Documentation/watchdog/watchdog-kernel-api.txt

   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 subsystem includes an registration deferral mechanism,
  40 which allows you to register an watchdog as early as you wish during
  41 the boot process.
  42
  43 The watchdog device structure looks like this:
  44
  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         struct notifier_block reboot_nb;
  56         struct notifier_block restart_nb;
  57         void *driver_data;
  58         struct watchdog_core_data *wd_data;
  59         unsigned long status;
  60         struct list_head deferred;
  61 };
  62
  63 It contains following fields:
  64 * id: set by watchdog_register_device, id 0 is special. It has both a
  65   /dev/watchdog0 cdev (dynamic major, minor 0) as well as the old
  66   /dev/watchdog miscdev. The id is set automatically when calling
  67   watchdog_register_device.
  68 * parent: set this to the parent device (or NULL) before calling
  69   watchdog_register_device.
  70 * groups: List of sysfs attribute groups to create when creating the watchdog
  71   device.
  72 * info: a pointer to a watchdog_info structure. This structure gives some
  73   additional information about the watchdog timer itself. (Like it's unique name)
  74 * ops: a pointer to the list of watchdog operations that the watchdog supports.
  75 * timeout: the watchdog timer's timeout value (in seconds).
  76 * min_timeout: the watchdog timer's minimum timeout value (in seconds).
  77 * max_timeout: the watchdog timer's maximum timeout value (in seconds).
  78 * reboot_nb: notifier block that is registered for reboot notifications, for
  79   internal use only. If the driver calls watchdog_stop_on_reboot, watchdog core
  80   will stop the watchdog on such notifications.
  81 * restart_nb: notifier block that is registered for machine restart, for
  82   internal use only. If a watchdog is capable of restarting the machine, it
  83   should define ops->restart. Priority can be changed through
  84   watchdog_set_restart_priority.
  85 * bootstatus: status of the device after booting (reported with watchdog
  86   WDIOF_* status bits).
  87 * driver_data: a pointer to the drivers private data of a watchdog device.
  88   This data should only be accessed via the watchdog_set_drvdata and
  89   watchdog_get_drvdata routines.
  90 * wd_data: a pointer to watchdog core internal data.
  91 * status: this field contains a number of status bits that give extra
  92   information about the status of the device (Like: is the watchdog timer
  93   running/active, or is the nowayout bit set).
  94 * deferred: entry in wtd_deferred_reg_list which is used to
  95   register early initialized watchdogs.
  96
  97 The list of watchdog operations is defined as:
  98
  99 struct watchdog_ops {
 100         struct module *owner;
 101         /* mandatory operations */
 102         int (*start)(struct watchdog_device *);
 103         int (*stop)(struct watchdog_device *);
 104         /* optional operations */
 105         int (*ping)(struct watchdog_device *);
 106         unsigned int (*status)(struct watchdog_device *);
 107         int (*set_timeout)(struct watchdog_device *, unsigned int);
 108         unsigned int (*get_timeleft)(struct watchdog_device *);
 109         int (*restart)(struct watchdog_device *);
 110         void (*ref)(struct watchdog_device *) __deprecated;
 111         void (*unref)(struct watchdog_device *) __deprecated;
 112         long (*ioctl)(struct watchdog_device *, unsigned int, unsigned long);
 113 };
 114
 115 It is important that you first define the module owner of the watchdog timer
 116 driver's operations. This module owner will be used to lock the module when
 117 the watchdog is active. (This to avoid a system crash when you unload the
 118 module and /dev/watchdog is still open).
 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 * restart: this routine restarts the machine. It returns 0 on success or a
 161   negative errno code for failure.
 162 * ioctl: if this routine is present then it will be called first before we do
 163   our own internal ioctl call handling. This routine should return -ENOIOCTLCMD
 164   if a command is not supported. The parameters that are passed to the ioctl
 165   call are: watchdog_device, cmd and arg.
 166
 167 The 'ref' and 'unref' operations are no longer used and deprecated.
 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_NO_WAY_OUT: this bit stores the nowayout setting for the watchdog.
 176   If this bit is set then the watchdog timer will not be able to stop.
 177
 178   To set the WDOG_NO_WAY_OUT status bit (before registering your watchdog
 179   timer device) you can either:
 180   * set it statically in your watchdog_device struct with
 181         .status = WATCHDOG_NOWAYOUT_INIT_STATUS,
 182     (this will set the value the same as CONFIG_WATCHDOG_NOWAYOUT) or
 183   * use the following helper function:
 184   static inline void watchdog_set_nowayout(struct watchdog_device *wdd, int nowayout)
 185
 186 Note: The WatchDog Timer Driver Core supports the magic close feature and
 187 the nowayout feature. To use the magic close feature you must set the
 188 WDIOF_MAGICCLOSE bit in the options field of the watchdog's info structure.
 189 The nowayout feature will overrule the magic close feature.
 190
 191 To get or set driver specific data the following two helper functions should be
 192 used:
 193
 194 static inline void watchdog_set_drvdata(struct watchdog_device *wdd, void *data)
 195 static inline void *watchdog_get_drvdata(struct watchdog_device *wdd)
 196
 197 The watchdog_set_drvdata function allows you to add driver specific data. The
 198 arguments of this function are the watchdog device where you want to add the
 199 driver specific data to and a pointer to the data itself.
 200
 201 The watchdog_get_drvdata function allows you to retrieve driver specific data.
 202 The argument of this function is the watchdog device where you want to retrieve
 203 data from. The function returns the pointer to the driver specific data.
 204
 205 To initialize the timeout field, the following function can be used:
 206
 207 extern int watchdog_init_timeout(struct watchdog_device *wdd,
 208                                   unsigned int timeout_parm, struct device *dev);
 209
 210 The watchdog_init_timeout function allows you to initialize the timeout field
 211 using the module timeout parameter or by retrieving the timeout-sec property from
 212 the device tree (if the module timeout parameter is invalid). Best practice is
 213 to set the default timeout value as timeout value in the watchdog_device and
 214 then use this function to set the user "preferred" timeout value.
 215 This routine returns zero on success and a negative errno code for failure.
 216
 217 To disable the watchdog on reboot, the user must call the following helper:
 218
 219 static inline void watchdog_stop_on_reboot(struct watchdog_device *wdd);
 220
 221 To change the priority of the restart handler the following helper should be
 222 used:
 223
 224 void watchdog_set_restart_priority(struct watchdog_device *wdd, int priority);
 225
 226 User should follow the following guidelines for setting the priority:
 227 * 0: should be called in last resort, has limited restart capabilities
 228 * 128: default restart handler, use if no other handler is expected to be
 229   available, and/or if restart is sufficient to restart the entire system
 230 * 255: highest priority, will preempt all other restart handlers