1 The Linux WatchDog Timer Driver Core kernel API.
2 ===============================================
3 Last reviewed: 21-May-2012
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 device structure looks like this:
41 struct watchdog_device {
44 const struct watchdog_info *info;
45 const struct watchdog_ops *ops;
46 unsigned int bootstatus;
48 unsigned int min_timeout;
49 unsigned int max_timeout;
54 It contains following fields:
55 * id: set by watchdog_register_device, id 0 is special. It has both a
56 /dev/watchdog0 cdev (dynamic major, minor 0) as well as the old
57 /dev/watchdog miscdev. The id is set automatically when calling
58 watchdog_register_device.
59 * cdev: cdev for the dynamic /dev/watchdog<id> device nodes. This
60 field is also populated by watchdog_register_device.
61 * info: a pointer to a watchdog_info structure. This structure gives some
62 additional information about the watchdog timer itself. (Like it's unique name)
63 * ops: a pointer to the list of watchdog operations that the watchdog supports.
64 * timeout: the watchdog timer's timeout value (in seconds).
65 * min_timeout: the watchdog timer's minimum timeout value (in seconds).
66 * max_timeout: the watchdog timer's maximum timeout value (in seconds).
67 * bootstatus: status of the device after booting (reported with watchdog
69 * driver_data: a pointer to the drivers private data of a watchdog device.
70 This data should only be accessed via the watchdog_set_drvdata and
71 watchdog_get_drvdata routines.
72 * status: this field contains a number of status bits that give extra
73 information about the status of the device (Like: is the watchdog timer
74 running/active, is the nowayout bit set, is the device opened via
75 the /dev/watchdog interface or not, ...).
77 The list of watchdog operations is defined as:
81 /* mandatory operations */
82 int (*start)(struct watchdog_device *);
83 int (*stop)(struct watchdog_device *);
84 /* optional operations */
85 int (*ping)(struct watchdog_device *);
86 unsigned int (*status)(struct watchdog_device *);
87 int (*set_timeout)(struct watchdog_device *, unsigned int);
88 unsigned int (*get_timeleft)(struct watchdog_device *);
89 long (*ioctl)(struct watchdog_device *, unsigned int, unsigned long);
92 It is important that you first define the module owner of the watchdog timer
93 driver's operations. This module owner will be used to lock the module when
94 the watchdog is active. (This to avoid a system crash when you unload the
95 module and /dev/watchdog is still open).
96 Some operations are mandatory and some are optional. The mandatory operations
98 * start: this is a pointer to the routine that starts the watchdog timer
100 The routine needs a pointer to the watchdog timer device structure as a
101 parameter. It returns zero on success or a negative errno code for failure.
102 * stop: with this routine the watchdog timer device is being stopped.
103 The routine needs a pointer to the watchdog timer device structure as a
104 parameter. It returns zero on success or a negative errno code for failure.
105 Some watchdog timer hardware can only be started and not be stopped. The
106 driver supporting this hardware needs to make sure that a start and stop
107 routine is being provided. This can be done by using a timer in the driver
108 that regularly sends a keepalive ping to the watchdog timer hardware.
110 Not all watchdog timer hardware supports the same functionality. That's why
111 all other routines/operations are optional. They only need to be provided if
112 they are supported. These optional routines/operations are:
113 * ping: this is the routine that sends a keepalive ping to the watchdog timer
115 The routine needs a pointer to the watchdog timer device structure as a
116 parameter. It returns zero on success or a negative errno code for failure.
117 Most hardware that does not support this as a separate function uses the
118 start function to restart the watchdog timer hardware. And that's also what
119 the watchdog timer driver core does: to send a keepalive ping to the watchdog
120 timer hardware it will either use the ping operation (when available) or the
121 start operation (when the ping operation is not available).
122 (Note: the WDIOC_KEEPALIVE ioctl call will only be active when the
123 WDIOF_KEEPALIVEPING bit has been set in the option field on the watchdog's
125 * status: this routine checks the status of the watchdog timer device. The
126 status of the device is reported with watchdog WDIOF_* status flags/bits.
127 * set_timeout: this routine checks and changes the timeout of the watchdog
128 timer device. It returns 0 on success, -EINVAL for "parameter out of range"
129 and -EIO for "could not write value to the watchdog". On success this
130 routine should set the timeout value of the watchdog_device to the
131 achieved timeout value (which may be different from the requested one
132 because the watchdog does not necessarily has a 1 second resolution).
133 (Note: the WDIOF_SETTIMEOUT needs to be set in the options field of the
134 watchdog's info structure).
135 * get_timeleft: this routines returns the time that's left before a reset.
136 * ioctl: if this routine is present then it will be called first before we do
137 our own internal ioctl call handling. This routine should return -ENOIOCTLCMD
138 if a command is not supported. The parameters that are passed to the ioctl
139 call are: watchdog_device, cmd and arg.
141 The status bits should (preferably) be set with the set_bit and clear_bit alike
142 bit-operations. The status bits that are defined are:
143 * WDOG_ACTIVE: this status bit indicates whether or not a watchdog timer device
144 is active or not. When the watchdog is active after booting, then you should
145 set this status bit (Note: when you register the watchdog timer device with
146 this bit set, then opening /dev/watchdog will skip the start operation)
147 * WDOG_DEV_OPEN: this status bit shows whether or not the watchdog device
148 was opened via /dev/watchdog.
149 (This bit should only be used by the WatchDog Timer Driver Core).
150 * WDOG_ALLOW_RELEASE: this bit stores whether or not the magic close character
151 has been sent (so that we can support the magic close feature).
152 (This bit should only be used by the WatchDog Timer Driver Core).
153 * WDOG_NO_WAY_OUT: this bit stores the nowayout setting for the watchdog.
154 If this bit is set then the watchdog timer will not be able to stop.
156 To set the WDOG_NO_WAY_OUT status bit (before registering your watchdog
157 timer device) you can either:
158 * set it statically in your watchdog_device struct with
159 .status = WATCHDOG_NOWAYOUT_INIT_STATUS,
160 (this will set the value the same as CONFIG_WATCHDOG_NOWAYOUT) or
161 * use the following helper function:
162 static inline void watchdog_set_nowayout(struct watchdog_device *wdd, int nowayout)
164 Note: The WatchDog Timer Driver Core supports the magic close feature and
165 the nowayout feature. To use the magic close feature you must set the
166 WDIOF_MAGICCLOSE bit in the options field of the watchdog's info structure.
167 The nowayout feature will overrule the magic close feature.
169 To get or set driver specific data the following two helper functions should be
172 static inline void watchdog_set_drvdata(struct watchdog_device *wdd, void *data)
173 static inline void *watchdog_get_drvdata(struct watchdog_device *wdd)
175 The watchdog_set_drvdata function allows you to add driver specific data. The
176 arguments of this function are the watchdog device where you want to add the
177 driver specific data to and a pointer to the data itself.
179 The watchdog_get_drvdata function allows you to retrieve driver specific data.
180 The argument of this function is the watchdog device where you want to retrieve
181 data from. The function returns the pointer to the driver specific data.