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 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.