1 /* 2 * i8253 PIT clocksource 3 */ 4 #include <linux/clocksource.h> 5 #include <linux/init.h> 6 #include <linux/io.h> 7 #include <linux/spinlock.h> 8 #include <linux/timex.h> 9 10 #include <asm/i8253.h> 11 12 /* 13 * Since the PIT overflows every tick, its not very useful 14 * to just read by itself. So use jiffies to emulate a free 15 * running counter: 16 */ 17 static cycle_t i8253_read(struct clocksource *cs) 18 { 19 static int old_count; 20 static u32 old_jifs; 21 unsigned long flags; 22 int count; 23 u32 jifs; 24 25 raw_spin_lock_irqsave(&i8253_lock, flags); 26 /* 27 * Although our caller may have the read side of xtime_lock, 28 * this is now a seqlock, and we are cheating in this routine 29 * by having side effects on state that we cannot undo if 30 * there is a collision on the seqlock and our caller has to 31 * retry. (Namely, old_jifs and old_count.) So we must treat 32 * jiffies as volatile despite the lock. We read jiffies 33 * before latching the timer count to guarantee that although 34 * the jiffies value might be older than the count (that is, 35 * the counter may underflow between the last point where 36 * jiffies was incremented and the point where we latch the 37 * count), it cannot be newer. 38 */ 39 jifs = jiffies; 40 outb_pit(0x00, PIT_MODE); /* latch the count ASAP */ 41 count = inb_pit(PIT_CH0); /* read the latched count */ 42 count |= inb_pit(PIT_CH0) << 8; 43 44 /* VIA686a test code... reset the latch if count > max + 1 */ 45 if (count > LATCH) { 46 outb_pit(0x34, PIT_MODE); 47 outb_pit(PIT_LATCH & 0xff, PIT_CH0); 48 outb_pit(PIT_LATCH >> 8, PIT_CH0); 49 count = PIT_LATCH - 1; 50 } 51 52 /* 53 * It's possible for count to appear to go the wrong way for a 54 * couple of reasons: 55 * 56 * 1. The timer counter underflows, but we haven't handled the 57 * resulting interrupt and incremented jiffies yet. 58 * 2. Hardware problem with the timer, not giving us continuous time, 59 * the counter does small "jumps" upwards on some Pentium systems, 60 * (see c't 95/10 page 335 for Neptun bug.) 61 * 62 * Previous attempts to handle these cases intelligently were 63 * buggy, so we just do the simple thing now. 64 */ 65 if (count > old_count && jifs == old_jifs) 66 count = old_count; 67 68 old_count = count; 69 old_jifs = jifs; 70 71 raw_spin_unlock_irqrestore(&i8253_lock, flags); 72 73 count = (PIT_LATCH - 1) - count; 74 75 return (cycle_t)(jifs * PIT_LATCH) + count; 76 } 77 78 static struct clocksource i8253_cs = { 79 .name = "pit", 80 .rating = 110, 81 .read = i8253_read, 82 .mask = CLOCKSOURCE_MASK(32), 83 }; 84 85 int __init clocksource_i8253_init(void) 86 { 87 return clocksource_register_hz(&i8253_cs, PIT_TICK_RATE); 88 } 89