1The Linux WatchDog Timer Driver Core kernel API.
  2===============================================
  3Last reviewed: 22-Jul-2011
  4
  5Wim Van Sebroeck <wim@iguana.be>
  6
  7Introduction
  8------------
  9This document does not describe what a WatchDog Timer (WDT) Driver or Device is.
 10It also does not describe the API which can be used by user space to communicate
 11with a WatchDog Timer. If you want to know this then please read the following
 12file: Documentation/watchdog/watchdog-api.txt .
 13
 14So what does this document describe? It describes the API that can be used by
 15WatchDog Timer Drivers that want to use the WatchDog Timer Driver Core
 16Framework. This framework provides all interfacing towards user space so that
 17the same code does not have to be reproduced each time. This also means that
 18a watchdog timer driver then only needs to provide the different routines
 19(operations) that control the watchdog timer (WDT).
 20
 21The API
 22-------
 23Each watchdog timer driver that wants to use the WatchDog Timer Driver Core
 24must #include <linux/watchdog.h> (you would have to do this anyway when
 25writing a watchdog device driver). This include file contains following
 26register/unregister routines:
 27
 28extern int watchdog_register_device(struct watchdog_device *);
 29extern void watchdog_unregister_device(struct watchdog_device *);
 30
 31The watchdog_register_device routine registers a watchdog timer device.
 32The parameter of this routine is a pointer to a watchdog_device structure.
 33This routine returns zero on success and a negative errno code for failure.
 34
 35The watchdog_unregister_device routine deregisters a registered watchdog timer
 36device. The parameter of this routine is the pointer to the registered
 37watchdog_device structure.
 38
 39The watchdog device structure looks like this:
 40
 41struct watchdog_device {
 42	const struct watchdog_info *info;
 43	const struct watchdog_ops *ops;
 44	unsigned int bootstatus;
 45	unsigned int timeout;
 46	unsigned int min_timeout;
 47	unsigned int max_timeout;
 48	void *driver_data;
 49	unsigned long status;
 50};
 51
 52It contains following fields:
 53* info: a pointer to a watchdog_info structure. This structure gives some
 54  additional information about the watchdog timer itself. (Like it's unique name)
 55* ops: a pointer to the list of watchdog operations that the watchdog supports.
 56* timeout: the watchdog timer's timeout value (in seconds).
 57* min_timeout: the watchdog timer's minimum timeout value (in seconds).
 58* max_timeout: the watchdog timer's maximum timeout value (in seconds).
 59* bootstatus: status of the device after booting (reported with watchdog
 60  WDIOF_* status bits).
 61* driver_data: a pointer to the drivers private data of a watchdog device.
 62  This data should only be accessed via the watchdog_set_drvadata and
 63  watchdog_get_drvdata routines.
 64* status: this field contains a number of status bits that give extra
 65  information about the status of the device (Like: is the watchdog timer
 66  running/active, is the nowayout bit set, is the device opened via
 67  the /dev/watchdog interface or not, ...).
 68
 69The list of watchdog operations is defined as:
 70
 71struct watchdog_ops {
 72	struct module *owner;
 73	/* mandatory operations */
 74	int (*start)(struct watchdog_device *);
 75	int (*stop)(struct watchdog_device *);
 76	/* optional operations */
 77	int (*ping)(struct watchdog_device *);
 78	unsigned int (*status)(struct watchdog_device *);
 79	int (*set_timeout)(struct watchdog_device *, unsigned int);
 80	long (*ioctl)(struct watchdog_device *, unsigned int, unsigned long);
 81};
 82
 83It is important that you first define the module owner of the watchdog timer
 84driver's operations. This module owner will be used to lock the module when
 85the watchdog is active. (This to avoid a system crash when you unload the
 86module and /dev/watchdog is still open).
 87Some operations are mandatory and some are optional. The mandatory operations
 88are:
 89* start: this is a pointer to the routine that starts the watchdog timer
 90  device.
 91  The routine needs a pointer to the watchdog timer device structure as a
 92  parameter. It returns zero on success or a negative errno code for failure.
