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 #ifdef CONFIG_PCSPKR_PLATFORM 20 EXPORT_SYMBOL(i8253_lock); 21 #endif 22 23 #ifdef CONFIG_CLKSRC_I8253 24 /* 25 * Since the PIT overflows every tick, its not very useful 26 * to just read by itself. So use jiffies to emulate a free 27 * running counter: 28 */ 29 static cycle_t i8253_read(struct clocksource *cs) 30 { 31 static int old_count; 32 static u32 old_jifs; 33 unsigned long flags; 34 int count; 35 u32 jifs; 36 37 raw_spin_lock_irqsave(&i8253_lock, flags); 38 /* 39 * Although our caller may have the read side of xtime_lock, 40 * this is now a seqlock, and we are cheating in this routine 41 * by having side effects on state that we cannot undo if 42 * there is a collision on the seqlock and our caller has to 43 * retry. (Namely, old_jifs and old_count.) So we must treat 44 * jiffies as volatile despite the lock. We read jiffies 45 * before latching the timer count to guarantee that although 46 * the jiffies value might be older than the count (that is, 47 * the counter may underflow between the last point where 48 * jiffies was incremented and the point where we latch the 49 * count), it cannot be newer. 50 */ 51 jifs = jiffies; 52 outb_pit(0x00, PIT_MODE); /* latch the count ASAP */ 53 count = inb_pit(PIT_CH0); /* read the latched count */ 54 count |= inb_pit(PIT_CH0) << 8; 55 56 /* VIA686a test code... reset the latch if count > max + 1 */ 57 if (count > LATCH) { 58 outb_pit(0x34, PIT_MODE); 59 outb_pit(PIT_LATCH & 0xff, PIT_CH0); 60 outb_pit(PIT_LATCH >> 8, PIT_CH0); 61 count = PIT_LATCH - 1; 62 } 63 64 /* 65 * It's possible for count to appear to go the wrong way for a 66 * couple of reasons: 67 * 68 * 1. The timer counter underflows, but we haven't handled the 69 * resulting interrupt and incremented jiffies yet. 70 * 2. Hardware problem with the timer, not giving us continuous time, 71 * the counter does small "jumps" upwards on some Pentium systems, 72 * (see c't 95/10 page 335 for Neptun bug.) 73 * 74 * Previous attempts to handle these cases intelligently were 75 * buggy, so we just do the simple thing now. 76 */ 77 if (count > old_count && jifs == old_jifs) 78 count = old_count; 79 80 old_count = count; 81 old_jifs = jifs; 82 83 raw_spin_unlock_irqrestore(&i8253_lock, flags); 84 85 count = (PIT_LATCH - 1) - count; 86 87 return (cycle_t)(jifs * PIT_LATCH) + count; 88 } 89 90 static struct clocksource i8253_cs = { 91 .name = "pit", 92 .rating = 110, 93 .read = i8253_read, 94 .mask = CLOCKSOURCE_MASK(32), 95 }; 96 97 int __init clocksource_i8253_init(void) 98 { 99 return clocksource_register_hz(&i8253_cs, PIT_TICK_RATE); 100 } 101 #endif 102