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 {
48 struct device *parent;
49 const struct watchdog_info *info;
50 const struct watchdog_ops *ops;
51 unsigned int bootstatus;
53 unsigned int min_timeout;
54 unsigned int max_timeout;
55 struct notifier_block reboot_nb;
56 struct notifier_block restart_nb;
58 struct watchdog_core_data *wd_data;
60 struct list_head deferred;
63 It contains following fields:
64 * id: set by watchdog_register_device, id 0 is special. It has both a
65 /dev/watchdog0 cdev (dynamic major, minor 0) as well as the old
66 /dev/watchdog miscdev. The id is set automatically when calling
67 watchdog_register_device.
68 * dev: device under the watchdog class (created by watchdog_register_device).
69 * parent: set this to the parent device (or NULL) before calling
70 watchdog_register_device.
71 * info: a pointer to a watchdog_info structure. This structure gives some
72 additional information about the watchdog timer itself. (Like it's unique name)
73 * ops: a pointer to the list of watchdog operations that the watchdog supports.
74 * timeout: the watchdog timer's timeout value (in seconds).
75 * min_timeout: the watchdog timer's minimum timeout value (in seconds).
76 * max_timeout: the watchdog timer's maximum timeout value (in seconds).
77 * reboot_nb: notifier block that is registered for reboot notifications, for
78 internal use only. If the driver calls watchdog_stop_on_reboot, watchdog core
79 will stop the watchdog on such notifications.
80 * restart_nb: notifier block that is registered for machine restart, for
81 internal use only. If a watchdog is capable of restarting the machine, it
82 should define ops->restart. Priority can be changed through
83 watchdog_set_restart_priority.
84 * bootstatus: status of the device after booting (reported with watchdog
86 * driver_data: a pointer to the drivers private data of a watchdog device.
87 This data should only be accessed via the watchdog_set_drvdata and
88 watchdog_get_drvdata routines.
89 * wd_data: a pointer to watchdog core internal data.
90 * status: this field contains a number of status bits that give extra
91 information about the status of the device (Like: is the watchdog timer
92 running/active, or is the nowayout bit set).
93 * deferred: entry in wtd_deferred_reg_list which is used to
94 register early initialized watchdogs.
96 The list of watchdog operations is defined as:
100 /* mandatory operations */
101 int (*start)(struct watchdog_device *);
102 int (*stop)(struct watchdog_device *);
103 /* optional operations */
104 int (*ping)(struct watchdog_device *);
105 unsigned int (*status)(struct watchdog_device *);
106 int (*set_timeout)(struct watchdog_device *, unsigned int);
107 unsigned int (*get_timeleft)(struct watchdog_device *);
108 int (*restart)(struct watchdog_device *);
109 void (*ref)(struct watchdog_device *) __deprecated;
110 void (*unref)(struct watchdog_device *) __deprecated;
111 long (*ioctl)(struct watchdog_device *, unsigned int, unsigned long);
114 It is important that you first define the module owner of the watchdog timer
115 driver's operations. This module owner will be used to lock the module when
116 the watchdog is active. (This to avoid a system crash when you unload the
117 module and /dev/watchdog is still open).
119 Some operations are mandatory and some are optional. The mandatory operations
121 * start: this is a pointer to the routine that starts the watchdog timer
123 The routine needs a pointer to the watchdog timer device structure as a
124 parameter. It returns zero on success or a negative errno code for failure.
125 * stop: with this routine the watchdog timer device is being stopped.
126 The routine needs a pointer to the watchdog timer device structure as a
127 parameter. It returns zero on success or a negative errno code for failure.
128 Some watchdog timer hardware can only be started and not be stopped. The
129 driver supporting this hardware needs to make sure that a start and stop
130 routine is being provided. This can be done by using a timer in the driver
131 that regularly sends a keepalive ping to the watchdog timer hardware.
133 Not all watchdog timer hardware supports the same functionality. That's why
134 all other routines/operations are optional. They only need to be provided if
135 they are supported. These optional routines/operations are:
136 * ping: this is the routine that sends a keepalive ping to the watchdog timer
138 The routine needs a pointer to the watchdog timer device structure as a
139 parameter. It returns zero on success or a negative errno code for failure.
140 Most hardware that does not support this as a separate function uses the
141 start function to restart the watchdog timer hardware. And that's also what
142 the watchdog timer driver core does: to send a keepalive ping to the watchdog
143 timer hardware it will either use the ping operation (when available) or the
144 start operation (when the ping operation is not available).
