path: root/Documentation/watchdog
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authorWim Van Sebroeck <wim@iguana.be>2011-07-22 18:55:18 (GMT)
committerWim Van Sebroeck <wim@iguana.be>2011-07-28 08:01:04 (GMT)
commit43316044d4f64da008d6aca7d4b60771b9a24eb8 (patch)
tree66d0d023a8713119d973e3c367efa21fb5a1908f /Documentation/watchdog
parent5efc7a6222f6408d29d6beb1142a302f31dc9eac (diff)
watchdog: WatchDog Timer Driver Core - Add basic framework
The WatchDog Timer Driver Core is a framework that contains the common code for all watchdog-driver's. It also introduces a watchdog device structure and the operations that go with it. This is the introduction of this framework. This part supports the minimal watchdog userspace API (or with other words: the functionality to use /dev/watchdog's open, release and write functionality as defined in the simplest watchdog API). Extra functionality will follow in the next set of patches. Signed-off-by: Alan Cox <alan@lxorguk.ukuu.org.uk> Signed-off-by: Wim Van Sebroeck <wim@iguana.be> Acked-by: Arnd Bergmann <arnd@arndb.de> Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Diffstat (limited to 'Documentation/watchdog')
2 files changed, 121 insertions, 0 deletions
diff --git a/Documentation/watchdog/00-INDEX b/Documentation/watchdog/00-INDEX
index ee99451..fc51128 100644
--- a/Documentation/watchdog/00-INDEX
+++ b/Documentation/watchdog/00-INDEX
@@ -8,6 +8,8 @@ src/
- directory holding watchdog related example programs.
- description of the Linux Watchdog driver API.
+ - description of the Linux WatchDog Timer Driver Core kernel API.
- information on driver parameters (for drivers other than
the ones that have driver-specific files here)
diff --git a/Documentation/watchdog/watchdog-kernel-api.txt b/Documentation/watchdog/watchdog-kernel-api.txt
new file mode 100644
index 0000000..3db67e7
--- /dev/null
+++ b/Documentation/watchdog/watchdog-kernel-api.txt
@@ -0,0 +1,119 @@
+The Linux WatchDog Timer Driver Core kernel API.
+Last reviewed: 22-Jul-2011
+Wim Van Sebroeck <wim@iguana.be>
+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;
+ 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.
+* 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 *);
+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
+* 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).
+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
+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.

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