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Based on kernel version 3.16. Page generated on 2014-08-06 21:41 EST.

1	The Linux WatchDog Timer Driver Core kernel API.
2	===============================================
3	Last reviewed: 12-Feb-2013
4	
5	Wim Van Sebroeck <wim@iguana.be>
6	
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 .
13	
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).
20	
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:
27	
28	extern int watchdog_register_device(struct watchdog_device *);
29	extern void watchdog_unregister_device(struct watchdog_device *);
30	
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.
34	
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.
38	
39	The watchdog device structure looks like this:
40	
41	struct watchdog_device {
42		int id;
43		struct cdev cdev;
44		struct device *dev;
45		struct device *parent;
46		const struct watchdog_info *info;
47		const struct watchdog_ops *ops;
48		unsigned int bootstatus;
49		unsigned int timeout;
50		unsigned int min_timeout;
51		unsigned int max_timeout;
52		void *driver_data;
53		struct mutex lock;
54		unsigned long status;
55	};
56	
57	It contains following fields:
58	* id: set by watchdog_register_device, id 0 is special. It has both a
59	  /dev/watchdog0 cdev (dynamic major, minor 0) as well as the old
60	  /dev/watchdog miscdev. The id is set automatically when calling
61	  watchdog_register_device.
62	* cdev: cdev for the dynamic /dev/watchdog<id> device nodes. This
63	  field is also populated by watchdog_register_device.
64	* dev: device under the watchdog class (created by watchdog_register_device).
65	* parent: set this to the parent device (or NULL) before calling
66	  watchdog_register_device.
67	* info: a pointer to a watchdog_info structure. This structure gives some
68	  additional information about the watchdog timer itself. (Like it's unique name)
69	* ops: a pointer to the list of watchdog operations that the watchdog supports.
70	* timeout: the watchdog timer's timeout value (in seconds).
71	* min_timeout: the watchdog timer's minimum timeout value (in seconds).
72	* max_timeout: the watchdog timer's maximum timeout value (in seconds).
73	* bootstatus: status of the device after booting (reported with watchdog
74	  WDIOF_* status bits).
75	* driver_data: a pointer to the drivers private data of a watchdog device.
76	  This data should only be accessed via the watchdog_set_drvdata and
77	  watchdog_get_drvdata routines.
78	* lock: Mutex for WatchDog Timer Driver Core internal use only.
79	* status: this field contains a number of status bits that give extra
80	  information about the status of the device (Like: is the watchdog timer
81	  running/active, is the nowayout bit set, is the device opened via
82	  the /dev/watchdog interface or not, ...).
83	
84	The list of watchdog operations is defined as:
85	
86	struct watchdog_ops {
87		struct module *owner;
88		/* mandatory operations */
89		int (*start)(struct watchdog_device *);
90		int (*stop)(struct watchdog_device *);
91		/* optional operations */
92		int (*ping)(struct watchdog_device *);
93		unsigned int (*status)(struct watchdog_device *);
94		int (*set_timeout)(struct watchdog_device *, unsigned int);
95		unsigned int (*get_timeleft)(struct watchdog_device *);
96		void (*ref)(struct watchdog_device *);
97		void (*unref)(struct watchdog_device *);
98		long (*ioctl)(struct watchdog_device *, unsigned int, unsigned long);
99	};
100	
101	It is important that you first define the module owner of the watchdog timer
102	driver's operations. This module owner will be used to lock the module when
103	the watchdog is active. (This to avoid a system crash when you unload the
104	module and /dev/watchdog is still open).
105	
106	If the watchdog_device struct is dynamically allocated, just locking the module
107	is not enough and a driver also needs to define the ref and unref operations to
108	ensure the structure holding the watchdog_device does not go away.
109	
110	The simplest (and usually sufficient) implementation of this is to:
111	1) Add a kref struct to the same structure which is holding the watchdog_device
112	2) Define a release callback for the kref which frees the struct holding both
113	3) Call kref_init on this kref *before* calling watchdog_register_device()
114	4) Define a ref operation calling kref_get on this kref
115	5) Define a unref operation calling kref_put on this kref
116	6) When it is time to cleanup:
117	 * Do not kfree() the struct holding both, the last kref_put will do this!
118	 * *After* calling watchdog_unregister_device() call kref_put on the kref
119	
120	Some operations are mandatory and some are optional. The mandatory operations
121	are:
122	* start: this is a pointer to the routine that starts the watchdog timer
123	  device.
124	  The routine needs a pointer to the watchdog timer device structure as a
125	  parameter. It returns zero on success or a negative errno code for failure.
126	* stop: with this routine the watchdog timer device is being stopped.
127	  The routine needs a pointer to the watchdog timer device structure as a
128	  parameter. It returns zero on success or a negative errno code for failure.
129	  Some watchdog timer hardware can only be started and not be stopped. The
130	  driver supporting this hardware needs to make sure that a start and stop
131	  routine is being provided. This can be done by using a timer in the driver
132	  that regularly sends a keepalive ping to the watchdog timer hardware.
