1 ===============================================
2 The Linux WatchDog Timer Driver Core kernel API
3 ===============================================
4
5 Last reviewed: 12-Feb-2013
6
7 Wim Van Sebroeck <wim@iguana.be>
8
9 Introduction
10 ------------
11 This document does not describe what a WatchDog Timer (WDT) Driver or Device is.
12 It also does not describe the API which can be used by user space to communicate
13 with a WatchDog Timer. If you want to know this then please read the following
14 file: Documentation/watchdog/watchdog-api.rst .
15
16 So what does this document describe? It describes the API that can be used by
17 WatchDog Timer Drivers that want to use the WatchDog Timer Driver Core
18 Framework. This framework provides all interfacing towards user space so that
19 the same code does not have to be reproduced each time. This also means that
20 a watchdog timer driver then only needs to provide the different routines
21 (operations) that control the watchdog timer (WDT).
22
23 The API
24 -------
25 Each watchdog timer driver that wants to use the WatchDog Timer Driver Core
26 must #include <linux/watchdog.h> (you would have to do this anyway when
27 writing a watchdog device driver). This include file contains following
28 register/unregister routines::
29
30 extern int watchdog_register_device(struct watchdog_device *);
31 extern void watchdog_unregister_device(struct watchdog_device *);
32
33 The watchdog_register_device routine registers a watchdog timer device.
34 The parameter of this routine is a pointer to a watchdog_device structure.
35 This routine returns zero on success and a negative errno code for failure.
36
37 The watchdog_unregister_device routine deregisters a registered watchdog timer
38 device. The parameter of this routine is the pointer to the registered
39 watchdog_device structure.
40
41 The watchdog subsystem includes an registration deferral mechanism,
42 which allows you to register an watchdog as early as you wish during
43 the boot process.
44
45 The watchdog device structure looks like this::
46
47 struct watchdog_device {
48 int id;
49 struct device *parent;
50 const struct attribute_group **groups;
51 const struct watchdog_info *info;
52 const struct watchdog_ops *ops;
53 const struct watchdog_governor *gov;
54 unsigned int bootstatus;
55 unsigned int timeout;
56 unsigned int pretimeout;
57 unsigned int min_timeout;
58 unsigned int max_timeout;
59 unsigned int min_hw_heartbeat_ms;
60 unsigned int max_hw_heartbeat_ms;
61 struct notifier_block reboot_nb;
62 struct notifier_block restart_nb;
63 void *driver_data;
64 struct watchdog_core_data *wd_data;
65 unsigned long status;
66 struct list_head deferred;
67 };
68
69 It contains following fields:
70
71 * id: set by watchdog_register_device, id 0 is special. It has both a
72 /dev/watchdog0 cdev (dynamic major, minor 0) as well as the old
73 /dev/watchdog miscdev. The id is set automatically when calling
74 watchdog_register_device.
75 * parent: set this to the parent device (or NULL) before calling
76 watchdog_register_device.
77 * groups: List of sysfs attribute groups to create when creating the watchdog
78 device.
79 * info: a pointer to a watchdog_info structure. This structure gives some
80 additional information about the watchdog timer itself. (Like its unique name)
81 * ops: a pointer to the list of watchdog operations that the watchdog supports.
82 * gov: a pointer to the assigned watchdog device pretimeout governor or NULL.
83 * timeout: the watchdog timer's timeout value (in seconds).
84 This is the time after which the system will reboot if user space does
85 not send a heartbeat request if WDOG_ACTIVE is set.
86 * pretimeout: the watchdog timer's pretimeout value (in seconds).
87 * min_timeout: the watchdog timer's minimum timeout value (in seconds).
88 If set, the minimum configurable value for 'timeout'.
89 * max_timeout: the watchdog timer's maximum timeout value (in seconds),
90 as seen from userspace. If set, the maximum configurable value for
91 'timeout'. Not used if max_hw_heartbeat_ms is non-zero.
92 * min_hw_heartbeat_ms: Hardware limit for minimum time between heartbeats,
93 in milli-seconds. This value is normally 0; it should only be provided
94 if the hardware can not tolerate lower intervals between heartbeats.
