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 cdev cdev;
  48         struct device *dev;
  49         struct device *parent;
  50         const struct watchdog_info *info;
  51         const struct watchdog_ops *ops;
  52         unsigned int bootstatus;
  53         unsigned int timeout;
  54         unsigned int min_timeout;
  55         unsigned int max_timeout;
  56         void *driver_data;
  57         struct mutex lock;
  58         unsigned long status;
  59         struct list_head deferred;
  60 };
  61
  62 It contains following fields:
  63 * id: set by watchdog_register_device, id 0 is special. It has both a
  64   /dev/watchdog0 cdev (dynamic major, minor 0) as well as the old
  65   /dev/watchdog miscdev. The id is set automatically when calling
  66   watchdog_register_device.
  67 * cdev: cdev for the dynamic /dev/watchdog<id> device nodes. This
  68   field is also populated by watchdog_register_device.
  69 * dev: device under the watchdog class (created by watchdog_register_device).
  70 * parent: set this to the parent device (or NULL) before calling
  71   watchdog_register_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 * bootstatus: status of the device after booting (reported with watchdog
  79   WDIOF_* status bits).
  80 * driver_data: a pointer to the drivers private data of a watchdog device.
  81   This data should only be accessed via the watchdog_set_drvdata and
  82   watchdog_get_drvdata routines.
  83 * lock: Mutex for WatchDog Timer Driver Core internal use only.
  84 * status: this field contains a number of status bits that give extra
  85   information about the status of the device (Like: is the watchdog timer
  86   running/active, is the nowayout bit set, is the device opened via
  87   the /dev/watchdog interface or not, ...).
  88 * deferred: entry in wtd_deferred_reg_list which is used to
  89   register early initialized watchdogs.
  90
  91 The list of watchdog operations is defined as:
  92
  93 struct watchdog_ops {
  94         struct module *owner;
  95         /* mandatory operations */
  96         int (*start)(struct watchdog_device *);
  97         int (*stop)(struct watchdog_device *);
  98         /* optional operations */
  99         int (*ping)(struct watchdog_device *);
 100         unsigned int (*status)(struct watchdog_device *);
 101         int (*set_timeout)(struct watchdog_device *, unsigned int);
 102         unsigned int (*get_timeleft)(struct watchdog_device *);
 103         void (*ref)(struct watchdog_device *);
 104         void (*unref)(struct watchdog_device *);
 105         long (*ioctl)(struct watchdog_device *, unsigned int, unsigned long);
 106 };
 107
 108 It is important that you first define the module owner of the watchdog timer
 109 driver's operations. This module owner will be used to lock the module when
 110 the watchdog is active. (This to avoid a system crash when you unload the
 111 module and /dev/watchdog is still open).
 112
 113 If the watchdog_device struct is dynamically allocated, just locking the module
 114 is not enough and a driver also needs to define the ref and unref operations to
 115 ensure the structure holding the watchdog_device does not go away.
 116
 117 The simplest (and usually sufficient) implementation of this is to:
 118 1) Add a kref struct to the same structure which is holding the watchdog_device
 119 2) Define a release callback for the kref which frees the struct holding both
 120 3) Call kref_init on this kref *before* calling watchdog_register_device()
 121 4) Define a ref operation calling kref_get on this kref
 122 5) Define a unref operation calling kref_put on this kref
 123 6) When it is time to cleanup:
 124  * Do not kfree() the struct holding both, the last kref_put will do this!
 125  * *After* calling watchdog_unregister_device() call kref_put on the kref
 126
 127 Some operations are mandatory and some are optional. The mandatory operations
 128 are:
 129 * start: this is a pointer to the routine that starts the watchdog timer
 130   device.
 131   The routine needs a pointer to the watchdog timer device structure as a
 132   parameter. It returns zero on success or a negative errno code for failure.
 133 * stop: with this routine the watchdog timer device is being stopped.
 134   The routine needs a pointer to the watchdog timer device structure as a
 135   parameter. It returns zero on success or a negative errno code for failure.
 136   Some watchdog timer hardware can only be started and not be stopped. The
 137   driver supporting this hardware needs to make sure that a start and stop
 138   routine is being provided. This can be done by using a timer in the driver
 139   that regularly sends a keepalive ping to the watchdog timer hardware.
 140
 141 Not all watchdog timer hardware supports the same functionality. That's why
 142 all other routines/operations are optional. They only need to be provided if
 143 they are supported. These optional routines/operations are:
 144 * ping: this is the routine that sends a keepalive ping to the watchdog timer
 145   hardware.
 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   Most hardware that does not support this as a separate function uses the
 149   start function to restart the watchdog timer hardware. And that's also what
 150   the watchdog timer driver core does: to send a keepalive ping to the watchdog
 151   timer hardware it will either use the ping operation (when available) or the
 152   start operation (when the ping operation is not available).
