watchdog: Add hrtimer-based pretimeout feature

This adds the option to use a hrtimer to generate a watchdog pretimeout
event for hardware watchdogs that do not natively support watchdog
pretimeouts.

With this enabled, all watchdogs will appear to have pretimeout support
in userspace. If no pretimeout value is set, there will be no change in
the watchdog's behavior. If a pretimeout value is set for a specific
watchdog that does not have built-in pretimeout support, a timer will be
started that should fire at the specified time before the watchdog
timeout would occur. When the watchdog is successfully pinged, the timer
will be restarted. If the timer is allowed to fire it will generate a
pretimeout event. However because a software timer is used, it may not
be able to fire in every circumstance.

If the watchdog does support a pretimeout natively, that functionality
will be used instead of the hrtimer.

The general design of this feaure was inspired by the software watchdog,
specifically its own pretimeout implementation. However the software
watchdog and this feature are completely independent. They can be used
together; with or without CONFIG_SOFT_WATCHDOG_PRETIMEOUT enabled.

The main advantage of using the hrtimer pretimeout with a hardware
watchdog, compared to running the software watchdog with a hardware
watchdog, is that if the hardware watchdog driver is unable to ping the
watchdog (e.g. due to a bus or communication error), then the hrtimer
pretimeout would still fire whereas the software watchdog would not.

Signed-off-by: Curtis Klein <[e-mail]>
Reviewed-by: Guenter Roeck <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Guenter Roeck <[e-mail]>
Signed-off-by: Wim Van Sebroeck <[e-mail]>

drivers/watchdog/Kconfig

···
 	  Say Y here if you want to enable watchdog device status read through
 	  sysfs attributes.
 
+config WATCHDOG_HRTIMER_PRETIMEOUT
+	bool "Enable watchdog hrtimer-based pretimeouts"
+	help
+	  Enable this if you want to use a hrtimer timer based pretimeout for
+	  watchdogs that do not natively support pretimeout support. Be aware
+	  that because this pretimeout functionality uses hrtimers, it may not
+	  be able to fire before the actual watchdog fires in some situations.
+
 comment "Watchdog Pretimeout Governors"
 
 config WATCHDOG_PRETIMEOUT_GOV

drivers/watchdog/Makefile

···
 watchdog-objs	+= watchdog_core.o watchdog_dev.o
 
 watchdog-$(CONFIG_WATCHDOG_PRETIMEOUT_GOV)	+= watchdog_pretimeout.o
+watchdog-$(CONFIG_WATCHDOG_HRTIMER_PRETIMEOUT)	+= watchdog_hrtimer_pretimeout.o
 
 obj-$(CONFIG_WATCHDOG_PRETIMEOUT_GOV_NOOP)	+= pretimeout_noop.o
 obj-$(CONFIG_WATCHDOG_PRETIMEOUT_GOV_PANIC)	+= pretimeout_panic.o

drivers/watchdog/watchdog_core.h

···
  *
  *	(c) Copyright 2008-2011 Wim Van Sebroeck <[e-mail]>.
  *
+ *	(c) Copyright 2021 Hewlett Packard Enterprise Development LP.
+ *
  *	This source code is part of the generic code that can be used
  *	by all the watchdog timer drivers.
  *
···
  *	This material is provided "AS-IS" and at no charge.
  */
 
+#include <linux/hrtimer.h>
+#include <linux/kthread.h>
+
 #define MAX_DOGS	32	/* Maximum number of watchdog devices */
+
+/*
+ * struct watchdog_core_data - watchdog core internal data
+ * @dev:	The watchdog's internal device
+ * @cdev:	The watchdog's Character device.
+ * @wdd:	Pointer to watchdog device.
+ * @lock:	Lock for watchdog core.
+ * @status:	Watchdog core internal status bits.
+ */
+struct watchdog_core_data {
+	struct device dev;
+	struct cdev cdev;
+	struct watchdog_device *wdd;
+	struct mutex lock;
+	ktime_t last_keepalive;
+	ktime_t last_hw_keepalive;
+	ktime_t open_deadline;
+	struct hrtimer timer;
+	struct kthread_work work;
+#if IS_ENABLED(CONFIG_WATCHDOG_HRTIMER_PRETIMEOUT)
+	struct hrtimer pretimeout_timer;
+#endif
+	unsigned long status;		/* Internal status bits */
+#define _WDOG_DEV_OPEN		0	/* Opened ? */
+#define _WDOG_ALLOW_RELEASE	1	/* Did we receive the magic char ? */
+#define _WDOG_KEEPALIVE		2	/* Did we receive a keepalive ? */
+};
 
