The Linux WatchDog Timer Driver Core kernel API.
===============================================
-Last reviewed: 16-Mar-2012
+Last reviewed: 22-May-2012
Wim Van Sebroeck <wim@iguana.be>
The watchdog device structure looks like this:
struct watchdog_device {
+ int id;
+ struct cdev cdev;
+ struct device *dev;
+ struct device *parent;
const struct watchdog_info *info;
const struct watchdog_ops *ops;
unsigned int bootstatus;
unsigned int min_timeout;
unsigned int max_timeout;
void *driver_data;
+ struct mutex lock;
unsigned long status;
};
It contains following fields:
+* id: set by watchdog_register_device, id 0 is special. It has both a
+ /dev/watchdog0 cdev (dynamic major, minor 0) as well as the old
+ /dev/watchdog miscdev. The id is set automatically when calling
+ watchdog_register_device.
+* cdev: cdev for the dynamic /dev/watchdog<id> device nodes. This
+ field is also populated by watchdog_register_device.
+* dev: device under the watchdog class (created by watchdog_register_device).
+* parent: set this to the parent device (or NULL) before calling
+ watchdog_register_device.
* 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
+ This data should only be accessed via the watchdog_set_drvdata and
watchdog_get_drvdata routines.
+* lock: Mutex for WatchDog Timer Driver Core internal use only.
* status: this field contains a number of status bits that give extra
information about the status of the device (Like: is the watchdog timer
running/active, is the nowayout bit set, is the device opened via
unsigned int (*status)(struct watchdog_device *);
int (*set_timeout)(struct watchdog_device *, unsigned int);
unsigned int (*get_timeleft)(struct watchdog_device *);
+ void (*ref)(struct watchdog_device *);
+ void (*unref)(struct watchdog_device *);
long (*ioctl)(struct watchdog_device *, unsigned int, unsigned long);
};
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).
+
+If the watchdog_device struct is dynamically allocated, just locking the module
+is not enough and a driver also needs to define the ref and unref operations to
+ensure the structure holding the watchdog_device does not go away.
+
+The simplest (and usually sufficient) implementation of this is to:
+1) Add a kref struct to the same structure which is holding the watchdog_device
+2) Define a release callback for the kref which frees the struct holding both
+3) Call kref_init on this kref *before* calling watchdog_register_device()
+4) Define a ref operation calling kref_get on this kref
+5) Define a unref operation calling kref_put on this kref
+6) When it is time to cleanup:
+ * Do not kfree() the struct holding both, the last kref_put will do this!
+ * *After* calling watchdog_unregister_device() call kref_put on the kref
+
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
(Note: the WDIOF_SETTIMEOUT needs to be set in the options field of the
watchdog's info structure).
* get_timeleft: this routines returns the time that's left before a reset.
+* ref: the operation that calls kref_get on the kref of a dynamically
+ allocated watchdog_device struct.
+* unref: the operation that calls kref_put on the kref of a dynamically
+ allocated watchdog_device struct.
* ioctl: if this routine is present then it will be called first before we do
our own internal ioctl call handling. This routine should return -ENOIOCTLCMD
if a command is not supported. The parameters that are passed to the ioctl
(This bit should only be used by the WatchDog Timer Driver Core).
* WDOG_NO_WAY_OUT: this bit stores the nowayout setting for the watchdog.
If this bit is set then the watchdog timer will not be able to stop.
+* WDOG_UNREGISTERED: this bit gets set by the WatchDog Timer Driver Core
+ after calling watchdog_unregister_device, and then checked before calling
+ any watchdog_ops, so that you can be sure that no operations (other then
+ unref) will get called after unregister, even if userspace still holds a
+ reference to /dev/watchdog
To set the WDOG_NO_WAY_OUT status bit (before registering your watchdog
timer device) you can either: