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

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

drivers/Kconfig

+3 −0
Original line number Diff line number Diff line
@@ -119,4 +119,7 @@ source "drivers/platform/Kconfig" 
source "drivers/clk/Kconfig"



source "drivers/hwspinlock/Kconfig"



source "drivers/clocksource/Kconfig"



endmenu

drivers/clocksource/Kconfig

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+2 −0
Original line number Diff line number Diff line 
config CLKSRC_I8253

	bool

drivers/clocksource/Makefile

+1 −0
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@@ -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

drivers/clocksource/i8253.c

0 → 100644
+88 −0
Original line number Diff line number Diff line 
/*

 * 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);

}