/
#include <linux/clocksource.h>
#include <linux/init.h>
#include <linux/io.h>
#include <linux/spinlock.h>
#include <linux/timex.h>
#include <asm/i8253.h>
/*
* 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 i8253_read(struct clocksource *cs)
{
static int old_count;
static u32 old_jifs;
unsigned long flags;
int count;
u32 jifs;
raw_spin_lock_irqsave(&i8253_lock, flags);
/*
* Although our caller may have the read side of xtime_lock,
* this is now a seqlock, and we are cheating in this routine
* by having side effects on state that we cannot undo if
* there is a collision on the seqlock and our caller has to
* retry. (Namely, old_jifs and old_count.) So we must treat
* jiffies as volatile despite the lock. We read jiffies
* before latching the timer count to guarantee that although
* the jiffies value might be older than the count (that is,
* the counter may underflow between the last point where
* jiffies was incremented and the point where we latch the
* count), it cannot be newer.
*/
jifs = jiffies;
outb_pit(0x00, PIT_MODE); /* latch the count ASAP */
count = inb_pit(PIT_CH0); /* read the latched count */
count |= inb_pit(PIT_CH0) << 8;
/* VIA686a test code... reset the latch if count > max + 1 */
if (count > LATCH) {
outb_pit(0x34, PIT_MODE);
outb_pit(PIT_LATCH & 0xff, PIT_CH0);
outb_pit(PIT_LATCH >> 8, PIT_CH0);
count = PIT_LATCH - 1;
}
/*
* It's possible for count to appear to go the wrong way for a
* couple of reasons:
*
* 1. The timer counter underflows, but we haven't handled the
* resulting interrupt and incremented jiffies yet.
* 2. Hardware problem with the timer, not giving us continuous time,
* the counter does small "jumps" upwards on some Pentium systems,
* (see c't 95/10 page 335 for Neptun bug.)
*
* Previous attempts to handle these cases intelligently were
* buggy, so we just do the simple thing now.
*/
if (count > old_count && jifs == old_jifs)
count = old_count;
old_count = count;
old_jifs = jifs;
raw_spin_unlock_irqrestore(&i8253_lock, flags);
count = (PIT_LATCH - 1) - count;
return (cycle_t)(jifs * PIT_LATCH) + count;
}
static struct clocksource i8253_cs = {
.name = "pit",
.rating = 110,
.read = i8253_read,
.mask = CLOCKSOURCE_MASK(32),
};
int __init clocksource_i8253_init(void)
{
return clocksource_register_hz(&i8253_cs, PIT_TICK_RATE);
}
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