1 files changed, 53 insertions, 0 deletions

diff --git a/arch/i386/kernel/i8253.c b/arch/i386/kernel/i8253.c
index 29cb2eb34363..a276bceade68 100644
--- a/arch/i386/kernel/i8253.c
+++ b/arch/i386/kernel/i8253.c
@@ -2,6 +2,7 @@
  * i8253.c  8253/PIT functions
  *
  */
+#include <linux/clocksource.h>
 #include <linux/spinlock.h>
 #include <linux/jiffies.h>
 #include <linux/sysdev.h>
@@ -30,3 +31,55 @@ void setup_pit_timer(void)
         outb(LATCH >> 8 , PIT_CH0);     /* MSB */
         spin_unlock_irqrestore(&i8253_lock, flags);
 }
+
+/*
+ * Since the PIT overflows every tick, its not very useful
+ * to just read by itself. So use jiffies to emulate a free
+ * running counter:
+ */
+static cycle_t pit_read(void)
+{
+        unsigned long flags;
+        int count;
+        u64 jifs;
+
+        spin_lock_irqsave(&i8253_lock, flags);
+        outb_p(0x00, PIT_MODE); /* latch the count ASAP */
+        count = inb_p(PIT_CH0); /* read the latched count */
+        count |= inb_p(PIT_CH0) << 8;
+
+        /* VIA686a test code... reset the latch if count > max + 1 */
+        if (count > LATCH) {
+                outb_p(0x34, PIT_MODE);
+                outb_p(LATCH & 0xff, PIT_CH0);
+                outb(LATCH >> 8, PIT_CH0);
+                count = LATCH - 1;
+        }
+        spin_unlock_irqrestore(&i8253_lock, flags);
+
+        jifs = jiffies_64;
+
+        jifs -= INITIAL_JIFFIES;
+        count = (LATCH-1) - count;
+
+        return (cycle_t)(jifs * LATCH) + count;
+}
+
+static struct clocksource clocksource_pit = {
+        .name   = "pit",
+        .rating = 110,
+        .read   = pit_read,
+        .mask   = (cycle_t)-1,
+        .mult   = 0,
+        .shift  = 20,
+};
+
+static int __init init_pit_clocksource(void)
+{
+        if (num_possible_cpus() > 4) /* PIT does not scale! */
+                return 0;
+
+        clocksource_pit.mult = clocksource_hz2mult(CLOCK_TICK_RATE, 20);
+        return register_clocksource(&clocksource_pit);
+}
+module_init(init_pit_clocksource);