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Linus Torvalds1da177e2005-04-16 15:20:36 -07001The Linux Watchdog driver API.
2
3Copyright 2002 Christer Weingel <wingel@nano-system.com>
4
5Some parts of this document are copied verbatim from the sbc60xxwdt
6driver which is (c) Copyright 2000 Jakob Oestergaard <jakob@ostenfeld.dk>
7
8This document describes the state of the Linux 2.4.18 kernel.
9
10Introduction:
11
12A Watchdog Timer (WDT) is a hardware circuit that can reset the
13computer system in case of a software fault. You probably knew that
14already.
15
16Usually a userspace daemon will notify the kernel watchdog driver via the
17/dev/watchdog special device file that userspace is still alive, at
18regular intervals. When such a notification occurs, the driver will
19usually tell the hardware watchdog that everything is in order, and
20that the watchdog should wait for yet another little while to reset
21the system. If userspace fails (RAM error, kernel bug, whatever), the
22notifications cease to occur, and the hardware watchdog will reset the
23system (causing a reboot) after the timeout occurs.
24
25The Linux watchdog API is a rather AD hoc construction and different
26drivers implement different, and sometimes incompatible, parts of it.
27This file is an attempt to document the existing usage and allow
28future driver writers to use it as a reference.
29
30The simplest API:
31
32All drivers support the basic mode of operation, where the watchdog
33activates as soon as /dev/watchdog is opened and will reboot unless
34the watchdog is pinged within a certain time, this time is called the
35timeout or margin. The simplest way to ping the watchdog is to write
36some data to the device. So a very simple watchdog daemon would look
37like this:
38
39int main(int argc, const char *argv[]) {
40 int fd=open("/dev/watchdog",O_WRONLY);
41 if (fd==-1) {
42 perror("watchdog");
43 exit(1);
44 }
45 while(1) {
46 write(fd, "\0", 1);
47 sleep(10);
48 }
49}
50
51A more advanced driver could for example check that a HTTP server is
52still responding before doing the write call to ping the watchdog.
53
54When the device is closed, the watchdog is disabled. This is not
55always such a good idea, since if there is a bug in the watchdog
56daemon and it crashes the system will not reboot. Because of this,
57some of the drivers support the configuration option "Disable watchdog
58shutdown on close", CONFIG_WATCHDOG_NOWAYOUT. If it is set to Y when
59compiling the kernel, there is no way of disabling the watchdog once
60it has been started. So, if the watchdog dameon crashes, the system
61will reboot after the timeout has passed.
62
63Some other drivers will not disable the watchdog, unless a specific
64magic character 'V' has been sent /dev/watchdog just before closing
65the file. If the userspace daemon closes the file without sending
66this special character, the driver will assume that the daemon (and
67userspace in general) died, and will stop pinging the watchdog without
68disabling it first. This will then cause a reboot.
69
70The ioctl API:
71
72All conforming drivers also support an ioctl API.
73
74Pinging the watchdog using an ioctl:
75
76All drivers that have an ioctl interface support at least one ioctl,
77KEEPALIVE. This ioctl does exactly the same thing as a write to the
78watchdog device, so the main loop in the above program could be
79replaced with:
80
81 while (1) {
82 ioctl(fd, WDIOC_KEEPALIVE, 0);
83 sleep(10);
84 }
85
86the argument to the ioctl is ignored.
87
88Setting and getting the timeout:
89
90For some drivers it is possible to modify the watchdog timeout on the
91fly with the SETTIMEOUT ioctl, those drivers have the WDIOF_SETTIMEOUT
92flag set in their option field. The argument is an integer
93representing the timeout in seconds. The driver returns the real
94timeout used in the same variable, and this timeout might differ from
95the requested one due to limitation of the hardware.
96
97 int timeout = 45;
98 ioctl(fd, WDIOC_SETTIMEOUT, &timeout);
99 printf("The timeout was set to %d seconds\n", timeout);
100
101This example might actually print "The timeout was set to 60 seconds"
102if the device has a granularity of minutes for its timeout.
103
104Starting with the Linux 2.4.18 kernel, it is possible to query the
105current timeout using the GETTIMEOUT ioctl.