133	
134	Not all watchdog timer hardware supports the same functionality. That's why
135	all other routines/operations are optional. They only need to be provided if
136	they are supported. These optional routines/operations are:
137	* ping: this is the routine that sends a keepalive ping to the watchdog timer
138	  hardware.
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.
141	  Most hardware that does not support this as a separate function uses the
142	  start function to restart the watchdog timer hardware. And that's also what
143	  the watchdog timer driver core does: to send a keepalive ping to the watchdog
144	  timer hardware it will either use the ping operation (when available) or the
145	  start operation (when the ping operation is not available).
146	  (Note: the WDIOC_KEEPALIVE ioctl call will only be active when the
147	  WDIOF_KEEPALIVEPING bit has been set in the option field on the watchdog's
148	  info structure).
149	* status: this routine checks the status of the watchdog timer device. The
150	  status of the device is reported with watchdog WDIOF_* status flags/bits.
151	* set_timeout: this routine checks and changes the timeout of the watchdog
152	  timer device. It returns 0 on success, -EINVAL for "parameter out of range"
153	  and -EIO for "could not write value to the watchdog". On success this
154	  routine should set the timeout value of the watchdog_device to the
155	  achieved timeout value (which may be different from the requested one
156	  because the watchdog does not necessarily has a 1 second resolution).
157	  (Note: the WDIOF_SETTIMEOUT needs to be set in the options field of the
158	  watchdog's info structure).
159	* get_timeleft: this routines returns the time that's left before a reset.
160	* ref: the operation that calls kref_get on the kref of a dynamically
161	  allocated watchdog_device struct.
162	* unref: the operation that calls kref_put on the kref of a dynamically
163	  allocated watchdog_device struct.
164	* ioctl: if this routine is present then it will be called first before we do
165	  our own internal ioctl call handling. This routine should return -ENOIOCTLCMD
166	  if a command is not supported. The parameters that are passed to the ioctl
167	  call are: watchdog_device, cmd and arg.
168	
169	The status bits should (preferably) be set with the set_bit and clear_bit alike
170	bit-operations. The status bits that are defined are:
171	* WDOG_ACTIVE: this status bit indicates whether or not a watchdog timer device
172	  is active or not. When the watchdog is active after booting, then you should
173	  set this status bit (Note: when you register the watchdog timer device with
174	  this bit set, then opening /dev/watchdog will skip the start operation)
175	* WDOG_DEV_OPEN: this status bit shows whether or not the watchdog device
176	  was opened via /dev/watchdog.
177	  (This bit should only be used by the WatchDog Timer Driver Core).
178	* WDOG_ALLOW_RELEASE: this bit stores whether or not the magic close character
179	  has been sent (so that we can support the magic close feature).
180	  (This bit should only be used by the WatchDog Timer Driver Core).
181	* WDOG_NO_WAY_OUT: this bit stores the nowayout setting for the watchdog.
182	  If this bit is set then the watchdog timer will not be able to stop.
183	* WDOG_UNREGISTERED: this bit gets set by the WatchDog Timer Driver Core
184	  after calling watchdog_unregister_device, and then checked before calling
185	  any watchdog_ops, so that you can be sure that no operations (other then
186	  unref) will get called after unregister, even if userspace still holds a
187	  reference to /dev/watchdog
188	
189	  To set the WDOG_NO_WAY_OUT status bit (before registering your watchdog
190	  timer device) you can either:
191	  * set it statically in your watchdog_device struct with
192		.status = WATCHDOG_NOWAYOUT_INIT_STATUS,
193	    (this will set the value the same as CONFIG_WATCHDOG_NOWAYOUT) or
194	  * use the following helper function:
195	  static inline void watchdog_set_nowayout(struct watchdog_device *wdd, int nowayout)
196	
197	Note: The WatchDog Timer Driver Core supports the magic close feature and
198	the nowayout feature. To use the magic close feature you must set the
199	WDIOF_MAGICCLOSE bit in the options field of the watchdog's info structure.
200	The nowayout feature will overrule the magic close feature.
201	
202	To get or set driver specific data the following two helper functions should be
203	used:
204	
205	static inline void watchdog_set_drvdata(struct watchdog_device *wdd, void *data)
206	static inline void *watchdog_get_drvdata(struct watchdog_device *wdd)
207	
208	The watchdog_set_drvdata function allows you to add driver specific data. The
209	arguments of this function are the watchdog device where you want to add the
210	driver specific data to and a pointer to the data itself.
211	
212	The watchdog_get_drvdata function allows you to retrieve driver specific data.
213	The argument of this function is the watchdog device where you want to retrieve
214	data from. The function returns the pointer to the driver specific data.
215	
216	To initialize the timeout field, the following function can be used:
217	
218	extern int watchdog_init_timeout(struct watchdog_device *wdd,
219	                                  unsigned int timeout_parm, struct device *dev);
220	
221	The watchdog_init_timeout function allows you to initialize the timeout field
222	using the module timeout parameter or by retrieving the timeout-sec property from
223	the device tree (if the module timeout parameter is invalid). Best practice is
224	to set the default timeout value as timeout value in the watchdog_device and
225	then use this function to set the user "preferred" timeout value.
226	This routine returns zero on success and a negative errno code for failure.
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