[PATCH v8 3/3] watchdog: make the device time out at open_deadline when open_timeout is used

[Rasmus Villemoes <rasmus.villemoes@prevas.dk>]

When the watchdog device is not open by userspace, the kernel takes
care of pinging it. When the open_timeout feature is in use, we should
ensure that the hardware fires close to open_timeout ms after the
kernel has assumed responsibility for the device (either at boot, or
after userspace has had it open and magic-closed it).

To do this, simply reuse the logic that is already in place for
ensuring the same thing when userspace is responsible for regularly
pinging the device:

- When watchdog_active(wdd), this patch doesn't change anything.

- When !watchdoc_active(wdd), the "virtual timeout" should be taken to
be ->open_deadline". When the open_timeout feature is not used (i.e.,
when open_timeout was 0 the last time watchdog_set_open_deadline was
called), ->open_deadline is KTIME_MAX, and the arithmetic ends up
returning keepalive_interval as we used to.

This has been tested on a Wandboard with various combinations of
open_timeout (including fractional-seconds settings) and timeout-sec
properties for the on-board watchdog by booting with 'init=/bin/sh',
timestamping the lines on the serial console, and comparing the
timestamp of the 'imx2-wdt 20bc000.wdog: timeout nnn sec' line with
the timestamp of the 'U-Boot SPL ...' line (which appears just after
reset).

Suggested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
---
 drivers/watchdog/watchdog_dev.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
index 804da5b2ce02..39dd2329cbea 100644
--- a/drivers/watchdog/watchdog_dev.c
+++ b/drivers/watchdog/watchdog_dev.c
@@ -133,14 +133,15 @@ static ktime_t watchdog_next_keepalive(struct watchdog_device *wdd)
 	ktime_t virt_timeout;
 	unsigned int hw_heartbeat_ms;
 
-	virt_timeout = ktime_add(wd_data->last_keepalive,
-				 ms_to_ktime(timeout_ms));
+	if (watchdog_active(wdd))
+		virt_timeout = ktime_add(wd_data->last_keepalive,
+					 ms_to_ktime(timeout_ms));
+	else
+		virt_timeout = wd_data->open_deadline;
+
 	hw_heartbeat_ms = min_not_zero(timeout_ms, wdd->max_hw_heartbeat_ms);
 	keepalive_interval = ms_to_ktime(hw_heartbeat_ms / 2);
 
-	if (!watchdog_active(wdd))
-		return keepalive_interval;
-
 	/*
 	 * To ensure that the watchdog times out wdd->timeout seconds
 	 * after the most recent ping from userspace, the last
-- 
2.20.1