watchdog: Add stop_on_reboot parameter to control reboot policy

Many watchdog drivers use watchdog_stop_on_reboot() helper in order
to stop the watchdog on system reboot. Unfortunately, this logic is
coded in driver's probe function and doesn't allows user to decide what
to do during shutdown/reboot.

On the other side, Xen and Qemu watchdog drivers (xen_wdt and i6300esb)
may be configured to either send NMI or turn off/reboot VM as
the watchdog action. As the kernel may stuck at any state, sending NMIs
can't reliably reboot the VM.

At Arista, we benefited from the following set-up: the emulated watchdogs
trigger VM reset and softdog is set to catch less severe conditions to
generate vmcore. Just before reboot watchdog's timeout is increased
to some good-enough value (3 mins). That keeps watchdog always running
and guarantees that VM doesn't stuck.

Provide new stop_on_reboot module parameter to let user control
watchdog's reboot policy.

Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Wim Van Sebroeck <wim@linux-watchdog.org>
Cc: linux-watchdog@vger.kernel.org
Signed-off-by: Dmitry Safonov <dima@arista.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20200223114939.194754-1-dima@arista.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

drivers/watchdog/watchdog_core.c

@@ -39,6 +39,10 @@
 
 static DEFINE_IDA(watchdog_ida);
 
+static int stop_on_reboot = -1;
+module_param(stop_on_reboot, int, 0444);
+MODULE_PARM_DESC(stop_on_reboot, "Stop watchdogs on reboot (0=keep watching, 1=stop)");
+
 /*
  * Deferred Registration infrastructure.
  *
@@ -254,6 +258,14 @@ static int __watchdog_register_device(struct watchdog_device *wdd)
 		}
 	}
 
+	/* Module parameter to force watchdog policy on reboot. */
+	if (stop_on_reboot != -1) {
+		if (stop_on_reboot)
+			set_bit(WDOG_STOP_ON_REBOOT, &wdd->status);
+		else
+			clear_bit(WDOG_STOP_ON_REBOOT, &wdd->status);
+	}
+
 	if (test_bit(WDOG_STOP_ON_REBOOT, &wdd->status)) {
 		wdd->reboot_nb.notifier_call = watchdog_reboot_notifier;