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 #include <linux/module.h> 10 #include <linux/i8253.h> 11 12 /* 13 * Protects access to I/O ports 14 * 15 * 0040-0043 : timer0, i8253 / i8254 16 * 0061-0061 : NMI Control Register which contains two speaker control bits. 17 */ 18 DEFINE_RAW_SPINLOCK(i8253_lock); 19 EXPORT_SYMBOL(i8253_lock); 20 21 #ifdef CONFIG_CLKSRC_I8253 22 /* 23 * Since the PIT overflows every tick, its not very useful 24 * to just read by itself. So use jiffies to emulate a free 25 * running counter: 26 */ 27 static cycle_t i8253_read(struct clocksource *cs) 28 { 29 static int old_count; 30 static u32 old_jifs; 31 unsigned long flags; 32 int count; 33 u32 jifs; 34 35 raw_spin_lock_irqsave(&i8253_lock, flags); 36 /* 37 * Although our caller may have the read side of xtime_lock, 38 * this is now a seqlock, and we are cheating in this routine 39 * by having side effects on state that we cannot undo if 40 * there is a collision on the seqlock and our caller has to 41 * retry. (Namely, old_jifs and old_count.) So we must treat 42 * jiffies as volatile despite the lock. We read jiffies 43 * before latching the timer count to guarantee that although 44 * the jiffies value might be older than the count (that is, 45 * the counter may underflow between the last point where 46 * jiffies was incremented and the point where we latch the 47 * count), it cannot be newer. 48 */ 49 jifs = jiffies; 50 outb_pit(0x00, PIT_MODE); /* latch the count ASAP */ 51 count = inb_pit(PIT_CH0); /* read the latched count */ 52 count |= inb_pit(PIT_CH0) << 8; 53 54 /* VIA686a test code... reset the latch if count > max + 1 */ 55 if (count > LATCH) { 56 outb_pit(0x34, PIT_MODE); 57 outb_pit(PIT_LATCH & 0xff, PIT_CH0); 58 outb_pit(PIT_LATCH >> 8, PIT_CH0); 59 count = PIT_LATCH - 1; 60 } 61 62 /* 63 * It's possible for count to appear to go the wrong way for a 64 * couple of reasons: 65 * 66 * 1. The timer counter underflows, but we haven't handled the 67 * resulting interrupt and incremented jiffies yet. 68 * 2. Hardware problem with the timer, not giving us continuous time, 69 * the counter does small "jumps" upwards on some Pentium systems, 70 * (see c't 95/10 page 335 for Neptun bug.) 71 * 72 * Previous attempts to handle these cases intelligently were 73 * buggy, so we just do the simple thing now. 74 */ 75 if (count > old_count && jifs == old_jifs) 76 count = old_count; 77 78 old_count = count; 79 old_jifs = jifs; 80 81 raw_spin_unlock_irqrestore(&i8253_lock, flags); 82 83 count = (PIT_LATCH - 1) - count; 84 85 return (cycle_t)(jifs * PIT_LATCH) + count; 86 } 87 88 static struct clocksource i8253_cs = { 89 .name = "pit", 90 .rating = 110, 91 .read = i8253_read, 92 .mask = CLOCKSOURCE_MASK(32), 93 }; 94 95 int __init clocksource_i8253_init(void) 96 { 97 return clocksource_register_hz(&i8253_cs, PIT_TICK_RATE); 98 } 99 #endif 100