145 (Note: the WDIOC_KEEPALIVE ioctl call will only be active when the
146 WDIOF_KEEPALIVEPING bit has been set in the option field on the watchdog's
148 * status: this routine checks the status of the watchdog timer device. The
149 status of the device is reported with watchdog WDIOF_* status flags/bits.
150 * set_timeout: this routine checks and changes the timeout of the watchdog
151 timer device. It returns 0 on success, -EINVAL for "parameter out of range"
152 and -EIO for "could not write value to the watchdog". On success this
153 routine should set the timeout value of the watchdog_device to the
154 achieved timeout value (which may be different from the requested one
155 because the watchdog does not necessarily has a 1 second resolution).
156 (Note: the WDIOF_SETTIMEOUT needs to be set in the options field of the
157 watchdog's info structure).
158 * get_timeleft: this routines returns the time that's left before a reset.
159 * restart: this routine restarts the machine. It returns 0 on success or a
160 negative errno code for failure.
161 * ioctl: if this routine is present then it will be called first before we do
162 our own internal ioctl call handling. This routine should return -ENOIOCTLCMD
163 if a command is not supported. The parameters that are passed to the ioctl
164 call are: watchdog_device, cmd and arg.
166 The 'ref' and 'unref' operations are no longer used and deprecated.
168 The status bits should (preferably) be set with the set_bit and clear_bit alike
169 bit-operations. The status bits that are defined are:
170 * WDOG_ACTIVE: this status bit indicates whether or not a watchdog timer device
171 is active or not. When the watchdog is active after booting, then you should
172 set this status bit (Note: when you register the watchdog timer device with
173 this bit set, then opening /dev/watchdog will skip the start operation)
174 * WDOG_NO_WAY_OUT: this bit stores the nowayout setting for the watchdog.
175 If this bit is set then the watchdog timer will not be able to stop.
177 To set the WDOG_NO_WAY_OUT status bit (before registering your watchdog
178 timer device) you can either:
179 * set it statically in your watchdog_device struct with
180 .status = WATCHDOG_NOWAYOUT_INIT_STATUS,
181 (this will set the value the same as CONFIG_WATCHDOG_NOWAYOUT) or
182 * use the following helper function:
183 static inline void watchdog_set_nowayout(struct watchdog_device *wdd, int nowayout)
185 Note: The WatchDog Timer Driver Core supports the magic close feature and
186 the nowayout feature. To use the magic close feature you must set the
187 WDIOF_MAGICCLOSE bit in the options field of the watchdog's info structure.
188 The nowayout feature will overrule the magic close feature.
190 To get or set driver specific data the following two helper functions should be
193 static inline void watchdog_set_drvdata(struct watchdog_device *wdd, void *data)
194 static inline void *watchdog_get_drvdata(struct watchdog_device *wdd)
196 The watchdog_set_drvdata function allows you to add driver specific data. The
197 arguments of this function are the watchdog device where you want to add the
198 driver specific data to and a pointer to the data itself.
200 The watchdog_get_drvdata function allows you to retrieve driver specific data.
201 The argument of this function is the watchdog device where you want to retrieve
202 data from. The function returns the pointer to the driver specific data.
204 To initialize the timeout field, the following function can be used:
206 extern int watchdog_init_timeout(struct watchdog_device *wdd,
207 unsigned int timeout_parm, struct device *dev);
209 The watchdog_init_timeout function allows you to initialize the timeout field
210 using the module timeout parameter or by retrieving the timeout-sec property from
211 the device tree (if the module timeout parameter is invalid). Best practice is
212 to set the default timeout value as timeout value in the watchdog_device and
213 then use this function to set the user "preferred" timeout value.
214 This routine returns zero on success and a negative errno code for failure.
216 To disable the watchdog on reboot, the user must call the following helper:
218 static inline void watchdog_stop_on_reboot(struct watchdog_device *wdd);
220 To change the priority of the restart handler the following helper should be
223 void watchdog_set_restart_priority(struct watchdog_device *wdd, int priority);
225 User should follow the following guidelines for setting the priority:
226 * 0: should be called in last resort, has limited restart capabilities
227 * 128: default restart handler, use if no other handler is expected to be
228 available, and/or if restart is sufficient to restart the entire system
229 * 255: highest priority, will preempt all other restart handlers