106
107 ioctl(fd, WDIOC_GETTIMEOUT, &timeout);
108 printf("The timeout was is %d seconds\n", timeout);
109
110Envinronmental monitoring:
111
112All watchdog drivers are required return more information about the system,
113some do temperature, fan and power level monitoring, some can tell you
114the reason for the last reboot of the system. The GETSUPPORT ioctl is
115available to ask what the device can do:
116
117 struct watchdog_info ident;
118 ioctl(fd, WDIOC_GETSUPPORT, &ident);
119
120the fields returned in the ident struct are:
121
122 identity a string identifying the watchdog driver
123 firmware_version the firmware version of the card if available
124 options a flags describing what the device supports
125
126the options field can have the following bits set, and describes what
127kind of information that the GET_STATUS and GET_BOOT_STATUS ioctls can
128return. [FIXME -- Is this correct?]
129
130 WDIOF_OVERHEAT Reset due to CPU overheat
131
132The machine was last rebooted by the watchdog because the thermal limit was
133exceeded
134
135 WDIOF_FANFAULT Fan failed
136
137A system fan monitored by the watchdog card has failed
138
139 WDIOF_EXTERN1 External relay 1
140
141External monitoring relay/source 1 was triggered. Controllers intended for
142real world applications include external monitoring pins that will trigger
143a reset.
144
145 WDIOF_EXTERN2 External relay 2
146
147External monitoring relay/source 2 was triggered
148
149 WDIOF_POWERUNDER Power bad/power fault
150
151The machine is showing an undervoltage status
152
153 WDIOF_CARDRESET Card previously reset the CPU
154
155The last reboot was caused by the watchdog card
156
157 WDIOF_POWEROVER Power over voltage
158
159The machine is showing an overvoltage status. Note that if one level is
160under and one over both bits will be set - this may seem odd but makes
161sense.
162
163 WDIOF_KEEPALIVEPING Keep alive ping reply
164
165The watchdog saw a keepalive ping since it was last queried.
166
167 WDIOF_SETTIMEOUT Can set/get the timeout
168
169
170For those drivers that return any bits set in the option field, the
171GETSTATUS and GETBOOTSTATUS ioctls can be used to ask for the current
172status, and the status at the last reboot, respectively.
173
174 int flags;
175 ioctl(fd, WDIOC_GETSTATUS, &flags);
176
177 or
178
179 ioctl(fd, WDIOC_GETBOOTSTATUS, &flags);
180
181Note that not all devices support these two calls, and some only
182support the GETBOOTSTATUS call.
183
184Some drivers can measure the temperature using the GETTEMP ioctl. The
185returned value is the temperature in degrees farenheit.
186
187 int temperature;
188 ioctl(fd, WDIOC_GETTEMP, &temperature);
189
190Finally the SETOPTIONS ioctl can be used to control some aspects of
191the cards operation; right now the pcwd driver is the only one
192supporting thiss ioctl.
193
194 int options = 0;
195 ioctl(fd, WDIOC_SETOPTIONS, options);
196
197The following options are available:
198
199 WDIOS_DISABLECARD Turn off the watchdog timer
200 WDIOS_ENABLECARD Turn on the watchdog timer
201 WDIOS_TEMPPANIC Kernel panic on temperature trip
202
203[FIXME -- better explanations]
204
205Implementations in the current drivers in the kernel tree:
206
207Here I have tried to summarize what the different drivers support and
208where they do strange things compared to the other drivers.
209
210acquirewdt.c -- Acquire Single Board Computer
211
212 This driver has a hardcoded timeout of 1 minute
213
214 Supports CONFIG_WATCHDOG_NOWAYOUT
215
216 GETSUPPORT returns KEEPALIVEPING. GETSTATUS will return 1 if
217 the device is open, 0 if not. [FIXME -- isn't this rather
218 silly? To be able to use the ioctl, the device must be open
219 and so GETSTATUS will always return 1].
220
221advantechwdt.c -- Advantech Single Board Computer
222
223 Timeout that defaults to 60 seconds, supports SETTIMEOUT.
224
225 Supports CONFIG_WATCHDOG_NOWAYOUT
226
227 GETSUPPORT returns WDIOF_KEEPALIVEPING and WDIOF_SETTIMEOUT.
228 The GETSTATUS call returns if the device is open or not.
229 [FIXME -- silliness again?]
