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