1The Linux WatchDog Timer Driver Core kernel API. 2=============================================== 3Last reviewed: 29-Nov-2011 4 5Wim Van Sebroeck <wim@iguana.be> 6 7Introduction 8------------ 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 . 13 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). 20 21The API 22------- 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: 27 28extern int watchdog_register_device(struct watchdog_device *); 29extern void watchdog_unregister_device(struct watchdog_device *); 30 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. 34 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. 38 39The watchdog device structure looks like this: 40 41struct watchdog_device { 42 const struct watchdog_info *info; 43 const struct watchdog_ops *ops; 44 unsigned int bootstatus; 45 unsigned int timeout; 46 unsigned int min_timeout; 47 unsigned int max_timeout; 48 void *driver_data; 49 unsigned long status; 50}; 51 52It contains following fields: 53* info: a pointer to a watchdog_info structure. This structure gives some 54 additional information about the watchdog timer itself. (Like it's unique name) 55* ops: a pointer to the list of watchdog operations that the watchdog supports. 56* timeout: the watchdog timer's timeout value (in seconds). 57* min_timeout: the watchdog timer's minimum timeout value (in seconds). 58* max_timeout: the watchdog timer's maximum timeout value (in seconds). 59* bootstatus: status of the device after booting (reported with watchdog 60 WDIOF_* status bits). 61* driver_data: a pointer to the drivers private data of a watchdog device. 62 This data should only be accessed via the watchdog_set_drvadata and 63 watchdog_get_drvdata routines. 64* status: this field contains a number of status bits that give extra 65 information about the status of the device (Like: is the watchdog timer 66 running/active, is the nowayout bit set, is the device opened via 67 the /dev/watchdog interface or not, ...). 68 69The list of watchdog operations is defined as: 70 71struct watchdog_ops { 72 struct module *owner; 73 /* mandatory operations */ 74 int (*start)(struct watchdog_device *); 75 int (*stop)(struct watchdog_device *); 76 /* optional operations */ 77 int (*ping)(struct watchdog_device *); 78 unsigned int (*status)(struct watchdog_device *); 79 int (*set_timeout)(struct watchdog_device *, unsigned int); 80 long (*ioctl)(struct watchdog_device *, unsigned int, unsigned long); 81}; 82 83It is important that you first define the module owner of the watchdog timer 84driver's operations. This module owner will be used to lock the module when 85the watchdog is active. (This to avoid a system crash when you unload the 86module and /dev/watchdog is still open). 87Some operations are mandatory and some are optional. The mandatory operations 88are: 89* start: this is a pointer to the routine that starts the watchdog timer 90 device. 91 The routine needs a pointer to the watchdog timer device structure as a 92 parameter. It returns zero on success or a negative errno code for failure. 93* stop: with this routine the watchdog timer device is being stopped. 94 The routine needs a pointer to the watchdog timer device structure as a 95 parameter. It returns zero on success or a negative errno code for failure. 96 Some watchdog timer hardware can only be started and not be stopped. The 97 driver supporting this hardware needs to make sure that a start and stop 98 routine is being provided. This can be done by using a timer in the driver 99 that regularly sends a keepalive ping to the watchdog timer hardware. 100 101Not all watchdog timer hardware supports the same functionality. That's why 102all other routines/operations are optional. They only need to be provided if 103they are supported. These optional routines/operations are: 104* ping: this is the routine that sends a keepalive ping to the watchdog timer 105 hardware. 106 The routine needs a pointer to the watchdog timer device structure as a 107 parameter. It returns zero on success or a negative errno code for failure. 108 Most hardware that does not support this as a separate function uses the 109 start function to restart the watchdog timer hardware. And that's also what 110 the watchdog timer driver core does: to send a keepalive ping to the watchdog 111 timer hardware it will either use the ping operation (when available) or the 112 start operation (when the ping operation is not available). 113 (Note: the WDIOC_KEEPALIVE ioctl call will only be active when the 114 WDIOF_KEEPALIVEPING bit has been set in the option field on the watchdog's 115 info structure). 116* status: this routine checks the status of the watchdog timer device. The 117 status of the device is reported with watchdog WDIOF_* status flags/bits. 118* set_timeout: this routine checks and changes the timeout of the watchdog 119 timer device. It returns 0 on success, -EINVAL for "parameter out of range" 120 and -EIO for "could not write value to the watchdog". On success the timeout 121 value of the watchdog_device will be changed to the value that was just used 122 to re-program the watchdog timer device. 123 (Note: the WDIOF_SETTIMEOUT needs to be set in the options field of the 124 watchdog's info structure). 125* ioctl: if this routine is present then it will be called first before we do 126 our own internal ioctl call handling. This routine should return -ENOIOCTLCMD 127 if a command is not supported. The parameters that are passed to the ioctl 128 call are: watchdog_device, cmd and arg. 129 130The status bits should (preferably) be set with the set_bit and clear_bit alike 131bit-operations. The status bits that are defined are: 132* WDOG_ACTIVE: this status bit indicates whether or not a watchdog timer device 133 is active or not. When the watchdog is active after booting, then you should 134 set this status bit (Note: when you register the watchdog timer device with 135 this bit set, then opening /dev/watchdog will skip the start operation) 136* WDOG_DEV_OPEN: this status bit shows whether or not the watchdog device 137 was opened via /dev/watchdog. 138 (This bit should only be used by the WatchDog Timer Driver Core). 139* WDOG_ALLOW_RELEASE: this bit stores whether or not the magic close character 140 has been sent (so that we can support the magic close feature). 141 (This bit should only be used by the WatchDog Timer Driver Core). 142* WDOG_NO_WAY_OUT: this bit stores the nowayout setting for the watchdog. 143 If this bit is set then the watchdog timer will not be able to stop. 144 145 To set the WDOG_NO_WAY_OUT status bit (before registering your watchdog 146 timer device) you can either: 147 * set it statically in your watchdog_device struct with 148 .status = WATCHDOG_NOWAYOUT_INIT_STATUS, 149 (this will set the value the same as CONFIG_WATCHDOG_NOWAYOUT) or 150 * use the following helper function: 151 static inline void watchdog_set_nowayout(struct watchdog_device *wdd, int nowayout) 152 153Note: The WatchDog Timer Driver Core supports the magic close feature and 154the nowayout feature. To use the magic close feature you must set the 155WDIOF_MAGICCLOSE bit in the options field of the watchdog's info structure. 156The nowayout feature will overrule the magic close feature. 157 158To get or set driver specific data the following two helper functions should be 159used: 160 161static inline void watchdog_set_drvdata(struct watchdog_device *wdd, void *data) 162static inline void *watchdog_get_drvdata(struct watchdog_device *wdd) 163 164The watchdog_set_drvdata function allows you to add driver specific data. The 165arguments of this function are the watchdog device where you want to add the 166driver specific data to and a pointer to the data itself. 167 168The watchdog_get_drvdata function allows you to retrieve driver specific data. 169The argument of this function is the watchdog device where you want to retrieve 170data from. The function retruns the pointer to the driver specific data. 171