1The Linux WatchDog Timer Driver Core kernel API.
   3Last reviewed: 22-May-2012
   5Wim Van Sebroeck <>
   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 .
  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).
  21The API
  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:
  28extern int watchdog_register_device(struct watchdog_device *);
  29extern void watchdog_unregister_device(struct watchdog_device *);
  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.
  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.
  39The watchdog device structure looks like this:
  41struct 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;
  57It 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, ...).
  84The list of watchdog operations is defined as:
  86struct 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);
 101It is important that you first define the module owner of the watchdog timer
 102driver's operations. This module owner will be used to lock the module when
 103the watchdog is active. (This to avoid a system crash when you unload the
 104module and /dev/watchdog is still open).
 106If the watchdog_device struct is dynamically allocated, just locking the module
 107is not enough and a driver also needs to define the ref and unref operations to
 108ensure the structure holding the watchdog_device does not go away.
 110The simplest (and usually sufficient) implementation of this is to:
 1111) Add a kref struct to the same structure which is holding the watchdog_device
 1122) Define a release callback for the kref which frees the struct holding both
 1133) Call kref_init on this kref *before* calling watchdog_register_device()
 1144) Define a ref operation calling kref_get on this kref
 1155) Define a unref operation calling kref_put on this kref
 1166) 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
 120Some operations are mandatory and some are optional. The mandatory operations
 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.
 134Not all watchdog timer hardware supports the same functionality. That's why
 135all other routines/operations are optional. They only need to be provided if
 136they 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.
 169The status bits should (preferably) be set with the set_bit and clear_bit alike
 170bit-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
 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)
 197Note: The WatchDog Timer Driver Core supports the magic close feature and
 198the nowayout feature. To use the magic close feature you must set the
 199WDIOF_MAGICCLOSE bit in the options field of the watchdog's info structure.
 200The nowayout feature will overrule the magic close feature.
 202To get or set driver specific data the following two helper functions should be
 205static inline void watchdog_set_drvdata(struct watchdog_device *wdd, void *data)
 206static inline void *watchdog_get_drvdata(struct watchdog_device *wdd)
 208The watchdog_set_drvdata function allows you to add driver specific data. The
 209arguments of this function are the watchdog device where you want to add the
 210driver specific data to and a pointer to the data itself.
 212The watchdog_get_drvdata function allows you to retrieve driver specific data.
 213The argument of this function is the watchdog device where you want to retrieve
 214data from. The function returns the pointer to the driver specific data.
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