About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / watchdog / watchdog-kernel-api.txt

Custom Search

Based on kernel version 4.7.2. Page generated on 2016-08-22 22:48 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: Minimum time between heartbeats sent to the chip,
86	  in milli-seconds.
87	* max_hw_heartbeat_ms: Maximum hardware heartbeat, in milli-seconds.
88	  If set, the infrastructure will send heartbeats to the watchdog driver
89	  if 'timeout' is larger than max_hw_heartbeat_ms, unless WDOG_ACTIVE
90	  is set and userspace failed to send a heartbeat for at least 'timeout'
91	  seconds. max_hw_heartbeat_ms must be set if a driver does not implement
92	  the stop function.
93	* reboot_nb: notifier block that is registered for reboot notifications, for
94	  internal use only. If the driver calls watchdog_stop_on_reboot, watchdog core
95	  will stop the watchdog on such notifications.
96	* restart_nb: notifier block that is registered for machine restart, for
97	  internal use only. If a watchdog is capable of restarting the machine, it
98	  should define ops->restart. Priority can be changed through
99	  watchdog_set_restart_priority.
100	* bootstatus: status of the device after booting (reported with watchdog
101	  WDIOF_* status bits).
102	* driver_data: a pointer to the drivers private data of a watchdog device.
103	  This data should only be accessed via the watchdog_set_drvdata and
104	  watchdog_get_drvdata routines.
105	* wd_data: a pointer to watchdog core internal data.
106	* status: this field contains a number of status bits that give extra
107	  information about the status of the device (Like: is the watchdog timer
108	  running/active, or is the nowayout bit set).
109	* deferred: entry in wtd_deferred_reg_list which is used to
110	  register early initialized watchdogs.
112	The list of watchdog operations is defined as:
114	struct watchdog_ops {
115		struct module *owner;
116		/* mandatory operations */
117		int (*start)(struct watchdog_device *);
118		int (*stop)(struct watchdog_device *);
119		/* optional operations */
120		int (*ping)(struct watchdog_device *);
121		unsigned int (*status)(struct watchdog_device *);
122		int (*set_timeout)(struct watchdog_device *, unsigned int);
123		unsigned int (*get_timeleft)(struct watchdog_device *);
124		int (*restart)(struct watchdog_device *);
125		void (*ref)(struct watchdog_device *) __deprecated;
126		void (*unref)(struct watchdog_device *) __deprecated;
127		long (*ioctl)(struct watchdog_device *, unsigned int, unsigned long);
128	};
130	It is important that you first define the module owner of the watchdog timer
131	driver's operations. This module owner will be used to lock the module when
132	the watchdog is active. (This to avoid a system crash when you unload the
133	module and /dev/watchdog is still open).
135	Some operations are mandatory and some are optional. The mandatory operations
136	are:
137	* start: this is a pointer to the routine that starts the watchdog timer
138	  device.
139	  The routine needs a pointer to the watchdog timer device structure as a
140	  parameter. It returns zero on success or a negative errno code for failure.
142	Not all watchdog timer hardware supports the same functionality. That's why
143	all other routines/operations are optional. They only need to be provided if
144	they are supported. These optional routines/operations are:
145	* stop: with this routine the watchdog timer device is being stopped.
146	  The routine needs a pointer to the watchdog timer device structure as a
147	  parameter. It returns zero on success or a negative errno code for failure.
148	  Some watchdog timer hardware can only be started and not be stopped. A
149	  driver supporting such hardware does not have to implement the stop routine.
150	  If a driver has no stop function, the watchdog core will set WDOG_HW_RUNNING
151	  and start calling the driver's keepalive pings function after the watchdog
152	  device is closed.
153	  If a watchdog driver does not implement the stop function, it must set
154	  max_hw_heartbeat_ms.
155	* ping: this is the routine that sends a keepalive ping to the watchdog timer
156	  hardware.
157	  The routine needs a pointer to the watchdog timer device structure as a
158	  parameter. It returns zero on success or a negative errno code for failure.
159	  Most hardware that does not support this as a separate function uses the
160	  start function to restart the watchdog timer hardware. And that's also what
161	  the watchdog timer driver core does: to send a keepalive ping to the watchdog
162	  timer hardware it will either use the ping operation (when available) or the
163	  start operation (when the ping operation is not available).
164	  (Note: the WDIOC_KEEPALIVE ioctl call will only be active when the
165	  WDIOF_KEEPALIVEPING bit has been set in the option field on the watchdog's
166	  info structure).
167	* status: this routine checks the status of the watchdog timer device. The
168	  status of the device is reported with watchdog WDIOF_* status flags/bits.
169	* set_timeout: this routine checks and changes the timeout of the watchdog
170	  timer device. It returns 0 on success, -EINVAL for "parameter out of range"
171	  and -EIO for "could not write value to the watchdog". On success this
172	  routine should set the timeout value of the watchdog_device to the
173	  achieved timeout value (which may be different from the requested one
174	  because the watchdog does not necessarily have a 1 second resolution).
