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Based on kernel version 4.8. Page generated on 2016-10-06 23:19 EST.

1	The Linux WatchDog Timer Driver Core kernel API.
2	===============================================
3	Last reviewed: 12-Feb-2013
5	Wim Van Sebroeck <wim@iguana.be>
7	Introduction
8	------------
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).
21	The API
22	-------
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
41	the boot process.
43	The watchdog device structure looks like this:
45	struct watchdog_device {
46		int id;
47		struct device *parent;
48		const struct attribute_group **groups;
49		const struct watchdog_info *info;
50		const struct watchdog_ops *ops;
51		unsigned int bootstatus;
52		unsigned int timeout;
53		unsigned int min_timeout;
54		unsigned int max_timeout;
55		unsigned int min_hw_heartbeat_ms;
56		unsigned int max_hw_heartbeat_ms;
57		struct notifier_block reboot_nb;
58		struct notifier_block restart_nb;
59		void *driver_data;
60		struct watchdog_core_data *wd_data;
61		unsigned long status;
62		struct list_head deferred;
63	};
65	It contains following fields:
66	* id: set by watchdog_register_device, id 0 is special. It has both a
67	  /dev/watchdog0 cdev (dynamic major, minor 0) as well as the old
68	  /dev/watchdog miscdev. The id is set automatically when calling
69	  watchdog_register_device.
70	* parent: set this to the parent device (or NULL) before calling
71	  watchdog_register_device.
72	* groups: List of sysfs attribute groups to create when creating the watchdog
73	  device.
74	* info: a pointer to a watchdog_info structure. This structure gives some
75	  additional information about the watchdog timer itself. (Like it's unique name)
76	* ops: a pointer to the list of watchdog operations that the watchdog supports.
77	* timeout: the watchdog timer's timeout value (in seconds).
78	  This is the time after which the system will reboot if user space does
79	  not send a heartbeat request if WDOG_ACTIVE is set.
80	* min_timeout: the watchdog timer's minimum timeout value (in seconds).
81	  If set, the minimum configurable value for 'timeout'.
82	* max_timeout: the watchdog timer's maximum timeout value (in seconds),
83	  as seen from userspace. If set, the maximum configurable value for
84	  'timeout'. Not used if max_hw_heartbeat_ms is non-zero.
85	* min_hw_heartbeat_ms: Hardware limit for minimum time between heartbeats,
86	  in milli-seconds. This value is normally 0; it should only be provided
87	  if the hardware can not tolerate lower intervals between heartbeats.
88	* max_hw_heartbeat_ms: Maximum hardware heartbeat, in milli-seconds.
89	  If set, the infrastructure will send heartbeats to the watchdog driver
90	  if 'timeout' is larger than max_hw_heartbeat_ms, unless WDOG_ACTIVE
91	  is set and userspace failed to send a heartbeat for at least 'timeout'
92	  seconds. max_hw_heartbeat_ms must be set if a driver does not implement
93	  the stop function.
94	* reboot_nb: notifier block that is registered for reboot notifications, for
95	  internal use only. If the driver calls watchdog_stop_on_reboot, watchdog core
96	  will stop the watchdog on such notifications.
97	* restart_nb: notifier block that is registered for machine restart, for
98	  internal use only. If a watchdog is capable of restarting the machine, it
99	  should define ops->restart. Priority can be changed through
100	  watchdog_set_restart_priority.
101	* bootstatus: status of the device after booting (reported with watchdog
102	  WDIOF_* status bits).
103	* driver_data: a pointer to the drivers private data of a watchdog device.
104	  This data should only be accessed via the watchdog_set_drvdata and
105	  watchdog_get_drvdata routines.
106	* wd_data: a pointer to watchdog core internal data.
107	* status: this field contains a number of status bits that give extra
108	  information about the status of the device (Like: is the watchdog timer
109	  running/active, or is the nowayout bit set).
110	* deferred: entry in wtd_deferred_reg_list which is used to
111	  register early initialized watchdogs.