 93* stop: with this routine the watchdog timer device is being stopped.
 94  The routine needs a pointer to the watchdog timer device structure as a
 95  parameter. It returns zero on success or a negative errno code for failure.
 96  Some watchdog timer hardware can only be started and not be stopped. The
 97  driver supporting this hardware needs to make sure that a start and stop
 98  routine is being provided. This can be done by using a timer in the driver
 99  that regularly sends a keepalive ping to the watchdog timer hardware.
100
101Not all watchdog timer hardware supports the same functionality. That's why
102all other routines/operations are optional. They only need to be provided if
103they are supported. These optional routines/operations are:
104* ping: this is the routine that sends a keepalive ping to the watchdog timer
105  hardware.
106  The routine needs a pointer to the watchdog timer device structure as a
107  parameter. It returns zero on success or a negative errno code for failure.
108  Most hardware that does not support this as a separate function uses the
109  start function to restart the watchdog timer hardware. And that's also what
110  the watchdog timer driver core does: to send a keepalive ping to the watchdog
111  timer hardware it will either use the ping operation (when available) or the
112  start operation (when the ping operation is not available).
113  (Note: the WDIOC_KEEPALIVE ioctl call will only be active when the
114  WDIOF_KEEPALIVEPING bit has been set in the option field on the watchdog's
115  info structure).
116* status: this routine checks the status of the watchdog timer device. The
117  status of the device is reported with watchdog WDIOF_* status flags/bits.
118* set_timeout: this routine checks and changes the timeout of the watchdog
119  timer device. It returns 0 on success, -EINVAL for "parameter out of range"
120  and -EIO for "could not write value to the watchdog". On success the timeout
121  value of the watchdog_device will be changed to the value that was just used
122  to re-program the watchdog timer device.
123  (Note: the WDIOF_SETTIMEOUT needs to be set in the options field of the
124  watchdog's info structure).
125* ioctl: if this routine is present then it will be called first before we do
126  our own internal ioctl call handling. This routine should return -ENOIOCTLCMD
127  if a command is not supported. The parameters that are passed to the ioctl
128  call are: watchdog_device, cmd and arg.
129
130The status bits should (preferably) be set with the set_bit and clear_bit alike
131bit-operations. The status bits that are defined are:
132* WDOG_ACTIVE: this status bit indicates whether or not a watchdog timer device
133  is active or not. When the watchdog is active after booting, then you should
134  set this status bit (Note: when you register the watchdog timer device with
135  this bit set, then opening /dev/watchdog will skip the start operation)
136* WDOG_DEV_OPEN: this status bit shows whether or not the watchdog device
137  was opened via /dev/watchdog.
138  (This bit should only be used by the WatchDog Timer Driver Core).
139* WDOG_ALLOW_RELEASE: this bit stores whether or not the magic close character
140  has been sent (so that we can support the magic close feature).
141  (This bit should only be used by the WatchDog Timer Driver Core).
142* WDOG_NO_WAY_OUT: this bit stores the nowayout setting for the watchdog.
143  If this bit is set then the watchdog timer will not be able to stop.
144
145Note: The WatchDog Timer Driver Core supports the magic close feature and
146the nowayout feature. To use the magic close feature you must set the
147WDIOF_MAGICCLOSE bit in the options field of the watchdog's info structure.
148The nowayout feature will overrule the magic close feature.
149
150To get or set driver specific data the following two helper functions should be
151used:
152
153static inline void watchdog_set_drvdata(struct watchdog_device *wdd, void *data)
154static inline void *watchdog_get_drvdata(struct watchdog_device *wdd)
155
156The watchdog_set_drvdata function allows you to add driver specific data. The
157arguments of this function are the watchdog device where you want to add the
158driver specific data to and a pointer to the data itself.
159
160The watchdog_get_drvdata function allows you to retrieve driver specific data.
161The argument of this function is the watchdog device where you want to retrieve
162data from. The function retruns the pointer to the driver specific data.