175	  Drivers implementing max_hw_heartbeat_ms set the hardware watchdog heartbeat
176	  to the minimum of timeout and max_hw_heartbeat_ms. Those drivers set the
177	  timeout value of the watchdog_device either to the requested timeout value
178	  (if it is larger than max_hw_heartbeat_ms), or to the achieved timeout value.
179	  (Note: the WDIOF_SETTIMEOUT needs to be set in the options field of the
180	  watchdog's info structure).
181	  If the watchdog driver does not have to perform any action but setting the
182	  watchdog_device.timeout, this callback can be omitted.
183	  If set_timeout is not provided but, WDIOF_SETTIMEOUT is set, the watchdog
184	  infrastructure updates the timeout value of the watchdog_device internally
185	  to the requested value.
186	* get_timeleft: this routines returns the time that's left before a reset.
187	* restart: this routine restarts the machine. It returns 0 on success or a
188	  negative errno code for failure.
189	* ioctl: if this routine is present then it will be called first before we do
190	  our own internal ioctl call handling. This routine should return -ENOIOCTLCMD
191	  if a command is not supported. The parameters that are passed to the ioctl
192	  call are: watchdog_device, cmd and arg.
194	The 'ref' and 'unref' operations are no longer used and deprecated.
196	The status bits should (preferably) be set with the set_bit and clear_bit alike
197	bit-operations. The status bits that are defined are:
198	* WDOG_ACTIVE: this status bit indicates whether or not a watchdog timer device
199	  is active or not from user perspective. User space is expected to send
200	  heartbeat requests to the driver while this flag is set.
201	* WDOG_NO_WAY_OUT: this bit stores the nowayout setting for the watchdog.
202	  If this bit is set then the watchdog timer will not be able to stop.
203	* WDOG_HW_RUNNING: Set by the watchdog driver if the hardware watchdog is
204	  running. The bit must be set if the watchdog timer hardware can not be
205	  stopped. The bit may also be set if the watchdog timer is running after
206	  booting, before the watchdog device is opened. If set, the watchdog
207	  infrastructure will send keepalives to the watchdog hardware while
208	  WDOG_ACTIVE is not set.
209	  Note: when you register the watchdog timer device with this bit set,
210	  then opening /dev/watchdog will skip the start operation but send a keepalive
211	  request instead.
213	  To set the WDOG_NO_WAY_OUT status bit (before registering your watchdog
214	  timer device) you can either:
215	  * set it statically in your watchdog_device struct with
217	    (this will set the value the same as CONFIG_WATCHDOG_NOWAYOUT) or
218	  * use the following helper function:
219	  static inline void watchdog_set_nowayout(struct watchdog_device *wdd, int nowayout)
221	Note: The WatchDog Timer Driver Core supports the magic close feature and
222	the nowayout feature. To use the magic close feature you must set the
223	WDIOF_MAGICCLOSE bit in the options field of the watchdog's info structure.
224	The nowayout feature will overrule the magic close feature.
226	To get or set driver specific data the following two helper functions should be
227	used:
229	static inline void watchdog_set_drvdata(struct watchdog_device *wdd, void *data)
230	static inline void *watchdog_get_drvdata(struct watchdog_device *wdd)
232	The watchdog_set_drvdata function allows you to add driver specific data. The
233	arguments of this function are the watchdog device where you want to add the
234	driver specific data to and a pointer to the data itself.
236	The watchdog_get_drvdata function allows you to retrieve driver specific data.
237	The argument of this function is the watchdog device where you want to retrieve
238	data from. The function returns the pointer to the driver specific data.
240	To initialize the timeout field, the following function can be used:
242	extern int watchdog_init_timeout(struct watchdog_device *wdd,
243	                                  unsigned int timeout_parm, struct device *dev);
245	The watchdog_init_timeout function allows you to initialize the timeout field
246	using the module timeout parameter or by retrieving the timeout-sec property from
247	the device tree (if the module timeout parameter is invalid). Best practice is
248	to set the default timeout value as timeout value in the watchdog_device and
249	then use this function to set the user "preferred" timeout value.
250	This routine returns zero on success and a negative errno code for failure.
252	To disable the watchdog on reboot, the user must call the following helper:
254	static inline void watchdog_stop_on_reboot(struct watchdog_device *wdd);
256	To change the priority of the restart handler the following helper should be
257	used:
259	void watchdog_set_restart_priority(struct watchdog_device *wdd, int priority);
261	User should follow the following guidelines for setting the priority:
262	* 0: should be called in last resort, has limited restart capabilities
263	* 128: default restart handler, use if no other handler is expected to be
264	  available, and/or if restart is sufficient to restart the entire system
265	* 255: highest priority, will preempt all other restart handlers
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.