95 * max_hw_heartbeat_ms: Maximum hardware heartbeat, in milli-seconds.
96 If set, the infrastructure will send heartbeats to the watchdog driver
97 if 'timeout' is larger than max_hw_heartbeat_ms, unless WDOG_ACTIVE
98 is set and userspace failed to send a heartbeat for at least 'timeout'
99 seconds. max_hw_heartbeat_ms must be set if a driver does not implement
100 the stop function.
101 * reboot_nb: notifier block that is registered for reboot notifications, for
102 internal use only. If the driver calls watchdog_stop_on_reboot, watchdog core
103 will stop the watchdog on such notifications.
104 * restart_nb: notifier block that is registered for machine restart, for
105 internal use only. If a watchdog is capable of restarting the machine, it
106 should define ops->restart. Priority can be changed through
107 watchdog_set_restart_priority.
108 * bootstatus: status of the device after booting (reported with watchdog
109 WDIOF_* status bits).
110 * driver_data: a pointer to the drivers private data of a watchdog device.
111 This data should only be accessed via the watchdog_set_drvdata and
112 watchdog_get_drvdata routines.
113 * wd_data: a pointer to watchdog core internal data.
114 * status: this field contains a number of status bits that give extra
115 information about the status of the device (Like: is the watchdog timer
116 running/active, or is the nowayout bit set).
117 * deferred: entry in wtd_deferred_reg_list which is used to
118 register early initialized watchdogs.
119
120 The list of watchdog operations is defined as::
121
122 struct watchdog_ops {
123 struct module *owner;
124 /* mandatory operations */
125 int (*start)(struct watchdog_device *);
126 /* optional operations */
127 int (*stop)(struct watchdog_device *);
128 int (*ping)(struct watchdog_device *);
129 unsigned int (*status)(struct watchdog_device *);
130 int (*set_timeout)(struct watchdog_device *, unsigned int);
131 int (*set_pretimeout)(struct watchdog_device *, unsigned int);
132 unsigned int (*get_timeleft)(struct watchdog_device *);
133 int (*restart)(struct watchdog_device *);
134 long (*ioctl)(struct watchdog_device *, unsigned int, unsigned long);
135 };
136
137 It is important that you first define the module owner of the watchdog timer
138 driver's operations. This module owner will be used to lock the module when
139 the watchdog is active. (This to avoid a system crash when you unload the
140 module and /dev/watchdog is still open).
141
142 Some operations are mandatory and some are optional. The mandatory operations
143 are:
144
145 * start: this is a pointer to the routine that starts the watchdog timer
146 device.
147 The routine needs a pointer to the watchdog timer device structure as a
148 parameter. It returns zero on success or a negative errno code for failure.
149
150 Not all watchdog timer hardware supports the same functionality. That's why
151 all other routines/operations are optional. They only need to be provided if
152 they are supported. These optional routines/operations are:
153
154 * stop: with this routine the watchdog timer device is being stopped.
155
156 The routine needs a pointer to the watchdog timer device structure as a
157 parameter. It returns zero on success or a negative errno code for failure.
158 Some watchdog timer hardware can only be started and not be stopped. A
159 driver supporting such hardware does not have to implement the stop routine.
160
161 If a driver has no stop function, the watchdog core will set WDOG_HW_RUNNING
162 and start calling the driver's keepalive pings function after the watchdog
163 device is closed.
164
165 If a watchdog driver does not implement the stop function, it must set
166 max_hw_heartbeat_ms.
167 * ping: this is the routine that sends a keepalive ping to the watchdog timer
168 hardware.
169
170 The routine needs a pointer to the watchdog timer device structure as a
171 parameter. It returns zero on success or a negative errno code for failure.
172
173 Most hardware that does not support this as a separate function uses the
174 start function to restart the watchdog timer hardware. And that's also what
175 the watchdog timer driver core does: to send a keepalive ping to the watchdog
176 timer hardware it will either use the ping operation (when available) or the
177 start operation (when the ping operation is not available).
178
179 (Note: the WDIOC_KEEPALIVE ioctl call will only be active when the
180 WDIOF_KEEPALIVEPING bit has been set in the option field on the watchdog's
181 info structure).
182 * status: this routine checks the status of the watchdog timer device. The
183 status of the device is reported with watchdog WDIOF_* status flags/bits.
184
185 WDIOF_MAGICCLOSE and WDIOF_KEEPALIVEPING are reported by the watchdog core;
186 it is not necessary to report those bits from the driver. Also, if no status
187 function is provided by the driver, the watchdog core reports the status bits
188 provided in the bootstatus variable of struct watchdog_device.
189
190 * set_timeout: this routine checks and changes the timeout of the watchdog
191 timer device. It returns 0 on success, -EINVAL for "parameter out of range"
192 and -EIO for "could not write value to the watchdog". On success this
193 routine should set the timeout value of the watchdog_device to the
194 achieved timeout value (which may be different from the requested one
195 because the watchdog does not necessarily have a 1 second resolution).
196
197 Drivers implementing max_hw_heartbeat_ms set the hardware watchdog heartbeat
198 to the minimum of timeout and max_hw_heartbeat_ms. Those drivers set the
199 timeout value of the watchdog_device either to the requested timeout value
200 (if it is larger than max_hw_heartbeat_ms), or to the achieved timeout value.
201 (Note: the WDIOF_SETTIMEOUT needs to be set in the options field of the
202 watchdog's info structure).
203
204 If the watchdog driver does not have to perform any action but setting the
205 watchdog_device.timeout, this callback can be omitted.
206
207 If set_timeout is not provided but, WDIOF_SETTIMEOUT is set, the watchdog
208 infrastructure updates the timeout value of the watchdog_device internally
209 to the requested value.
210
211 If the pretimeout feature is used (WDIOF_PRETIMEOUT), then set_timeout must
212 also take care of checking if pretimeout is still valid and set up the timer
213 accordingly. This can't be done in the core without races, so it is the
214 duty of the driver.
215 * set_pretimeout: this routine checks and changes the pretimeout value of
216 the watchdog. It is optional because not all watchdogs support pretimeout
217 notification. The timeout value is not an absolute time, but the number of
218 seconds before the actual timeout would happen. It returns 0 on success,
219 -EINVAL for "parameter out of range" and -EIO for "could not write value to
220 the watchdog". A value of 0 disables pretimeout notification.
221
222 (Note: the WDIOF_PRETIMEOUT needs to be set in the options field of the
223 watchdog's info structure).
224
225 If the watchdog driver does not have to perform any action but setting the
226 watchdog_device.pretimeout, this callback can be omitted. That means if
227 set_pretimeout is not provided but WDIOF_PRETIMEOUT is set, the watchdog
228 infrastructure updates the pretimeout value of the watchdog_device internally
229 to the requested value.
230
231 * get_timeleft: this routines returns the time that's left before a reset.
232 * restart: this routine restarts the machine. It returns 0 on success or a
233 negative errno code for failure.
234 * ioctl: if this routine is present then it will be called first before we do
235 our own internal ioctl call handling. This routine should return -ENOIOCTLCMD
236 if a command is not supported. The parameters that are passed to the ioctl
237 call are: watchdog_device, cmd and arg.
238
239 The status bits should (preferably) be set with the set_bit and clear_bit alike
240 bit-operations. The status bits that are defined are:
241
242 * WDOG_ACTIVE: this status bit indicates whether or not a watchdog timer device
243 is active or not from user perspective. User space is expected to send
244 heartbeat requests to the driver while this flag is set.
245 * WDOG_NO_WAY_OUT: this bit stores the nowayout setting for the watchdog.
246 If this bit is set then the watchdog timer will not be able to stop.
247 * WDOG_HW_RUNNING: Set by the watchdog driver if the hardware watchdog is
248 running. The bit must be set if the watchdog timer hardware can not be
249 stopped. The bit may also be set if the watchdog timer is running after
250 booting, before the watchdog device is opened. If set, the watchdog
251 infrastructure will send keepalives to the watchdog hardware while
252 WDOG_ACTIVE is not set.
253 Note: when you register the watchdog timer device with this bit set,
254 then opening /dev/watchdog will skip the start operation but send a keepalive
255 request instead.