113	The list of watchdog operations is defined as:
115	struct watchdog_ops {
116		struct module *owner;
117		/* mandatory operations */
118		int (*start)(struct watchdog_device *);
119		int (*stop)(struct watchdog_device *);
120		/* optional operations */
121		int (*ping)(struct watchdog_device *);
122		unsigned int (*status)(struct watchdog_device *);
123		int (*set_timeout)(struct watchdog_device *, unsigned int);
124		unsigned int (*get_timeleft)(struct watchdog_device *);
125		int (*restart)(struct watchdog_device *);
126		void (*ref)(struct watchdog_device *) __deprecated;
127		void (*unref)(struct watchdog_device *) __deprecated;
128		long (*ioctl)(struct watchdog_device *, unsigned int, unsigned long);
129	};
131	It is important that you first define the module owner of the watchdog timer
132	driver's operations. This module owner will be used to lock the module when
133	the watchdog is active. (This to avoid a system crash when you unload the
134	module and /dev/watchdog is still open).
136	Some operations are mandatory and some are optional. The mandatory operations
137	are:
138	* start: this is a pointer to the routine that starts the watchdog timer
139	  device.
140	  The routine needs a pointer to the watchdog timer device structure as a
141	  parameter. It returns zero on success or a negative errno code for failure.
143	Not all watchdog timer hardware supports the same functionality. That's why
144	all other routines/operations are optional. They only need to be provided if
145	they are supported. These optional routines/operations are:
146	* stop: with this routine the watchdog timer device is being stopped.
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	  Some watchdog timer hardware can only be started and not be stopped. A
150	  driver supporting such hardware does not have to implement the stop routine.
151	  If a driver has no stop function, the watchdog core will set WDOG_HW_RUNNING
152	  and start calling the driver's keepalive pings function after the watchdog
153	  device is closed.
154	  If a watchdog driver does not implement the stop function, it must set
155	  max_hw_heartbeat_ms.
156	* ping: this is the routine that sends a keepalive ping to the watchdog timer
157	  hardware.
158	  The routine needs a pointer to the watchdog timer device structure as a
159	  parameter. It returns zero on success or a negative errno code for failure.
160	  Most hardware that does not support this as a separate function uses the
161	  start function to restart the watchdog timer hardware. And that's also what
162	  the watchdog timer driver core does: to send a keepalive ping to the watchdog
163	  timer hardware it will either use the ping operation (when available) or the
164	  start operation (when the ping operation is not available).
165	  (Note: the WDIOC_KEEPALIVE ioctl call will only be active when the
166	  WDIOF_KEEPALIVEPING bit has been set in the option field on the watchdog's
167	  info structure).
168	* status: this routine checks the status of the watchdog timer device. The
169	  status of the device is reported with watchdog WDIOF_* status flags/bits.
170	  WDIOF_MAGICCLOSE and WDIOF_KEEPALIVEPING are reported by the watchdog core;
171	  it is not necessary to report those bits from the driver. Also, if no status
172	  function is provided by the driver, the watchdog core reports the status bits
173	  provided in the bootstatus variable of struct watchdog_device.
174	* set_timeout: this routine checks and changes the timeout of the watchdog
175	  timer device. It returns 0 on success, -EINVAL for "parameter out of range"
176	  and -EIO for "could not write value to the watchdog". On success this
177	  routine should set the timeout value of the watchdog_device to the
178	  achieved timeout value (which may be different from the requested one
179	  because the watchdog does not necessarily have a 1 second resolution).
180	  Drivers implementing max_hw_heartbeat_ms set the hardware watchdog heartbeat
181	  to the minimum of timeout and max_hw_heartbeat_ms. Those drivers set the
182	  timeout value of the watchdog_device either to the requested timeout value
183	  (if it is larger than max_hw_heartbeat_ms), or to the achieved timeout value.
184	  (Note: the WDIOF_SETTIMEOUT needs to be set in the options field of the
185	  watchdog's info structure).
186	  If the watchdog driver does not have to perform any action but setting the
187	  watchdog_device.timeout, this callback can be omitted.
