[PATCH v4 2/3] watchdog: introduce watchdog.open_timeout commandline parameter

[Rasmus Villemoes]

The watchdog framework takes care of feeding a hardware watchdog until
userspace opens /dev/watchdogN. If that never happens for some reason
(buggy init script, corrupt root filesystem or whatnot) but the kernel
itself is fine, the machine stays up indefinitely. This patch allows
setting an upper limit for how long the kernel will take care of the
watchdog, thus ensuring that the watchdog will eventually reset the
machine.

This is particularly useful for embedded devices where some fallback
logic is implemented in the bootloader (e.g., use a different root
partition, boot from network, ...).

The open timeout is also used as a maximum time for an application to
re-open /dev/watchdogN after closing it.

A value of 0 (the default) means infinite timeout, preserving the
current behaviour.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@xxxxxxxxx>
---
 Documentation/watchdog/watchdog-parameters.txt |  9 +++++++++
 drivers/watchdog/watchdog_dev.c                | 26 +++++++++++++++++++++++++-
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/Documentation/watchdog/watchdog-parameters.txt b/Documentation/watchdog/watchdog-parameters.txt
index e21850e..2ae0fdf 100644
--- a/Documentation/watchdog/watchdog-parameters.txt
+++ b/Documentation/watchdog/watchdog-parameters.txt
@@ -8,6 +8,15 @@ See Documentation/admin-guide/kernel-parameters.rst for information on
 providing kernel parameters for builtin drivers versus loadable
 modules.
 
+The watchdog core currently understands one parameter,
+watchdog.open_timeout. This is the maximum time, in milliseconds, for
+which the watchdog framework will take care of pinging a hardware
+watchdog until userspace opens the corresponding /dev/watchdogN
+device. A value of 0 (the default) means an infinite timeout. Setting
+this to a non-zero value can be useful to ensure that either userspace
+comes up properly, or the board gets reset and allows fallback logic
+in the bootloader to try something else.
+
 
 -------------------------------------------------
 acquirewdt:
diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
index 21809e4..8bc9f24 100644
--- a/drivers/watchdog/watchdog_dev.c
+++ b/drivers/watchdog/watchdog_dev.c
@@ -66,6 +66,7 @@ struct watchdog_core_data {
 	struct mutex lock;
 	unsigned long last_keepalive;
 	unsigned long last_hw_keepalive;
+	unsigned long open_deadline;
 	struct delayed_work work;
 	unsigned long status;		/* Internal status bits */
 #define _WDOG_DEV_OPEN		0	/* Opened ? */
@@ -80,6 +81,21 @@ static struct watchdog_core_data *old_wd_data;
 
 static struct workqueue_struct *watchdog_wq;
 
+static unsigned open_timeout;
+module_param(open_timeout, uint, 0644);
+
+static bool watchdog_past_open_deadline(struct watchdog_core_data *data)
+{
+	if (!open_timeout)
+		return false;
+	return time_is_before_jiffies(data->open_deadline);
+}
+
+static void watchdog_set_open_deadline(struct watchdog_core_data *data)
+{
+	data->open_deadline = jiffies + msecs_to_jiffies(open_timeout);
+}
+
 static inline bool watchdog_need_worker(struct watchdog_device *wdd)
 {
 	/* All variables in milli-seconds */
@@ -196,7 +212,13 @@ static bool watchdog_worker_should_ping(struct watchdog_core_data *wd_data)
 {
 	struct watchdog_device *wdd = wd_data->wdd;
 
-	return wdd && (watchdog_active(wdd) || watchdog_hw_running(wdd));
+	if (!wdd)
+		return false;
+
+	if (watchdog_active(wdd))
+		return true;
+
+	return watchdog_hw_running(wdd) && !watchdog_past_open_deadline(wd_data);
 }
 
 static void watchdog_ping_work(struct work_struct *work)
@@ -857,6 +879,7 @@ static int watchdog_release(struct inode *inode, struct file *file)
 		watchdog_ping(wdd);
 	}
 
+	watchdog_set_open_deadline(wd_data);
 	watchdog_update_worker(wdd);
 
 	/* make sure that /dev/watchdog can be re-opened */
@@ -955,6 +978,7 @@ static int watchdog_cdev_register(struct watchdog_device *wdd, dev_t devno)
 
 	/* Record time of most recent heartbeat as 'just before now'. */
 	wd_data->last_hw_keepalive = jiffies - 1;
+	watchdog_set_open_deadline(wd_data);
 
 	/*
 	 * If the watchdog is running, prevent its driver from being unloaded,
-- 
2.7.4