1 The Linux WatchDog Timer Driver Core kernel API.
2 ===============================================
3 Last reviewed: 12-Feb-2013
5 Wim Van Sebroeck <wim@iguana.be>
9 This document does not describe what a WatchDog Timer (WDT) Driver or Device is.
10 It also does not describe the API which can be used by user space to communicate
11 with a WatchDog Timer. If you want to know this then please read the following
12 file: Documentation/watchdog/watchdog-api.txt .
14 So what does this document describe? It describes the API that can be used by
15 WatchDog Timer Drivers that want to use the WatchDog Timer Driver Core
16 Framework. This framework provides all interfacing towards user space so that
17 the same code does not have to be reproduced each time. This also means that
18 a watchdog timer driver then only needs to provide the different routines
19 (operations) that control the watchdog timer (WDT).
23 Each watchdog timer driver that wants to use the WatchDog Timer Driver Core
24 must #include <linux/watchdog.h> (you would have to do this anyway when
25 writing a watchdog device driver). This include file contains following
26 register/unregister routines:
28 extern int watchdog_register_device(struct watchdog_device *);
29 extern void watchdog_unregister_device(struct watchdog_device *);
31 The watchdog_register_device routine registers a watchdog timer device.
32 The parameter of this routine is a pointer to a watchdog_device structure.
33 This routine returns zero on success and a negative errno code for failure.
35 The watchdog_unregister_device routine deregisters a registered watchdog timer
36 device. The parameter of this routine is the pointer to the registered
37 watchdog_device structure.
39 The watchdog subsystem includes an registration deferral mechanism,
40 which allows you to register an watchdog as early as you wish during
43 The watchdog device structure looks like this:
45 struct watchdog_device {
47 struct device *parent;
48 const struct attribute_group **groups;
49 const struct watchdog_info *info;
50 const struct watchdog_ops *ops;
51 const struct watchdog_governor *gov;
52 unsigned int bootstatus;
54 unsigned int pretimeout;
55 unsigned int min_timeout;
56 unsigned int max_timeout;
57 unsigned int min_hw_heartbeat_ms;
58 unsigned int max_hw_heartbeat_ms;
59 struct notifier_block reboot_nb;
60 struct notifier_block restart_nb;
62 struct watchdog_core_data *wd_data;
64 struct list_head deferred;
67 It contains following fields:
68 * id: set by watchdog_register_device, id 0 is special. It has both a
69 /dev/watchdog0 cdev (dynamic major, minor 0) as well as the old
70 /dev/watchdog miscdev. The id is set automatically when calling
71 watchdog_register_device.
72 * parent: set this to the parent device (or NULL) before calling
73 watchdog_register_device.
74 * groups: List of sysfs attribute groups to create when creating the watchdog
76 * info: a pointer to a watchdog_info structure. This structure gives some
77 additional information about the watchdog timer itself. (Like it's unique name)
78 * ops: a pointer to the list of watchdog operations that the watchdog supports.
79 * gov: a pointer to the assigned watchdog device pretimeout governor or NULL.
80 * timeout: the watchdog timer's timeout value (in seconds).
81 This is the time after which the system will reboot if user space does
82 not send a heartbeat request if WDOG_ACTIVE is set.
83 * pretimeout: the watchdog timer's pretimeout value (in seconds).
84 * min_timeout: the watchdog timer's minimum timeout value (in seconds).
85 If set, the minimum configurable value for 'timeout'.
86 * max_timeout: the watchdog timer's maximum timeout value (in seconds),
87 as seen from userspace. If set, the maximum configurable value for
88 'timeout'. Not used if max_hw_heartbeat_ms is non-zero.
89 * min_hw_heartbeat_ms: Hardware limit for minimum time between heartbeats,
90 in milli-seconds. This value is normally 0; it should only be provided
91 if the hardware can not tolerate lower intervals between heartbeats.
92 * max_hw_heartbeat_ms: Maximum hardware heartbeat, in milli-seconds.
93 If set, the infrastructure will send heartbeats to the watchdog driver
94 if 'timeout' is larger than max_hw_heartbeat_ms, unless WDOG_ACTIVE
95 is set and userspace failed to send a heartbeat for at least 'timeout'
96 seconds. max_hw_heartbeat_ms must be set if a driver does not implement
98 * reboot_nb: notifier block that is registered for reboot notifications, for
99 internal use only. If the driver calls watchdog_stop_on_reboot, watchdog core
100 will stop the watchdog on such notifications.
101 * restart_nb: notifier block that is registered for machine restart, for
102 internal use only. If a watchdog is capable of restarting the machine, it
103 should define ops->restart. Priority can be changed through
104 watchdog_set_restart_priority.
105 * bootstatus: status of the device after booting (reported with watchdog
106 WDIOF_* status bits).
107 * driver_data: a pointer to the drivers private data of a watchdog device.
108 This data should only be accessed via the watchdog_set_drvdata and
109 watchdog_get_drvdata routines.
110 * wd_data: a pointer to watchdog core internal data.
