The Linux WatchDog Timer Driver Core kernel API.
===============================================
Last reviewed: 22-Jul-2011

Wim Van Sebroeck <wim@iguana.be>

Introduction
------------
This document does not describe what a WatchDog Timer (WDT) Driver or Device is.
It also does not describe the API which can be used by user space to communicate
with a WatchDog Timer. If you want to know this then please read the following
file: Documentation/watchdog/watchdog-api.txt .

So what does this document describe? It describes the API that can be used by
WatchDog Timer Drivers that want to use the WatchDog Timer Driver Core
Framework. This framework provides all interfacing towards user space so that
the same code does not have to be reproduced each time. This also means that
a watchdog timer driver then only needs to provide the different routines
(operations) that control the watchdog timer (WDT).

The API
-------
Each watchdog timer driver that wants to use the WatchDog Timer Driver Core
must #include <linux/watchdog.h> (you would have to do this anyway when
writing a watchdog device driver). This include file contains following
register/unregister routines:

extern int watchdog_register_device(struct watchdog_device *);
extern void watchdog_unregister_device(struct watchdog_device *);

The watchdog_register_device routine registers a watchdog timer device.
The parameter of this routine is a pointer to a watchdog_device structure.
This routine returns zero on success and a negative errno code for failure.

The watchdog_unregister_device routine deregisters a registered watchdog timer
device. The parameter of this routine is the pointer to the registered
watchdog_device structure.

The watchdog device structure looks like this:

struct watchdog_device {
	const struct watchdog_info *info;
	const struct watchdog_ops *ops;
	unsigned int bootstatus;
	void *driver_data;
	unsigned long status;
};

It contains following fields:
* info: a pointer to a watchdog_info structure. This structure gives some
  additional information about the watchdog timer itself. (Like it's unique name)
* ops: a pointer to the list of watchdog operations that the watchdog supports.
* bootstatus: status of the device after booting (reported with watchdog
  WDIOF_* status bits).
* driver_data: a pointer to the drivers private data of a watchdog device.
  This data should only be accessed via the watchdog_set_drvadata and
  watchdog_get_drvdata routines.
* status: this field contains a number of status bits that give extra
  information about the status of the device (Like: is the device opened via
  the /dev/watchdog interface or not, ...).

The list of watchdog operations is defined as:

struct watchdog_ops {
	struct module *owner;
	/* mandatory operations */
	int (*start)(struct watchdog_device *);
	int (*stop)(struct watchdog_device *);
	/* optional operations */
	int (*ping)(struct watchdog_device *);
	unsigned int (*status)(struct watchdog_device *);
};

It is important that you first define the module owner of the watchdog timer
driver's operations. This module owner will be used to lock the module when
the watchdog is active. (This to avoid a system crash when you unload the
module and /dev/watchdog is still open).
Some operations are mandatory and some are optional. The mandatory operations
are:
* start: this is a pointer to the routine that starts the watchdog timer
  device.
  The routine needs a pointer to the watchdog timer device structure as a
  parameter. It returns zero on success or a negative errno code for failure.
* stop: with this routine the watchdog timer device is being stopped.
  The routine needs a pointer to the watchdog timer device structure as a
  parameter. It returns zero on success or a negative errno code for failure.
  Some watchdog timer hardware can only be started and not be stopped. The
  driver supporting this hardware needs to make sure that a start and stop
  routine is being provided. This can be done by using a timer in the driver
  that regularly sends a keepalive ping to the watchdog timer hardware.

Not all watchdog timer hardware supports the same functionality. That's why
all other routines/operations are optional. They only need to be provided if
they are supported. These optional routines/operations are:
* ping: this is the routine that sends a keepalive ping to the watchdog timer
  hardware.
  The routine needs a pointer to the watchdog timer device structure as a
  parameter. It returns zero on success or a negative errno code for failure.
  Most hardware that does not support this as a separate function uses the
  start function to restart the watchdog timer hardware. And that's also what
  the watchdog timer driver core does: to send a keepalive ping to the watchdog
  timer hardware it will either use the ping operation (when available) or the
  start operation (when the ping operation is not available).
* status: this routine checks the status of the watchdog timer device. The
  status of the device is reported with watchdog WDIOF_* status flags/bits.

The status bits should (preferably) be set with the set_bit and clear_bit alike
bit-operations. The status bits that are defined are:
* WDOG_DEV_OPEN: this status bit shows whether or not the watchdog device
  was opened via /dev/watchdog.
  (This bit should only be used by the WatchDog Timer Driver Core).

To get or set driver specific data the following two helper functions should be
used:

static inline void watchdog_set_drvdata(struct watchdog_device *wdd, void *data)
static inline void *watchdog_get_drvdata(struct watchdog_device *wdd)

The watchdog_set_drvdata function allows you to add driver specific data. The
arguments of this function are the watchdog device where you want to add the
driver specific data to and a pointer to the data itself.

The watchdog_get_drvdata function allows you to retrieve driver specific data.
The argument of this function is the watchdog device where you want to retrieve
data from. The function retruns the pointer to the driver specific data.