[PATCH] HPET RTC emulation: add watchdog timer

[Clemens Ladisch <clemens@ladisch.de>]

To prevent the emulated RTC timer from stopping when interrupts are
disabled for too long, implement the watchdog timer to restart it when
needed.

Index: linux-2.6.15/arch/i386/kernel/time_hpet.c
===================================================================
--- linux-2.6.15.orig/arch/i386/kernel/time_hpet.c	2006-01-06 16:11:10.000000000 +0100
+++ linux-2.6.15/arch/i386/kernel/time_hpet.c	2006-01-08 21:31:55.000000000 +0100
@@ -413,9 +413,45 @@ int hpet_set_periodic_freq(unsigned long
 
 int hpet_rtc_dropped_irq(void)
 {
+	unsigned int cnt, ticks_per_int, lost_ints;
+
 	if (!is_hpet_enabled())
 		return 0;
 
+	if (UIE_on | PIE_on | AIE_on) {
+		/*
+		 * The interrupt handler schedules the next interrupt at a
+		 * constant offset from the time the current interrupt was
+		 * scheduled, without regard to the actual time. When the
+		 * handler is delayed too long, it tries to schedule the next
+		 * interrupt in the past and the hardware would not interrupt
+		 * until the counter had wrapped around. We catch it here.
+		 */
+		cnt = hpet_readl(HPET_COUNTER);
+		/* was the comparator set to a time in the past? */
+		if ((int)(cnt - hpet_t1_cmp) > 0) {
+			/* determine how many interrupts were actually lost */
+			ticks_per_int = (hpet_tick * HZ) / hpet_rtc_int_freq;
+			lost_ints = (cnt - hpet_t1_cmp) / ticks_per_int + 1;
+			/*
+			 * Make sure that, even with the time needed to execute
+			 * this code, the next scheduled interrupt has been
+			 * moved back to the future.
+			 */
+			lost_ints++;
+
+			cnt = hpet_t1_cmp + lost_ints * ticks_per_int;
+			hpet_writel(cnt, HPET_T1_CMP);
+			hpet_t1_cmp = cnt;
+
+			if (PIE_on)
+				PIE_count += lost_ints;
+
+			printk(KERN_WARNING "rtc: lost some interrupts"
+			       " at %ldHz.\n", hpet_rtc_int_freq);
+		}
+	}
+
 	return 1;
 }
 
Index: linux-2.6.15/arch/x86_64/kernel/time.c
===================================================================
--- linux-2.6.15.orig/arch/x86_64/kernel/time.c	2006-01-06 16:11:13.000000000 +0100
+++ linux-2.6.15/arch/x86_64/kernel/time.c	2006-01-08 21:42:16.000000000 +0100
@@ -1234,9 +1234,45 @@ int hpet_set_periodic_freq(unsigned long
 
 int hpet_rtc_dropped_irq(void)
 {
+	unsigned int cnt, ticks_per_int, lost_ints;
+
 	if (!is_hpet_enabled())
 		return 0;
 
+	if (UIE_on | PIE_on | AIE_on) {
+		/*
+		 * The interrupt handler schedules the next interrupt at a
+		 * constant offset from the time the current interrupt was
+		 * scheduled, without regard to the actual time. When the
+		 * handler is delayed too long, it tries to schedule the next
+		 * interrupt in the past and the hardware would not interrupt
+		 * until the counter had wrapped around. We catch it here.
+		 */
+		cnt = hpet_readl(HPET_COUNTER);
+		/* was the comparator set to a time in the past? */
+		if ((int)(cnt - hpet_t1_cmp) > 0) {
+			/* determine how many interrupts were actually lost */
+			ticks_per_int = (hpet_tick * HZ) / hpet_rtc_int_freq;
+			lost_ints = (cnt - hpet_t1_cmp) / ticks_per_int + 1;
+			/*
+			 * Make sure that, even with the time needed to execute
+			 * this code, the next scheduled interrupt has been
+			 * moved back to the future.
+			 */
+			lost_ints++;
+
+			cnt = hpet_t1_cmp + lost_ints * ticks_per_int;
+			hpet_writel(cnt, HPET_T1_CMP);
+			hpet_t1_cmp = cnt;
+
+			if (PIE_on)
+				PIE_count += lost_ints;
+
+			printk(KERN_WARNING "rtc: lost some interrupts"
+			       " at %ldHz.\n", hpet_rtc_int_freq);
+		}
+	}
+
 	return 1;
 }
 
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