drivers/clocksource/i8253.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * i8253 PIT clocksource
   4  */
   5 #include <linux/clockchips.h>
   6 #include <linux/init.h>
   7 #include <linux/io.h>
   8 #include <linux/spinlock.h>
   9 #include <linux/timex.h>
  10 #include <linux/module.h>
  11 #include <linux/i8253.h>
  12 #include <linux/smp.h>
  13
  14 /*
  15  * Protects access to I/O ports
  16  *
  17  * 0040-0043 : timer0, i8253 / i8254
  18  * 0061-0061 : NMI Control Register which contains two speaker control bits.
  19  */
  20 DEFINE_RAW_SPINLOCK(i8253_lock);
  21 EXPORT_SYMBOL(i8253_lock);
  22
  23 /*
  24  * Handle PIT quirk in pit_shutdown() where zeroing the counter register
  25  * restarts the PIT, negating the shutdown. On platforms with the quirk,
  26  * platform specific code can set this to false.
  27  */
  28 bool i8253_clear_counter_on_shutdown __ro_after_init = true;
  29
  30 #ifdef CONFIG_CLKSRC_I8253
  31 /*
  32  * Since the PIT overflows every tick, its not very useful
  33  * to just read by itself. So use jiffies to emulate a free
  34  * running counter:
  35  */
  36 static u64 i8253_read(struct clocksource *cs)
  37 {
  38         static int old_count;
  39         static u32 old_jifs;
  40         unsigned long flags;
  41         int count;
  42         u32 jifs;
  43
  44         raw_spin_lock_irqsave(&i8253_lock, flags);
  45         /*
  46          * Although our caller may have the read side of jiffies_lock,
  47          * this is now a seqlock, and we are cheating in this routine
  48          * by having side effects on state that we cannot undo if
  49          * there is a collision on the seqlock and our caller has to
  50          * retry.  (Namely, old_jifs and old_count.)  So we must treat
  51          * jiffies as volatile despite the lock.  We read jiffies
  52          * before latching the timer count to guarantee that although
  53          * the jiffies value might be older than the count (that is,
  54          * the counter may underflow between the last point where
  55          * jiffies was incremented and the point where we latch the
  56          * count), it cannot be newer.
  57          */
  58         jifs = jiffies;
  59         outb_p(0x00, PIT_MODE); /* latch the count ASAP */
  60         count = inb_p(PIT_CH0); /* read the latched count */
  61         count |= inb_p(PIT_CH0) << 8;
  62
  63         /* VIA686a test code... reset the latch if count > max + 1 */
  64         if (count > PIT_LATCH) {
  65                 outb_p(0x34, PIT_MODE);
  66                 outb_p(PIT_LATCH & 0xff, PIT_CH0);
  67                 outb_p(PIT_LATCH >> 8, PIT_CH0);
  68                 count = PIT_LATCH - 1;
  69         }
  70
  71         /*
  72          * It's possible for count to appear to go the wrong way for a
  73          * couple of reasons:
  74          *
  75          *  1. The timer counter underflows, but we haven't handled the
  76          *     resulting interrupt and incremented jiffies yet.
  77          *  2. Hardware problem with the timer, not giving us continuous time,
  78          *     the counter does small "jumps" upwards on some Pentium systems,
  79          *     (see c't 95/10 page 335 for Neptun bug.)
  80          *
  81          * Previous attempts to handle these cases intelligently were
  82          * buggy, so we just do the simple thing now.
  83          */
  84         if (count > old_count && jifs == old_jifs)
  85                 count = old_count;
  86
  87         old_count = count;
  88         old_jifs = jifs;
  89
  90         raw_spin_unlock_irqrestore(&i8253_lock, flags);
  91
  92         count = (PIT_LATCH - 1) - count;
  93
  94         return (u64)(jifs * PIT_LATCH) + count;
  95 }
  96
  97 static struct clocksource i8253_cs = {
  98         .name           = "pit",
  99         .rating         = 110,
 100         .read           = i8253_read,
 101         .mask           = CLOCKSOURCE_MASK(32),
 102 };
 103
 104 int __init clocksource_i8253_init(void)
 105 {
 106         return clocksource_register_hz(&i8253_cs, PIT_TICK_RATE);
 107 }
 108 #endif
 109
 110 #ifdef CONFIG_CLKEVT_I8253
 111 static int pit_shutdown(struct clock_event_device *evt)
 112 {
 113         if (!clockevent_state_oneshot(evt) && !clockevent_state_periodic(evt))
 114                 return 0;
 115
 116         raw_spin_lock(&i8253_lock);
 117
 118         outb_p(0x30, PIT_MODE);
 119
 120         if (i8253_clear_counter_on_shutdown) {
 121                 outb_p(0, PIT_CH0);
 122                 outb_p(0, PIT_CH0);
 123         }
 124
 125         raw_spin_unlock(&i8253_lock);
 126         return 0;
 127 }
 128
 129 static int pit_set_oneshot(struct clock_event_device *evt)
 130 {
 131         raw_spin_lock(&i8253_lock);
 132         outb_p(0x38, PIT_MODE);
 133         raw_spin_unlock(&i8253_lock);
 134         return 0;
 135 }
 136
 137 static int pit_set_periodic(struct clock_event_device *evt)
 138 {
 139         raw_spin_lock(&i8253_lock);
 140
 141         /* binary, mode 2, LSB/MSB, ch 0 */
 142         outb_p(0x34, PIT_MODE);
 143         outb_p(PIT_LATCH & 0xff, PIT_CH0);      /* LSB */
 144         outb_p(PIT_LATCH >> 8, PIT_CH0);        /* MSB */
 145
 146         raw_spin_unlock(&i8253_lock);
 147         return 0;
 148 }
 149
 150 /*
 151  * Program the next event in oneshot mode
 152  *
 153  * Delta is given in PIT ticks
 154  */
 155 static int pit_next_event(unsigned long delta, struct clock_event_device *evt)
 156 {
 157         raw_spin_lock(&i8253_lock);
 158         outb_p(delta & 0xff , PIT_CH0); /* LSB */
 159         outb_p(delta >> 8 , PIT_CH0);           /* MSB */
 160         raw_spin_unlock(&i8253_lock);
 161
 162         return 0;
 163 }
 164
 165 /*
 166  * On UP the PIT can serve all of the possible timer functions. On SMP systems
 167  * it can be solely used for the global tick.
 168  */
 169 struct clock_event_device i8253_clockevent = {
 170         .name                   = "pit",
 171         .features               = CLOCK_EVT_FEAT_PERIODIC,
 172         .set_state_shutdown     = pit_shutdown,
 173         .set_state_periodic     = pit_set_periodic,
 174         .set_next_event         = pit_next_event,
 175 };
 176
 177 /*
 178  * Initialize the conversion factor and the min/max deltas of the clock event
 179  * structure and register the clock event source with the framework.
 180  */
 181 void __init clockevent_i8253_init(bool oneshot)
 182 {
 183         if (oneshot) {
 184                 i8253_clockevent.features |= CLOCK_EVT_FEAT_ONESHOT;
 185                 i8253_clockevent.set_state_oneshot = pit_set_oneshot;
 186         }
 187         /*
 188          * Start pit with the boot cpu mask. x86 might make it global
 189          * when it is used as broadcast device later.
 190          */
 191         i8253_clockevent.cpumask = cpumask_of(smp_processor_id());
 192
 193         clockevents_config_and_register(&i8253_clockevent, PIT_TICK_RATE,
 194                                         0xF, 0x7FFF);
 195 }
 196 #endif