188	  If set_timeout is not provided but, WDIOF_SETTIMEOUT is set, the watchdog
189	  infrastructure updates the timeout value of the watchdog_device internally
190	  to the requested value.
191	* get_timeleft: this routines returns the time that's left before a reset.
192	* restart: this routine restarts the machine. It returns 0 on success or a
193	  negative errno code for failure.
194	* ioctl: if this routine is present then it will be called first before we do
195	  our own internal ioctl call handling. This routine should return -ENOIOCTLCMD
196	  if a command is not supported. The parameters that are passed to the ioctl
197	  call are: watchdog_device, cmd and arg.
199	The 'ref' and 'unref' operations are no longer used and deprecated.
201	The status bits should (preferably) be set with the set_bit and clear_bit alike
202	bit-operations. The status bits that are defined are:
203	* WDOG_ACTIVE: this status bit indicates whether or not a watchdog timer device
204	  is active or not from user perspective. User space is expected to send
205	  heartbeat requests to the driver while this flag is set.
206	* WDOG_NO_WAY_OUT: this bit stores the nowayout setting for the watchdog.
207	  If this bit is set then the watchdog timer will not be able to stop.
208	* WDOG_HW_RUNNING: Set by the watchdog driver if the hardware watchdog is
209	  running. The bit must be set if the watchdog timer hardware can not be
210	  stopped. The bit may also be set if the watchdog timer is running after
211	  booting, before the watchdog device is opened. If set, the watchdog
212	  infrastructure will send keepalives to the watchdog hardware while
213	  WDOG_ACTIVE is not set.
214	  Note: when you register the watchdog timer device with this bit set,
215	  then opening /dev/watchdog will skip the start operation but send a keepalive
216	  request instead.
218	  To set the WDOG_NO_WAY_OUT status bit (before registering your watchdog
219	  timer device) you can either:
220	  * set it statically in your watchdog_device struct with
222	    (this will set the value the same as CONFIG_WATCHDOG_NOWAYOUT) or
223	  * use the following helper function:
224	  static inline void watchdog_set_nowayout(struct watchdog_device *wdd, int nowayout)
226	Note: The WatchDog Timer Driver Core supports the magic close feature and
227	the nowayout feature. To use the magic close feature you must set the
228	WDIOF_MAGICCLOSE bit in the options field of the watchdog's info structure.
229	The nowayout feature will overrule the magic close feature.
231	To get or set driver specific data the following two helper functions should be
232	used:
234	static inline void watchdog_set_drvdata(struct watchdog_device *wdd, void *data)
235	static inline void *watchdog_get_drvdata(struct watchdog_device *wdd)
237	The watchdog_set_drvdata function allows you to add driver specific data. The
238	arguments of this function are the watchdog device where you want to add the
239	driver specific data to and a pointer to the data itself.
241	The watchdog_get_drvdata function allows you to retrieve driver specific data.
242	The argument of this function is the watchdog device where you want to retrieve
243	data from. The function returns the pointer to the driver specific data.
245	To initialize the timeout field, the following function can be used:
247	extern int watchdog_init_timeout(struct watchdog_device *wdd,
248	                                  unsigned int timeout_parm, struct device *dev);
250	The watchdog_init_timeout function allows you to initialize the timeout field
251	using the module timeout parameter or by retrieving the timeout-sec property from
252	the device tree (if the module timeout parameter is invalid). Best practice is
253	to set the default timeout value as timeout value in the watchdog_device and
254	then use this function to set the user "preferred" timeout value.
255	This routine returns zero on success and a negative errno code for failure.
257	To disable the watchdog on reboot, the user must call the following helper:
259	static inline void watchdog_stop_on_reboot(struct watchdog_device *wdd);
261	To change the priority of the restart handler the following helper should be
262	used:
264	void watchdog_set_restart_priority(struct watchdog_device *wdd, int priority);
266	User should follow the following guidelines for setting the priority:
267	* 0: should be called in last resort, has limited restart capabilities
268	* 128: default restart handler, use if no other handler is expected to be
269	  available, and/or if restart is sufficient to restart the entire system
270	* 255: highest priority, will preempt all other restart handlers
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