Revert "clocksource: Avoid accidental unstable marking of clocksources"

This reverts commit fd99aeb978 which is
commit c86ff8c55b8ae68837b2fa59dc0c203907e9a15f upstream.

It breaks the Android kernel ABI and is not an issue for Android
systems, so revert it.

Bug: 161946584
Fixes: fd99aeb978 ("clocksource: Avoid accidental unstable marking of clocksources")
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I6ea81231f082921f85c3c671f120724a40a191fe

kernel/time/clocksource.c

@@ -105,7 +105,7 @@ static u64 suspend_start;
  * This delay could be due to SMIs, NMIs, or to VCPU preemptions.  Used as
  * a lower bound for cs->uncertainty_margin values when registering clocks.
  */
-#define WATCHDOG_MAX_SKEW (100 * NSEC_PER_USEC)
+#define WATCHDOG_MAX_SKEW (50 * NSEC_PER_USEC)
 
 #ifdef CONFIG_CLOCKSOURCE_WATCHDOG
 static void clocksource_watchdog_work(struct work_struct *work);
@@ -200,24 +200,17 @@ void clocksource_mark_unstable(struct clocksource *cs)
 static ulong max_cswd_read_retries = 3;
 module_param(max_cswd_read_retries, ulong, 0644);
 
-enum wd_read_status {
+static bool cs_watchdog_read(struct clocksource *cs, u64 *csnow, u64 *wdnow)
-	WD_READ_SUCCESS,
-	WD_READ_UNSTABLE,
-	WD_READ_SKIP
-};
-
-static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow, u64 *wdnow)
 {
 	unsigned int nretries;
-	u64 wd_end, wd_end2, wd_delta;
+	u64 wd_end, wd_delta;
-	int64_t wd_delay, wd_seq_delay;
+	int64_t wd_delay;
 
 	for (nretries = 0; nretries <= max_cswd_read_retries; nretries++) {
 		local_irq_disable();
 		*wdnow = watchdog->read(watchdog);
 		*csnow = cs->read(cs);
 		wd_end = watchdog->read(watchdog);
-		wd_end2 = watchdog->read(watchdog);
 		local_irq_enable();
 
 		wd_delta = clocksource_delta(wd_end, *wdnow, watchdog->mask);
@@ -228,34 +221,13 @@ static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow,
 				pr_warn("timekeeping watchdog on CPU%d: %s retried %d times before success\n",
 					smp_processor_id(), watchdog->name, nretries);
 			}
-			return WD_READ_SUCCESS;
+			return true;
 		}
-
-		/*
-		 * Now compute delay in consecutive watchdog read to see if
-		 * there is too much external interferences that cause
-		 * significant delay in reading both clocksource and watchdog.
-		 *
-		 * If consecutive WD read-back delay > WATCHDOG_MAX_SKEW/2,
-		 * report system busy, reinit the watchdog and skip the current
-		 * watchdog test.
-		 */
-		wd_delta = clocksource_delta(wd_end2, wd_end, watchdog->mask);
-		wd_seq_delay = clocksource_cyc2ns(wd_delta, watchdog->mult, watchdog->shift);
-		if (wd_seq_delay > WATCHDOG_MAX_SKEW/2)
-			goto skip_test;
 	}
 
 	pr_warn("timekeeping watchdog on CPU%d: %s read-back delay of %lldns, attempt %d, marking unstable\n",
 		smp_processor_id(), watchdog->name, wd_delay, nretries);
-	return WD_READ_UNSTABLE;
+	return false;
-
-skip_test:
-	pr_info("timekeeping watchdog on CPU%d: %s wd-wd read-back delay of %lldns\n",
-		smp_processor_id(), watchdog->name, wd_seq_delay);
-	pr_info("wd-%s-wd read-back delay of %lldns, clock-skew test skipped!\n",
-		cs->name, wd_delay);
-	return WD_READ_SKIP;
 }
 
 static u64 csnow_mid;
@@ -318,7 +290,6 @@ static void clocksource_watchdog(struct timer_list *unused)
 	int next_cpu, reset_pending;
 	int64_t wd_nsec, cs_nsec;
 	struct clocksource *cs;
-	enum wd_read_status read_ret;
 	u32 md;
 
 	spin_lock(&watchdog_lock);
@@ -336,12 +307,9 @@ static void clocksource_watchdog(struct timer_list *unused)
 			continue;
 		}
 
-		read_ret = cs_watchdog_read(cs, &csnow, &wdnow);
+		if (!cs_watchdog_read(cs, &csnow, &wdnow)) {
-
+			/* Clock readout unreliable, so give it up. */
-		if (read_ret != WD_READ_SUCCESS) {
+			__clocksource_unstable(cs);
-			if (read_ret == WD_READ_UNSTABLE)
-				/* Clock readout unreliable, so give it up. */
-				__clocksource_unstable(cs);
 			continue;
 		}