 153   (Note: the WDIOC_KEEPALIVE ioctl call will only be active when the
 154   WDIOF_KEEPALIVEPING bit has been set in the option field on the watchdog's
 155   info structure).
 156 * status: this routine checks the status of the watchdog timer device. The
 157   status of the device is reported with watchdog WDIOF_* status flags/bits.
 158 * set_timeout: this routine checks and changes the timeout of the watchdog
 159   timer device. It returns 0 on success, -EINVAL for "parameter out of range"
 160   and -EIO for "could not write value to the watchdog". On success this
 161   routine should set the timeout value of the watchdog_device to the
 162   achieved timeout value (which may be different from the requested one
 163   because the watchdog does not necessarily has a 1 second resolution).
 164   (Note: the WDIOF_SETTIMEOUT needs to be set in the options field of the
 165   watchdog's info structure).
 166 * get_timeleft: this routines returns the time that's left before a reset.
 167 * ref: the operation that calls kref_get on the kref of a dynamically
 168   allocated watchdog_device struct.
 169 * unref: the operation that calls kref_put on the kref of a dynamically
 170   allocated watchdog_device struct.
 171 * ioctl: if this routine is present then it will be called first before we do
 172   our own internal ioctl call handling. This routine should return -ENOIOCTLCMD
 173   if a command is not supported. The parameters that are passed to the ioctl
 174   call are: watchdog_device, cmd and arg.
 175
 176 The status bits should (preferably) be set with the set_bit and clear_bit alike
 177 bit-operations. The status bits that are defined are:
 178 * WDOG_ACTIVE: this status bit indicates whether or not a watchdog timer device
 179   is active or not. When the watchdog is active after booting, then you should
 180   set this status bit (Note: when you register the watchdog timer device with
 181   this bit set, then opening /dev/watchdog will skip the start operation)
 182 * WDOG_DEV_OPEN: this status bit shows whether or not the watchdog device
 183   was opened via /dev/watchdog.
 184   (This bit should only be used by the WatchDog Timer Driver Core).
 185 * WDOG_ALLOW_RELEASE: this bit stores whether or not the magic close character
 186   has been sent (so that we can support the magic close feature).
 187   (This bit should only be used by the WatchDog Timer Driver Core).
 188 * WDOG_NO_WAY_OUT: this bit stores the nowayout setting for the watchdog.
 189   If this bit is set then the watchdog timer will not be able to stop.
 190 * WDOG_UNREGISTERED: this bit gets set by the WatchDog Timer Driver Core
 191   after calling watchdog_unregister_device, and then checked before calling
 192   any watchdog_ops, so that you can be sure that no operations (other then
 193   unref) will get called after unregister, even if userspace still holds a
 194   reference to /dev/watchdog
 195
 196   To set the WDOG_NO_WAY_OUT status bit (before registering your watchdog
 197   timer device) you can either:
 198   * set it statically in your watchdog_device struct with
 199         .status = WATCHDOG_NOWAYOUT_INIT_STATUS,
 200     (this will set the value the same as CONFIG_WATCHDOG_NOWAYOUT) or
 201   * use the following helper function:
 202   static inline void watchdog_set_nowayout(struct watchdog_device *wdd, int nowayout)
 203
 204 Note: The WatchDog Timer Driver Core supports the magic close feature and
 205 the nowayout feature. To use the magic close feature you must set the
 206 WDIOF_MAGICCLOSE bit in the options field of the watchdog's info structure.
 207 The nowayout feature will overrule the magic close feature.
 208
 209 To get or set driver specific data the following two helper functions should be
 210 used:
 211
 212 static inline void watchdog_set_drvdata(struct watchdog_device *wdd, void *data)
 213 static inline void *watchdog_get_drvdata(struct watchdog_device *wdd)
 214
 215 The watchdog_set_drvdata function allows you to add driver specific data. The
 216 arguments of this function are the watchdog device where you want to add the
 217 driver specific data to and a pointer to the data itself.
 218
 219 The watchdog_get_drvdata function allows you to retrieve driver specific data.
 220 The argument of this function is the watchdog device where you want to retrieve
 221 data from. The function returns the pointer to the driver specific data.
 222
 223 To initialize the timeout field, the following function can be used:
 224
 225 extern int watchdog_init_timeout(struct watchdog_device *wdd,
 226                                   unsigned int timeout_parm, struct device *dev);
 227
 228 The watchdog_init_timeout function allows you to initialize the timeout field
 229 using the module timeout parameter or by retrieving the timeout-sec property from
 230 the device tree (if the module timeout parameter is invalid). Best practice is
 231 to set the default timeout value as timeout value in the watchdog_device and
 232 then use this function to set the user "preferred" timeout value.
 233 This routine returns zero on success and a negative errno code for failure.