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watchdog-kernel-api.txt (16102B)


      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 	const struct watchdog_governor *gov;
     52 	unsigned int bootstatus;
     53 	unsigned int timeout;
     54 	unsigned int pretimeout;
     55 	unsigned int min_timeout;
     56 	unsigned int max_timeout;
     57 	unsigned int min_hw_heartbeat_ms;
     58 	unsigned int max_hw_heartbeat_ms;
     59 	struct notifier_block reboot_nb;
     60 	struct notifier_block restart_nb;
     61 	void *driver_data;
     62 	struct watchdog_core_data *wd_data;
     63 	unsigned long status;
     64 	struct list_head deferred;
     65 };
     66 
     67 It contains following fields:
     68 * id: set by watchdog_register_device, id 0 is special. It has both a
     69   /dev/watchdog0 cdev (dynamic major, minor 0) as well as the old
     70   /dev/watchdog miscdev. The id is set automatically when calling
     71   watchdog_register_device.
     72 * parent: set this to the parent device (or NULL) before calling
     73   watchdog_register_device.
     74 * groups: List of sysfs attribute groups to create when creating the watchdog
     75   device.
     76 * info: a pointer to a watchdog_info structure. This structure gives some
     77   additional information about the watchdog timer itself. (Like it's unique name)
     78 * ops: a pointer to the list of watchdog operations that the watchdog supports.
     79 * gov: a pointer to the assigned watchdog device pretimeout governor or NULL.
     80 * timeout: the watchdog timer's timeout value (in seconds).
     81   This is the time after which the system will reboot if user space does
     82   not send a heartbeat request if WDOG_ACTIVE is set.
     83 * pretimeout: the watchdog timer's pretimeout value (in seconds).
     84 * min_timeout: the watchdog timer's minimum timeout value (in seconds).
     85   If set, the minimum configurable value for 'timeout'.
     86 * max_timeout: the watchdog timer's maximum timeout value (in seconds),
     87   as seen from userspace. If set, the maximum configurable value for
     88   'timeout'. Not used if max_hw_heartbeat_ms is non-zero.
     89 * min_hw_heartbeat_ms: Hardware limit for minimum time between heartbeats,
     90   in milli-seconds. This value is normally 0; it should only be provided
     91   if the hardware can not tolerate lower intervals between heartbeats.
     92 * max_hw_heartbeat_ms: Maximum hardware heartbeat, in milli-seconds.
     93   If set, the infrastructure will send heartbeats to the watchdog driver
     94   if 'timeout' is larger than max_hw_heartbeat_ms, unless WDOG_ACTIVE
     95   is set and userspace failed to send a heartbeat for at least 'timeout'
     96   seconds. max_hw_heartbeat_ms must be set if a driver does not implement
     97   the stop function.
     98 * reboot_nb: notifier block that is registered for reboot notifications, for
     99   internal use only. If the driver calls watchdog_stop_on_reboot, watchdog core
    100   will stop the watchdog on such notifications.
    101 * restart_nb: notifier block that is registered for machine restart, for
    102   internal use only. If a watchdog is capable of restarting the machine, it
    103   should define ops->restart. Priority can be changed through
    104   watchdog_set_restart_priority.
    105 * bootstatus: status of the device after booting (reported with watchdog
    106   WDIOF_* status bits).
    107 * driver_data: a pointer to the drivers private data of a watchdog device.
    108   This data should only be accessed via the watchdog_set_drvdata and
    109   watchdog_get_drvdata routines.
    110 * wd_data: a pointer to watchdog core internal data.
    111 * status: this field contains a number of status bits that give extra
    112   information about the status of the device (Like: is the watchdog timer
    113   running/active, or is the nowayout bit set).
    114 * deferred: entry in wtd_deferred_reg_list which is used to
    115   register early initialized watchdogs.
    116 
    117 The list of watchdog operations is defined as:
    118 
    119 struct watchdog_ops {
    120 	struct module *owner;
    121 	/* mandatory operations */
    122 	int (*start)(struct watchdog_device *);
    123 	int (*stop)(struct watchdog_device *);
    124 	/* optional operations */
    125 	int (*ping)(struct watchdog_device *);
    126 	unsigned int (*status)(struct watchdog_device *);
    127 	int (*set_timeout)(struct watchdog_device *, unsigned int);
    128 	int (*set_pretimeout)(struct watchdog_device *, unsigned int);
    129 	unsigned int (*get_timeleft)(struct watchdog_device *);
    130 	int (*restart)(struct watchdog_device *);
    131 	long (*ioctl)(struct watchdog_device *, unsigned int, unsigned long);
    132 };
    133 
    134 It is important that you first define the module owner of the watchdog timer
    135 driver's operations. This module owner will be used to lock the module when
    136 the watchdog is active. (This to avoid a system crash when you unload the
    137 module and /dev/watchdog is still open).