111 * status: this field contains a number of status bits that give extra
112 information about the status of the device (Like: is the watchdog timer
113 running/active, or is the nowayout bit set).
114 * deferred: entry in wtd_deferred_reg_list which is used to
115 register early initialized watchdogs.
117 The list of watchdog operations is defined as:
119 struct watchdog_ops {
120 struct module *owner;
121 /* mandatory operations */
122 int (*start)(struct watchdog_device *);
123 int (*stop)(struct watchdog_device *);
124 /* optional operations */
125 int (*ping)(struct watchdog_device *);
126 unsigned int (*status)(struct watchdog_device *);
127 int (*set_timeout)(struct watchdog_device *, unsigned int);
128 int (*set_pretimeout)(struct watchdog_device *, unsigned int);
129 unsigned int (*get_timeleft)(struct watchdog_device *);
130 int (*restart)(struct watchdog_device *);
131 long (*ioctl)(struct watchdog_device *, unsigned int, unsigned long);
134 It is important that you first define the module owner of the watchdog timer
135 driver's operations. This module owner will be used to lock the module when
136 the watchdog is active. (This to avoid a system crash when you unload the
137 module and /dev/watchdog is still open).
139 Some operations are mandatory and some are optional. The mandatory operations
141 * start: this is a pointer to the routine that starts the watchdog timer
143 The routine needs a pointer to the watchdog timer device structure as a
144 parameter. It returns zero on success or a negative errno code for failure.
146 Not all watchdog timer hardware supports the same functionality. That's why
147 all other routines/operations are optional. They only need to be provided if
148 they are supported. These optional routines/operations are:
149 * stop: with this routine the watchdog timer device is being stopped.
150 The routine needs a pointer to the watchdog timer device structure as a
151 parameter. It returns zero on success or a negative errno code for failure.
152 Some watchdog timer hardware can only be started and not be stopped. A
153 driver supporting such hardware does not have to implement the stop routine.
154 If a driver has no stop function, the watchdog core will set WDOG_HW_RUNNING
155 and start calling the driver's keepalive pings function after the watchdog
157 If a watchdog driver does not implement the stop function, it must set
159 * ping: this is the routine that sends a keepalive ping to the watchdog timer
161 The routine needs a pointer to the watchdog timer device structure as a
162 parameter. It returns zero on success or a negative errno code for failure.
163 Most hardware that does not support this as a separate function uses the
164 start function to restart the watchdog timer hardware. And that's also what
165 the watchdog timer driver core does: to send a keepalive ping to the watchdog
166 timer hardware it will either use the ping operation (when available) or the
167 start operation (when the ping operation is not available).
168 (Note: the WDIOC_KEEPALIVE ioctl call will only be active when the
169 WDIOF_KEEPALIVEPING bit has been set in the option field on the watchdog's
171 * status: this routine checks the status of the watchdog timer device. The
172 status of the device is reported with watchdog WDIOF_* status flags/bits.
173 WDIOF_MAGICCLOSE and WDIOF_KEEPALIVEPING are reported by the watchdog core;
174 it is not necessary to report those bits from the driver. Also, if no status
175 function is provided by the driver, the watchdog core reports the status bits
176 provided in the bootstatus variable of struct watchdog_device.
177 * set_timeout: this routine checks and changes the timeout of the watchdog
178 timer device. It returns 0 on success, -EINVAL for "parameter out of range"
179 and -EIO for "could not write value to the watchdog". On success this
180 routine should set the timeout value of the watchdog_device to the
181 achieved timeout value (which may be different from the requested one
182 because the watchdog does not necessarily have a 1 second resolution).
183 Drivers implementing max_hw_heartbeat_ms set the hardware watchdog heartbeat
184 to the minimum of timeout and max_hw_heartbeat_ms. Those drivers set the
185 timeout value of the watchdog_device either to the requested timeout value
186 (if it is larger than max_hw_heartbeat_ms), or to the achieved timeout value.
187 (Note: the WDIOF_SETTIMEOUT needs to be set in the options field of the
188 watchdog's info structure).
189 If the watchdog driver does not have to perform any action but setting the
190 watchdog_device.timeout, this callback can be omitted.
191 If set_timeout is not provided but, WDIOF_SETTIMEOUT is set, the watchdog
192 infrastructure updates the timeout value of the watchdog_device internally
193 to the requested value.
194 If the pretimeout feature is used (WDIOF_PRETIMEOUT), then set_timeout must
195 also take care of checking if pretimeout is still valid and set up the timer
196 accordingly. This can't be done in the core without races, so it is the
198 * set_pretimeout: this routine checks and changes the pretimeout value of
199 the watchdog. It is optional because not all watchdogs support pretimeout
200 notification. The timeout value is not an absolute time, but the number of
201 seconds before the actual timeout would happen. It returns 0 on success,
202 -EINVAL for "parameter out of range" and -EIO for "could not write value to
203 the watchdog". A value of 0 disables pretimeout notification.
204 (Note: the WDIOF_PRETIMEOUT needs to be set in the options field of the
205 watchdog's info structure).
