125
125
setup_irq(0, &irq0);
129
* Since the PIT overflows every tick, its not very useful
130
* to just read by itself. So use jiffies to emulate a free
133
static cycle_t pit_read(struct clocksource *cs)
138
static int old_count;
141
raw_spin_lock_irqsave(&i8253_lock, flags);
143
* Although our caller may have the read side of xtime_lock,
144
* this is now a seqlock, and we are cheating in this routine
145
* by having side effects on state that we cannot undo if
146
* there is a collision on the seqlock and our caller has to
147
* retry. (Namely, old_jifs and old_count.) So we must treat
148
* jiffies as volatile despite the lock. We read jiffies
149
* before latching the timer count to guarantee that although
150
* the jiffies value might be older than the count (that is,
151
* the counter may underflow between the last point where
152
* jiffies was incremented and the point where we latch the
153
* count), it cannot be newer.
156
outb_p(0x00, PIT_MODE); /* latch the count ASAP */
157
count = inb_p(PIT_CH0); /* read the latched count */
158
count |= inb_p(PIT_CH0) << 8;
160
/* VIA686a test code... reset the latch if count > max + 1 */
162
outb_p(0x34, PIT_MODE);
163
outb_p(LATCH & 0xff, PIT_CH0);
164
outb(LATCH >> 8, PIT_CH0);
169
* It's possible for count to appear to go the wrong way for a
172
* 1. The timer counter underflows, but we haven't handled the
173
* resulting interrupt and incremented jiffies yet.
174
* 2. Hardware problem with the timer, not giving us continuous time,
175
* the counter does small "jumps" upwards on some Pentium systems,
176
* (see c't 95/10 page 335 for Neptun bug.)
178
* Previous attempts to handle these cases intelligently were
179
* buggy, so we just do the simple thing now.
181
if (count > old_count && jifs == old_jifs) {
187
raw_spin_unlock_irqrestore(&i8253_lock, flags);
189
count = (LATCH - 1) - count;
191
return (cycle_t)(jifs * LATCH) + count;
194
static struct clocksource clocksource_pit = {
198
.mask = CLOCKSOURCE_MASK(32),
203
128
static int __init init_pit_clocksource(void)
205
130
if (num_possible_cpus() > 1) /* PIT does not scale! */
208
clocksource_pit.mult = clocksource_hz2mult(CLOCK_TICK_RATE, 20);
209
return clocksource_register(&clocksource_pit);
133
return clocksource_i8253_init();
211
135
arch_initcall(init_pit_clocksource);