 /*
  *	Functions/procedures to be called by the core
···
 extern void watchdog_dev_unregister(struct watchdog_device *);
 extern int __init watchdog_dev_init(void);
 extern void __exit watchdog_dev_exit(void);
+
+static inline bool watchdog_have_pretimeout(struct watchdog_device *wdd)
+{
+	return wdd->info->options & WDIOF_PRETIMEOUT ||
+	       IS_ENABLED(CONFIG_WATCHDOG_HRTIMER_PRETIMEOUT);
+}
+
+#if IS_ENABLED(CONFIG_WATCHDOG_HRTIMER_PRETIMEOUT)
+void watchdog_hrtimer_pretimeout_init(struct watchdog_device *wdd);
+void watchdog_hrtimer_pretimeout_start(struct watchdog_device *wdd);
+void watchdog_hrtimer_pretimeout_stop(struct watchdog_device *wdd);
+#else
+static inline void watchdog_hrtimer_pretimeout_init(struct watchdog_device *wdd) {}
+static inline void watchdog_hrtimer_pretimeout_start(struct watchdog_device *wdd) {}
+static inline void watchdog_hrtimer_pretimeout_stop(struct watchdog_device *wdd) {}
+#endif

drivers/watchdog/watchdog_dev.c

···
  *
  *	(c) Copyright 2008-2011 Wim Van Sebroeck <[e-mail]>.
  *
+ *	(c) Copyright 2021 Hewlett Packard Enterprise Development LP.
  *
  *	This source code is part of the generic code that can be used
  *	by all the watchdog timer drivers.
···
 
 #include "watchdog_core.h"
 #include "watchdog_pretimeout.h"
-
-/*
- * struct watchdog_core_data - watchdog core internal data
- * @dev:	The watchdog's internal device
- * @cdev:	The watchdog's Character device.
- * @wdd:	Pointer to watchdog device.
- * @lock:	Lock for watchdog core.
- * @status:	Watchdog core internal status bits.
- */
-struct watchdog_core_data {
-	struct device dev;
-	struct cdev cdev;
-	struct watchdog_device *wdd;
-	struct mutex lock;
-	ktime_t last_keepalive;
-	ktime_t last_hw_keepalive;
-	ktime_t open_deadline;
-	struct hrtimer timer;
-	struct kthread_work work;
-	unsigned long status;		/* Internal status bits */
-#define _WDOG_DEV_OPEN		0	/* Opened ? */
-#define _WDOG_ALLOW_RELEASE	1	/* Did we receive the magic char ? */
-#define _WDOG_KEEPALIVE		2	/* Did we receive a keepalive ? */
-};
 
 /* the dev_t structure to store the dynamically allocated watchdog devices */
 static dev_t watchdog_devt;
···
 	else
 		err = wdd->ops->start(wdd); /* restart watchdog */
 
+	if (err == 0)
+		watchdog_hrtimer_pretimeout_start(wdd);
+
 	watchdog_update_worker(wdd);
 
 	return err;
···
 	started_at = ktime_get();
 	if (watchdog_hw_running(wdd) && wdd->ops->ping) {
 		err = __watchdog_ping(wdd);
-		if (err == 0)
+		if (err == 0) {
 			set_bit(WDOG_ACTIVE, &wdd->status);
+			watchdog_hrtimer_pretimeout_start(wdd);
+		}
 	} else {
 		err = wdd->ops->start(wdd);
 		if (err == 0) {
···
 			wd_data->last_keepalive = started_at;
 			wd_data->last_hw_keepalive = started_at;
 			watchdog_update_worker(wdd);
+			watchdog_hrtimer_pretimeout_start(wdd);
 		}
 	}
 
···
 	if (err == 0) {
 		clear_bit(WDOG_ACTIVE, &wdd->status);
 		watchdog_update_worker(wdd);
+		watchdog_hrtimer_pretimeout_stop(wdd);
 	}
 
 	return err;
···
 
 	if (test_and_clear_bit(_WDOG_KEEPALIVE, &wd_data->status))
 		status |= WDIOF_KEEPALIVEPING;
+
+	if (IS_ENABLED(CONFIG_WATCHDOG_HRTIMER_PRETIMEOUT))
+		status |= WDIOF_PRETIMEOUT;
 
 	return status;
 }
···
 {
 	int err = 0;
 