    138 
    139 Some operations are mandatory and some are optional. The mandatory operations
    140 are:
    141 * start: this is a pointer to the routine that starts the watchdog timer
    142   device.
    143   The routine needs a pointer to the watchdog timer device structure as a
    144   parameter. It returns zero on success or a negative errno code for failure.
    145 
    146 Not all watchdog timer hardware supports the same functionality. That's why
    147 all other routines/operations are optional. They only need to be provided if
    148 they are supported. These optional routines/operations are:
    149 * stop: with this routine the watchdog timer device is being stopped.
    150   The routine needs a pointer to the watchdog timer device structure as a
    151   parameter. It returns zero on success or a negative errno code for failure.
    152   Some watchdog timer hardware can only be started and not be stopped. A
    153   driver supporting such hardware does not have to implement the stop routine.
    154   If a driver has no stop function, the watchdog core will set WDOG_HW_RUNNING
    155   and start calling the driver's keepalive pings function after the watchdog
    156   device is closed.
    157   If a watchdog driver does not implement the stop function, it must set
    158   max_hw_heartbeat_ms.
    159 * ping: this is the routine that sends a keepalive ping to the watchdog timer
    160   hardware.
    161   The routine needs a pointer to the watchdog timer device structure as a
    162   parameter. It returns zero on success or a negative errno code for failure.
    163   Most hardware that does not support this as a separate function uses the
    164   start function to restart the watchdog timer hardware. And that's also what
    165   the watchdog timer driver core does: to send a keepalive ping to the watchdog
    166   timer hardware it will either use the ping operation (when available) or the
    167   start operation (when the ping operation is not available).
    168   (Note: the WDIOC_KEEPALIVE ioctl call will only be active when the
    169   WDIOF_KEEPALIVEPING bit has been set in the option field on the watchdog's
    170   info structure).
    171 * status: this routine checks the status of the watchdog timer device. The
    172   status of the device is reported with watchdog WDIOF_* status flags/bits.
    173   WDIOF_MAGICCLOSE and WDIOF_KEEPALIVEPING are reported by the watchdog core;
    174   it is not necessary to report those bits from the driver. Also, if no status
    175   function is provided by the driver, the watchdog core reports the status bits
    176   provided in the bootstatus variable of struct watchdog_device.
    177 * set_timeout: this routine checks and changes the timeout of the watchdog
    178   timer device. It returns 0 on success, -EINVAL for "parameter out of range"
    179   and -EIO for "could not write value to the watchdog". On success this
    180   routine should set the timeout value of the watchdog_device to the
    181   achieved timeout value (which may be different from the requested one
    182   because the watchdog does not necessarily have a 1 second resolution).
    183   Drivers implementing max_hw_heartbeat_ms set the hardware watchdog heartbeat
    184   to the minimum of timeout and max_hw_heartbeat_ms. Those drivers set the
    185   timeout value of the watchdog_device either to the requested timeout value
    186   (if it is larger than max_hw_heartbeat_ms), or to the achieved timeout value.
    187   (Note: the WDIOF_SETTIMEOUT needs to be set in the options field of the
    188   watchdog's info structure).
    189   If the watchdog driver does not have to perform any action but setting the
    190   watchdog_device.timeout, this callback can be omitted.
    191   If set_timeout is not provided but, WDIOF_SETTIMEOUT is set, the watchdog
    192   infrastructure updates the timeout value of the watchdog_device internally
    193   to the requested value.
    194   If the pretimeout feature is used (WDIOF_PRETIMEOUT), then set_timeout must
    195   also take care of checking if pretimeout is still valid and set up the timer
    196   accordingly. This can't be done in the core without races, so it is the
    197   duty of the driver.
    198 * set_pretimeout: this routine checks and changes the pretimeout value of
    199   the watchdog. It is optional because not all watchdogs support pretimeout
    200   notification. The timeout value is not an absolute time, but the number of
    201   seconds before the actual timeout would happen. It returns 0 on success,
    202   -EINVAL for "parameter out of range" and -EIO for "could not write value to
    203   the watchdog". A value of 0 disables pretimeout notification.
    204   (Note: the WDIOF_PRETIMEOUT needs to be set in the options field of the
    205   watchdog's info structure).
    206   If the watchdog driver does not have to perform any action but setting the
    207   watchdog_device.pretimeout, this callback can be omitted. That means if
    208   set_pretimeout is not provided but WDIOF_PRETIMEOUT is set, the watchdog
    209   infrastructure updates the pretimeout value of the watchdog_device internally
    210   to the requested value.
    211 * get_timeleft: this routines returns the time that's left before a reset.
    212 * restart: this routine restarts the machine. It returns 0 on success or a
    213   negative errno code for failure.