206 If the watchdog driver does not have to perform any action but setting the
207 watchdog_device.pretimeout, this callback can be omitted. That means if
208 set_pretimeout is not provided but WDIOF_PRETIMEOUT is set, the watchdog
209 infrastructure updates the pretimeout value of the watchdog_device internally
210 to the requested value.
211 * get_timeleft: this routines returns the time that's left before a reset.
212 * restart: this routine restarts the machine. It returns 0 on success or a
213 negative errno code for failure.
214 * ioctl: if this routine is present then it will be called first before we do
215 our own internal ioctl call handling. This routine should return -ENOIOCTLCMD
216 if a command is not supported. The parameters that are passed to the ioctl
217 call are: watchdog_device, cmd and arg.
219 The status bits should (preferably) be set with the set_bit and clear_bit alike
220 bit-operations. The status bits that are defined are:
221 * WDOG_ACTIVE: this status bit indicates whether or not a watchdog timer device
222 is active or not from user perspective. User space is expected to send
223 heartbeat requests to the driver while this flag is set.
224 * WDOG_NO_WAY_OUT: this bit stores the nowayout setting for the watchdog.
225 If this bit is set then the watchdog timer will not be able to stop.
226 * WDOG_HW_RUNNING: Set by the watchdog driver if the hardware watchdog is
227 running. The bit must be set if the watchdog timer hardware can not be
228 stopped. The bit may also be set if the watchdog timer is running after
229 booting, before the watchdog device is opened. If set, the watchdog
230 infrastructure will send keepalives to the watchdog hardware while
231 WDOG_ACTIVE is not set.
232 Note: when you register the watchdog timer device with this bit set,
233 then opening /dev/watchdog will skip the start operation but send a keepalive
236 To set the WDOG_NO_WAY_OUT status bit (before registering your watchdog
237 timer device) you can either:
238 * set it statically in your watchdog_device struct with
239 .status = WATCHDOG_NOWAYOUT_INIT_STATUS,
240 (this will set the value the same as CONFIG_WATCHDOG_NOWAYOUT) or
241 * use the following helper function:
242 static inline void watchdog_set_nowayout(struct watchdog_device *wdd, int nowayout)
244 Note: The WatchDog Timer Driver Core supports the magic close feature and
245 the nowayout feature. To use the magic close feature you must set the
246 WDIOF_MAGICCLOSE bit in the options field of the watchdog's info structure.
247 The nowayout feature will overrule the magic close feature.
249 To get or set driver specific data the following two helper functions should be
252 static inline void watchdog_set_drvdata(struct watchdog_device *wdd, void *data)
253 static inline void *watchdog_get_drvdata(struct watchdog_device *wdd)
255 The watchdog_set_drvdata function allows you to add driver specific data. The
256 arguments of this function are the watchdog device where you want to add the
257 driver specific data to and a pointer to the data itself.
259 The watchdog_get_drvdata function allows you to retrieve driver specific data.
260 The argument of this function is the watchdog device where you want to retrieve
261 data from. The function returns the pointer to the driver specific data.
263 To initialize the timeout field, the following function can be used:
265 extern int watchdog_init_timeout(struct watchdog_device *wdd,
266 unsigned int timeout_parm, struct device *dev);
268 The watchdog_init_timeout function allows you to initialize the timeout field
269 using the module timeout parameter or by retrieving the timeout-sec property from
270 the device tree (if the module timeout parameter is invalid). Best practice is
271 to set the default timeout value as timeout value in the watchdog_device and
272 then use this function to set the user "preferred" timeout value.
273 This routine returns zero on success and a negative errno code for failure.
275 To disable the watchdog on reboot, the user must call the following helper:
277 static inline void watchdog_stop_on_reboot(struct watchdog_device *wdd);
279 To disable the watchdog when unregistering the watchdog, the user must call
280 the following helper. Note that this will only stop the watchdog if the
281 nowayout flag is not set.
283 static inline void watchdog_stop_on_unregister(struct watchdog_device *wdd);
285 To change the priority of the restart handler the following helper should be
288 void watchdog_set_restart_priority(struct watchdog_device *wdd, int priority);
290 User should follow the following guidelines for setting the priority:
291 * 0: should be called in last resort, has limited restart capabilities
292 * 128: default restart handler, use if no other handler is expected to be
293 available, and/or if restart is sufficient to restart the entire system
294 * 255: highest priority, will preempt all other restart handlers
296 To raise a pretimeout notification, the following function should be used:
298 void watchdog_notify_pretimeout(struct watchdog_device *wdd)
300 The function can be called in the interrupt context. If watchdog pretimeout
301 governor framework (kbuild CONFIG_WATCHDOG_PRETIMEOUT_GOV symbol) is enabled,
302 an action is taken by a preconfigured pretimeout governor preassigned to
303 the watchdog device. If watchdog pretimeout governor framework is not
304 enabled, watchdog_notify_pretimeout() prints a notification message to
305 the kernel log buffer.