Documentation/watchdog/watchdog-kernel-api.txt

   1 The Linux WatchDog Timer Driver Core kernel API.
   2 ===============================================
   3 Last reviewed: 22-Jul-2011
   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         const struct watchdog_info *info;
  43         const struct watchdog_ops *ops;
  44         unsigned int bootstatus;
  45         void *driver_data;
  46         unsigned long status;
  47 };
  48
  49 It contains following fields:
  50 * info: a pointer to a watchdog_info structure. This structure gives some
  51   additional information about the watchdog timer itself. (Like it's unique name)
  52 * ops: a pointer to the list of watchdog operations that the watchdog supports.
  53 * bootstatus: status of the device after booting (reported with watchdog
  54   WDIOF_* status bits).
  55 * driver_data: a pointer to the drivers private data of a watchdog device.
  56   This data should only be accessed via the watchdog_set_drvadata and
  57   watchdog_get_drvdata routines.
  58 * status: this field contains a number of status bits that give extra
  59   information about the status of the device (Like: is the device opened via
  60   the /dev/watchdog interface or not, ...).
  61
  62 The list of watchdog operations is defined as:
  63
  64 struct watchdog_ops {
  65         struct module *owner;
  66         /* mandatory operations */
  67         int (*start)(struct watchdog_device *);
  68         int (*stop)(struct watchdog_device *);
  69         /* optional operations */
  70         int (*ping)(struct watchdog_device *);
  71         unsigned int (*status)(struct watchdog_device *);
  72 };
  73
  74 It is important that you first define the module owner of the watchdog timer
  75 driver's operations. This module owner will be used to lock the module when
  76 the watchdog is active. (This to avoid a system crash when you unload the
  77 module and /dev/watchdog is still open).
  78 Some operations are mandatory and some are optional. The mandatory operations
  79 are:
  80 * start: this is a pointer to the routine that starts the watchdog timer
  81   device.
  82   The routine needs a pointer to the watchdog timer device structure as a
  83   parameter. It returns zero on success or a negative errno code for failure.
  84 * stop: with this routine the watchdog timer device is being stopped.
  85   The routine needs a pointer to the watchdog timer device structure as a
  86   parameter. It returns zero on success or a negative errno code for failure.
  87   Some watchdog timer hardware can only be started and not be stopped. The
  88   driver supporting this hardware needs to make sure that a start and stop
  89   routine is being provided. This can be done by using a timer in the driver
  90   that regularly sends a keepalive ping to the watchdog timer hardware.
  91
  92 Not all watchdog timer hardware supports the same functionality. That's why
  93 all other routines/operations are optional. They only need to be provided if
  94 they are supported. These optional routines/operations are:
  95 * ping: this is the routine that sends a keepalive ping to the watchdog timer
  96   hardware.
  97   The routine needs a pointer to the watchdog timer device structure as a
  98   parameter. It returns zero on success or a negative errno code for failure.
  99   Most hardware that does not support this as a separate function uses the
 100   start function to restart the watchdog timer hardware. And that's also what
 101   the watchdog timer driver core does: to send a keepalive ping to the watchdog
 102   timer hardware it will either use the ping operation (when available) or the
 103   start operation (when the ping operation is not available).
 104 * status: this routine checks the status of the watchdog timer device. The
 105   status of the device is reported with watchdog WDIOF_* status flags/bits.
 106
 107 The status bits should (preferably) be set with the set_bit and clear_bit alike
 108 bit-operations. The status bits that are defined are:
 109 * WDOG_DEV_OPEN: this status bit shows whether or not the watchdog device
 110   was opened via /dev/watchdog.
 111   (This bit should only be used by the WatchDog Timer Driver Core).
 112
 113 To get or set driver specific data the following two helper functions should be
 114 used:
 115
 116 static inline void watchdog_set_drvdata(struct watchdog_device *wdd, void *data)
 117 static inline void *watchdog_get_drvdata(struct watchdog_device *wdd)
 118
 119 The watchdog_set_drvdata function allows you to add driver specific data. The
 120 arguments of this function are the watchdog device where you want to add the
 121 driver specific data to and a pointer to the data itself.
 122
 123 The watchdog_get_drvdata function allows you to retrieve driver specific data.
 124 The argument of this function is the watchdog device where you want to retrieve
 125 data from. The function retruns the pointer to the driver specific data.