[PATCH] hpet rtc emulation: add watchdog timer

To prevent the emulated RTC timer from stopping when interrupts are delayed
for too long, disable interrupts around all of the register initialization,
and check that the interrupt handler did not schedule the next interrupt in
the past.
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Cc: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Cc: Andi Kleen <ak@muc.de>
Cc: Vojtech Pavlik <vojtech@suse.cz>
Cc: Robert Picco <Robert.Picco@hp.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/i386/kernel/time_hpet.c

@@ -301,23 +301,25 @@ int hpet_rtc_timer_init(void)
 		hpet_rtc_int_freq = DEFAULT_RTC_INT_FREQ;
 
 	local_irq_save(flags);
+
 	cnt = hpet_readl(HPET_COUNTER);
 	cnt += ((hpet_tick*HZ)/hpet_rtc_int_freq);
 	hpet_writel(cnt, HPET_T1_CMP);
 	hpet_t1_cmp = cnt;
-	local_irq_restore(flags);
 
 	cfg = hpet_readl(HPET_T1_CFG);
 	cfg &= ~HPET_TN_PERIODIC;
 	cfg |= HPET_TN_ENABLE | HPET_TN_32BIT;
 	hpet_writel(cfg, HPET_T1_CFG);
 
+	local_irq_restore(flags);
+
 	return 1;
 }
 
 static void hpet_rtc_timer_reinit(void)
 {
-	unsigned int cfg, cnt;
+	unsigned int cfg, cnt, ticks_per_int, lost_ints;
 
 	if (unlikely(!(PIE_on | AIE_on | UIE_on))) {
 		cfg = hpet_readl(HPET_T1_CFG);
@@ -332,10 +334,33 @@ static void hpet_rtc_timer_reinit(void)
 		hpet_rtc_int_freq = DEFAULT_RTC_INT_FREQ;
 
 	/* It is more accurate to use the comparator value than current count.*/
-	cnt = hpet_t1_cmp;
-	cnt += hpet_tick*HZ/hpet_rtc_int_freq;
-	hpet_writel(cnt, HPET_T1_CMP);
-	hpet_t1_cmp = cnt;
+	ticks_per_int = hpet_tick * HZ / hpet_rtc_int_freq;
+	hpet_t1_cmp += ticks_per_int;
+	hpet_writel(hpet_t1_cmp, HPET_T1_CMP);
+
+	/*
+	 * If the interrupt handler was delayed too long, the write above tries
+	 * to schedule the next interrupt in the past and the hardware would
+	 * not interrupt until the counter had wrapped around.
+	 * So we have to check that the comparator wasn't set to a past time.
+	 */
+	cnt = hpet_readl(HPET_COUNTER);
+	if (unlikely((int)(cnt - hpet_t1_cmp) > 0)) {
+		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++;
+
+		hpet_t1_cmp += lost_ints * ticks_per_int;
+		hpet_writel(hpet_t1_cmp, HPET_T1_CMP);
+
+		if (PIE_on)
+			PIE_count += lost_ints;
+
+		printk(KERN_WARNING "rtc: lost some interrupts at %ldHz.\n",
+		       hpet_rtc_int_freq);
+	}
 }
 
 /*

arch/x86_64/kernel/time.c

@@ -1148,23 +1148,25 @@ int hpet_rtc_timer_init(void)
 		hpet_rtc_int_freq = DEFAULT_RTC_INT_FREQ;
 
 	local_irq_save(flags);
+
 	cnt = hpet_readl(HPET_COUNTER);
 	cnt += ((hpet_tick*HZ)/hpet_rtc_int_freq);
 	hpet_writel(cnt, HPET_T1_CMP);
 	hpet_t1_cmp = cnt;
-	local_irq_restore(flags);
 
 	cfg = hpet_readl(HPET_T1_CFG);
 	cfg &= ~HPET_TN_PERIODIC;
 	cfg |= HPET_TN_ENABLE | HPET_TN_32BIT;
 	hpet_writel(cfg, HPET_T1_CFG);
 
+	local_irq_restore(flags);
+
 	return 1;
 }
 
 static void hpet_rtc_timer_reinit(void)
 {
-	unsigned int cfg, cnt;
+	unsigned int cfg, cnt, ticks_per_int, lost_ints;
 
 	if (unlikely(!(PIE_on | AIE_on | UIE_on))) {
 		cfg = hpet_readl(HPET_T1_CFG);
@@ -1179,10 +1181,33 @@ static void hpet_rtc_timer_reinit(void)
 		hpet_rtc_int_freq = DEFAULT_RTC_INT_FREQ;
 
 	/* It is more accurate to use the comparator value than current count.*/
-	cnt = hpet_t1_cmp;
-	cnt += hpet_tick*HZ/hpet_rtc_int_freq;
-	hpet_writel(cnt, HPET_T1_CMP);
-	hpet_t1_cmp = cnt;
+	ticks_per_int = hpet_tick * HZ / hpet_rtc_int_freq;
+	hpet_t1_cmp += ticks_per_int;
+	hpet_writel(hpet_t1_cmp, HPET_T1_CMP);
+
+	/*
+	 * If the interrupt handler was delayed too long, the write above tries
+	 * to schedule the next interrupt in the past and the hardware would
+	 * not interrupt until the counter had wrapped around.
+	 * So we have to check that the comparator wasn't set to a past time.
+	 */
+	cnt = hpet_readl(HPET_COUNTER);
+	if (unlikely((int)(cnt - hpet_t1_cmp) > 0)) {
+		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++;
+
+		hpet_t1_cmp += lost_ints * ticks_per_int;
+		hpet_writel(hpet_t1_cmp, HPET_T1_CMP);
+
+		if (PIE_on)
+			PIE_count += lost_ints;
+
+		printk(KERN_WARNING "rtc: lost some interrupts at %ldHz.\n",
+		       hpet_rtc_int_freq);
+	}
 }
 
 /*