    214 * ioctl: if this routine is present then it will be called first before we do
    215   our own internal ioctl call handling. This routine should return -ENOIOCTLCMD
    216   if a command is not supported. The parameters that are passed to the ioctl
    217   call are: watchdog_device, cmd and arg.
    218 
    219 The status bits should (preferably) be set with the set_bit and clear_bit alike
    220 bit-operations. The status bits that are defined are:
    221 * WDOG_ACTIVE: this status bit indicates whether or not a watchdog timer device
    222   is active or not from user perspective. User space is expected to send
    223   heartbeat requests to the driver while this flag is set.
    224 * WDOG_NO_WAY_OUT: this bit stores the nowayout setting for the watchdog.
    225   If this bit is set then the watchdog timer will not be able to stop.
    226 * WDOG_HW_RUNNING: Set by the watchdog driver if the hardware watchdog is
    227   running. The bit must be set if the watchdog timer hardware can not be
    228   stopped. The bit may also be set if the watchdog timer is running after
    229   booting, before the watchdog device is opened. If set, the watchdog
    230   infrastructure will send keepalives to the watchdog hardware while
    231   WDOG_ACTIVE is not set.
    232   Note: when you register the watchdog timer device with this bit set,
    233   then opening /dev/watchdog will skip the start operation but send a keepalive
    234   request instead.
    235 
    236   To set the WDOG_NO_WAY_OUT status bit (before registering your watchdog
    237   timer device) you can either:
    238   * set it statically in your watchdog_device struct with
    239 	.status = WATCHDOG_NOWAYOUT_INIT_STATUS,
    240     (this will set the value the same as CONFIG_WATCHDOG_NOWAYOUT) or
    241   * use the following helper function:
    242   static inline void watchdog_set_nowayout(struct watchdog_device *wdd, int nowayout)
    243 
    244 Note: The WatchDog Timer Driver Core supports the magic close feature and
    245 the nowayout feature. To use the magic close feature you must set the
    246 WDIOF_MAGICCLOSE bit in the options field of the watchdog's info structure.
    247 The nowayout feature will overrule the magic close feature.
    248 
    249 To get or set driver specific data the following two helper functions should be
    250 used:
    251 
    252 static inline void watchdog_set_drvdata(struct watchdog_device *wdd, void *data)
    253 static inline void *watchdog_get_drvdata(struct watchdog_device *wdd)
    254 
    255 The watchdog_set_drvdata function allows you to add driver specific data. The
    256 arguments of this function are the watchdog device where you want to add the
    257 driver specific data to and a pointer to the data itself.
    258 
    259 The watchdog_get_drvdata function allows you to retrieve driver specific data.
    260 The argument of this function is the watchdog device where you want to retrieve
    261 data from. The function returns the pointer to the driver specific data.
    262 
    263 To initialize the timeout field, the following function can be used:
    264 
    265 extern int watchdog_init_timeout(struct watchdog_device *wdd,
    266                                   unsigned int timeout_parm, struct device *dev);
    267 
    268 The watchdog_init_timeout function allows you to initialize the timeout field
    269 using the module timeout parameter or by retrieving the timeout-sec property from
    270 the device tree (if the module timeout parameter is invalid). Best practice is
    271 to set the default timeout value as timeout value in the watchdog_device and
    272 then use this function to set the user "preferred" timeout value.
    273 This routine returns zero on success and a negative errno code for failure.
    274 
    275 To disable the watchdog on reboot, the user must call the following helper:
    276 
    277 static inline void watchdog_stop_on_reboot(struct watchdog_device *wdd);
    278 
    279 To disable the watchdog when unregistering the watchdog, the user must call
    280 the following helper. Note that this will only stop the watchdog if the
    281 nowayout flag is not set.
    282 
    283 static inline void watchdog_stop_on_unregister(struct watchdog_device *wdd);
    284 
    285 To change the priority of the restart handler the following helper should be
    286 used:
    287 
    288 void watchdog_set_restart_priority(struct watchdog_device *wdd, int priority);
    289 
    290 User should follow the following guidelines for setting the priority:
    291 * 0: should be called in last resort, has limited restart capabilities
    292 * 128: default restart handler, use if no other handler is expected to be
    293   available, and/or if restart is sufficient to restart the entire system
    294 * 255: highest priority, will preempt all other restart handlers
    295 
    296 To raise a pretimeout notification, the following function should be used:
    297 
    298 void watchdog_notify_pretimeout(struct watchdog_device *wdd)
    299 
    300 The function can be called in the interrupt context. If watchdog pretimeout
    301 governor framework (kbuild CONFIG_WATCHDOG_PRETIMEOUT_GOV symbol) is enabled,
    302 an action is taken by a preconfigured pretimeout governor preassigned to
    303 the watchdog device. If watchdog pretimeout governor framework is not
    304 enabled, watchdog_notify_pretimeout() prints a notification message to
    305 the kernel log buffer.