[PATCH] rtc: armada38x: do not advertise update interrupt (UIE) support

[Ioannis Fountzoulas]

Problem:
chrony enables RTC update interrupts via the RTC_UIE_ON ioctl to track
RTC drift. On the armada38x driver this request is served by the RTC
core's native path, which arms a 1 second periodic timer that is
re-programmed on the alarm and serviced by rtc_timer_do_work().
If the RTC time is then stepped forward by a large amount while this
timer is active, its scheduled expiry ends up far in the past compared
to the freshly read time.

Why the CPU hangs:
When rtc_timer_do_work() runs, it expires every timer whose expiry is
not in the future, advancing periodic timers by one period each pass:
	while ((next = timerqueue_getnext(&rtc->timerqueue))) {
		if (next->expires > now)
			break;
		...
		timer->node.expires += timer->period;   /* += 1s */
		timerqueue_add(&rtc->timerqueue, &timer->node);
	}
With a large forward step (seen when the RTC starts far in the past and
chrony corrects it after the first NTP sync), the periodic UIE timer is
overdue by the size of the jump, so this loop must run one iteration per
elapsed second before it can exit.
It never yields in that time, so the workqueue worker pins the CPU and
the watchdog reports a soft lockup / RCU stall, after which the
board reboots:
	watchdog: BUG: soft lockup - CPU#1 stuck for 48s! [kworker/1:3:432]
	Kernel panic - not syncing: softlockup: hung tasks
	Workqueue: events rtc_timer_do_work
	 rtc_handle_legacy_irq from rtc_timer_do_work
	 rtc_timer_do_work from process_one_work
	 process_one_work from worker_thread

Fix:
Clear RTC_FEATURE_UPDATE_INTERRUPT at probe time so the driver stops
advertising native UIE. RTC_UIE_ON is then served by the poll-based UIE
emulation in rtc-dev (CONFIG_RTC_INTF_DEV_UIE_EMUL), which delivers the
1 Hz update notifications chrony needs without ever queuing the runaway
periodic timer.

Testing:
Tested on a Marvell Armada 38x board (ARM Cortex-A9). Before the change,
stepping the clock forward while UIE was enabled reproduced the soft
lockup (about 1 in 4 boots). After the change, rtc_timer_do_work() runs
zero loop iterations and returns immediately, chrony still receives
update interrupts via emulation, and no lockups occur over repeated
reboot cycles.

Signed-off-by: Ioannis Fountzoulas <ioannis.fountzoulas at nokia.com>
---
 drivers/rtc/rtc-armada38x.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/rtc/rtc-armada38x.c b/drivers/rtc/rtc-armada38x.c
index 245290ae1a8d..da036d819649 100644
--- a/drivers/rtc/rtc-armada38x.c
+++ b/drivers/rtc/rtc-armada38x.c
@@ -526,6 +526,14 @@ static __init int armada38x_rtc_probe(struct platform_device *pdev)
 	else
 		clear_bit(RTC_FEATURE_ALARM, rtc->rtc_dev->features);
 
+	/*
+	 * A large forward step of the RTC time makes
+	 * rtc_timer_do_work() replay one period per elapsed second and can
+	 * loop long enough to trigger a soft lockup. Do not advertise
+	 * native UIE; RTC_UIE_ON then uses the poll-based emulation.
+	 */
+	clear_bit(RTC_FEATURE_UPDATE_INTERRUPT, rtc->rtc_dev->features);
+
 	/* Update RTC-MBUS bridge timing parameters */
 	rtc->data->update_mbus_timing(rtc);
 
-- 
2.34.1