256
257 To set the WDOG_NO_WAY_OUT status bit (before registering your watchdog
258 timer device) you can either:
259
260 * set it statically in your watchdog_device struct with
261
262 .status = WATCHDOG_NOWAYOUT_INIT_STATUS,
263
264 (this will set the value the same as CONFIG_WATCHDOG_NOWAYOUT) or
265 * use the following helper function::
266
267 static inline void watchdog_set_nowayout(struct watchdog_device *wdd,
268 int nowayout)
269
270 Note:
271 The WatchDog Timer Driver Core supports the magic close feature and
272 the nowayout feature. To use the magic close feature you must set the
273 WDIOF_MAGICCLOSE bit in the options field of the watchdog's info structure.
274
275 The nowayout feature will overrule the magic close feature.
276
277 To get or set driver specific data the following two helper functions should be
278 used::
279
280 static inline void watchdog_set_drvdata(struct watchdog_device *wdd,
281 void *data)
282 static inline void *watchdog_get_drvdata(struct watchdog_device *wdd)
283
284 The watchdog_set_drvdata function allows you to add driver specific data. The
285 arguments of this function are the watchdog device where you want to add the
286 driver specific data to and a pointer to the data itself.
287
288 The watchdog_get_drvdata function allows you to retrieve driver specific data.
289 The argument of this function is the watchdog device where you want to retrieve
290 data from. The function returns the pointer to the driver specific data.
291
292 To initialize the timeout field, the following function can be used::
293
294 extern int watchdog_init_timeout(struct watchdog_device *wdd,
295 unsigned int timeout_parm,
296 struct device *dev);
297
298 The watchdog_init_timeout function allows you to initialize the timeout field
299 using the module timeout parameter or by retrieving the timeout-sec property from
300 the device tree (if the module timeout parameter is invalid). Best practice is
301 to set the default timeout value as timeout value in the watchdog_device and
302 then use this function to set the user "preferred" timeout value.
303 This routine returns zero on success and a negative errno code for failure.
304
305 To disable the watchdog on reboot, the user must call the following helper::
306
307 static inline void watchdog_stop_on_reboot(struct watchdog_device *wdd);
308
309 To disable the watchdog when unregistering the watchdog, the user must call
310 the following helper. Note that this will only stop the watchdog if the
311 nowayout flag is not set.
312
313 ::
314
315 static inline void watchdog_stop_on_unregister(struct watchdog_device *wdd);
316
317 To change the priority of the restart handler the following helper should be
318 used::
319
320 void watchdog_set_restart_priority(struct watchdog_device *wdd, int priority);
321
322 User should follow the following guidelines for setting the priority:
323
324 * 0: should be called in last resort, has limited restart capabilities
325 * 128: default restart handler, use if no other handler is expected to be
326 available, and/or if restart is sufficient to restart the entire system
327 * 255: highest priority, will preempt all other restart handlers
328
329 To raise a pretimeout notification, the following function should be used::
330
331 void watchdog_notify_pretimeout(struct watchdog_device *wdd)
332
333 The function can be called in the interrupt context. If watchdog pretimeout
334 governor framework (kbuild CONFIG_WATCHDOG_PRETIMEOUT_GOV symbol) is enabled,
335 an action is taken by a preconfigured pretimeout governor preassigned to
336 the watchdog device. If watchdog pretimeout governor framework is not
337 enabled, watchdog_notify_pretimeout() prints a notification message to
338 the kernel log buffer.
339
340 To set the last known HW keepalive time for a watchdog, the following function
341 should be used::
342
343 int watchdog_set_last_hw_keepalive(struct watchdog_device *wdd,
344 unsigned int last_ping_ms)
345
346 This function must be called immediately after watchdog registration. It
347 sets the last known hardware heartbeat to have happened last_ping_ms before
348 current time. Calling this is only needed if the watchdog is already running
349 when probe is called, and the watchdog can only be pinged after the
350 min_hw_heartbeat_ms time has passed from the last ping.
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.