-	if (!(wdd->info->options & WDIOF_PRETIMEOUT))
+	if (!watchdog_have_pretimeout(wdd))
 		return -EOPNOTSUPP;
 
 	if (watchdog_pretimeout_invalid(wdd, timeout))
···
 
 	if (attr == &dev_attr_timeleft.attr && !wdd->ops->get_timeleft)
 		mode = 0;
-	else if (attr == &dev_attr_pretimeout.attr &&
-		 !(wdd->info->options & WDIOF_PRETIMEOUT))
+	else if (attr == &dev_attr_pretimeout.attr && !watchdog_have_pretimeout(wdd))
 		mode = 0;
 	else if ((attr == &dev_attr_pretimeout_governor.attr ||
 		  attr == &dev_attr_pretimeout_available_governors.attr) &&
-		 (!(wdd->info->options & WDIOF_PRETIMEOUT) ||
-		  !IS_ENABLED(CONFIG_WATCHDOG_PRETIMEOUT_GOV)))
+		 (!watchdog_have_pretimeout(wdd) || !IS_ENABLED(CONFIG_WATCHDOG_PRETIMEOUT_GOV)))
 		mode = 0;
 
 	return mode;
···
 	kthread_init_work(&wd_data->work, watchdog_ping_work);
 	hrtimer_init(&wd_data->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
 	wd_data->timer.function = watchdog_timer_expired;
+	watchdog_hrtimer_pretimeout_init(wdd);
 
 	if (wdd->id == 0) {
 		old_wd_data = wd_data;
···
 
 	hrtimer_cancel(&wd_data->timer);
 	kthread_cancel_work_sync(&wd_data->work);
+	watchdog_hrtimer_pretimeout_stop(wdd);
 
 	put_device(&wd_data->dev);
 }

drivers/watchdog/watchdog_hrtimer_pretimeout.c

···
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * (c) Copyright 2021 Hewlett Packard Enterprise Development LP.
+ */
+
+#include <linux/hrtimer.h>
+#include <linux/watchdog.h>
+
+#include "watchdog_core.h"
+#include "watchdog_pretimeout.h"
+
+static enum hrtimer_restart watchdog_hrtimer_pretimeout(struct hrtimer *timer)
+{
+	struct watchdog_core_data *wd_data;
+
+	wd_data = container_of(timer, struct watchdog_core_data, pretimeout_timer);
+
+	watchdog_notify_pretimeout(wd_data->wdd);
+	return HRTIMER_NORESTART;
+}
+
+void watchdog_hrtimer_pretimeout_init(struct watchdog_device *wdd)
+{
+	struct watchdog_core_data *wd_data = wdd->wd_data;
+
+	hrtimer_init(&wd_data->pretimeout_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	wd_data->pretimeout_timer.function = watchdog_hrtimer_pretimeout;
+}
+
+void watchdog_hrtimer_pretimeout_start(struct watchdog_device *wdd)
+{
+	if (!(wdd->info->options & WDIOF_PRETIMEOUT) &&
+	    !watchdog_pretimeout_invalid(wdd, wdd->pretimeout))
+		hrtimer_start(&wdd->wd_data->pretimeout_timer,
+			      ktime_set(wdd->timeout - wdd->pretimeout, 0),
+			      HRTIMER_MODE_REL);
+	else
+		hrtimer_cancel(&wdd->wd_data->pretimeout_timer);
+}
+
+void watchdog_hrtimer_pretimeout_stop(struct watchdog_device *wdd)
+{
+	hrtimer_cancel(&wdd->wd_data->pretimeout_timer);
+}

drivers/watchdog/watchdog_pretimeout.c

···
 #include <linux/string.h>
 #include <linux/watchdog.h>
 
+#include "watchdog_core.h"
 #include "watchdog_pretimeout.h"
 
 /* Default watchdog pretimeout governor */
···
 {
 	struct watchdog_pretimeout *p;
 
-	if (!(wdd->info->options & WDIOF_PRETIMEOUT))
+	if (!watchdog_have_pretimeout(wdd))
 		return 0;
 
 	p = kzalloc(sizeof(*p), GFP_KERNEL);
···
 {
 	struct watchdog_pretimeout *p, *t;
 
-	if (!(wdd->info->options & WDIOF_PRETIMEOUT))
+	if (!watchdog_have_pretimeout(wdd))
 		return;
 
 	spin_lock_irq(&pretimeout_lock);