230
Kumar Galaa2f40cc2005-09-03 15:55:33 -0700231booke_wdt.c -- PowerPC BookE Watchdog Timer
232
233 Timeout default varies according to frequency, supports
234 SETTIMEOUT
235
236 Watchdog can not be turned off, CONFIG_WATCHDOG_NOWAYOUT
237 does not make sense
238
239 GETSUPPORT returns the watchdog_info struct, and
240 GETSTATUS returns the supported options. GETBOOTSTATUS
241 returns a 1 if the last reset was caused by the
242 watchdog and a 0 otherwise. This watchdog can not be
243 disabled once it has been started. The wdt_period kernel
244 parameter selects which bit of the time base changing
245 from 0->1 will trigger the watchdog exception. Changing
246 the timeout from the ioctl calls will change the
247 wdt_period as defined above. Finally if you would like to
248 replace the default Watchdog Handler you can implement the
249 WatchdogHandler() function in your own code.
250
Linus Torvalds1da177e2005-04-16 15:20:36 -0700251eurotechwdt.c -- Eurotech CPU-1220/1410
252
253 The timeout can be set using the SETTIMEOUT ioctl and defaults
254 to 60 seconds.
255
256 Also has a module parameter "ev", event type which controls
257 what should happen on a timeout, the string "int" or anything
258 else that causes a reboot. [FIXME -- better description]
259
260 Supports CONFIG_WATCHDOG_NOWAYOUT
261
262 GETSUPPORT returns CARDRESET and WDIOF_SETTIMEOUT but
263 GETSTATUS is not supported and GETBOOTSTATUS just returns 0.
264
265i810-tco.c -- Intel 810 chipset
266
267 Also has support for a lot of other i8x0 stuff, but the
268 watchdog is one of the things.
269
270 The timeout is set using the module parameter "i810_margin",
271 which is in steps of 0.6 seconds where 2<i810_margin<64. The
272 driver supports the SETTIMEOUT ioctl.
273
274 Supports CONFIG_WATCHDOG_NOWAYOUT.
275
276 GETSUPPORT returns WDIOF_SETTIMEOUT. The GETSTATUS call
277 returns some kind of timer value which ist not compatible with
278 the other drivers. GETBOOT status returns some kind of
279 hardware specific boot status. [FIXME -- describe this]
280
281ib700wdt.c -- IB700 Single Board Computer
282
283 Default timeout of 30 seconds and the timeout is settable
284 using the SETTIMEOUT ioctl. Note that only a few timeout
285 values are supported.
286
287 Supports CONFIG_WATCHDOG_NOWAYOUT
288
289 GETSUPPORT returns WDIOF_KEEPALIVEPING and WDIOF_SETTIMEOUT.
290 The GETSTATUS call returns if the device is open or not.
291 [FIXME -- silliness again?]
292
293machzwd.c -- MachZ ZF-Logic
294
295 Hardcoded timeout of 10 seconds
296
297 Has a module parameter "action" that controls what happens
298 when the timeout runs out which can be 0 = RESET (default),
299 1 = SMI, 2 = NMI, 3 = SCI.
300
301 Supports CONFIG_WATCHDOG_NOWAYOUT and the magic character
302 'V' close handling.
303
304 GETSUPPORT returns WDIOF_KEEPALIVEPING, and the GETSTATUS call
305 returns if the device is open or not. [FIXME -- silliness
306 again?]
307
308mixcomwd.c -- MixCom Watchdog
309
310 [FIXME -- I'm unable to tell what the timeout is]
311
312 Supports CONFIG_WATCHDOG_NOWAYOUT
313
314 GETSUPPORT returns WDIOF_KEEPALIVEPING, GETSTATUS returns if
315 the device is opened or not [FIXME -- I'm not really sure how
316 this works, there seems to be some magic connected to
317 CONFIG_WATCHDOG_NOWAYOUT]
318
319pcwd.c -- Berkshire PC Watchdog
320
321 Hardcoded timeout of 1.5 seconds
322
323 Supports CONFIG_WATCHDOG_NOWAYOUT
324
325 GETSUPPORT returns WDIOF_OVERHEAT|WDIOF_CARDRESET and both
326 GETSTATUS and GETBOOTSTATUS return something useful.
327
328 The SETOPTIONS call can be used to enable and disable the card
329 and to ask the driver to call panic if the system overheats.
330
331sbc60xxwdt.c -- 60xx Single Board Computer
332
333 Hardcoded timeout of 10 seconds
334
335 Does not support CONFIG_WATCHDOG_NOWAYOUT, but has the magic
336 character 'V' close handling.
337
338 No bits set in GETSUPPORT
339
340scx200.c -- National SCx200 CPUs
341
342 Not in the kernel yet.
343
344 The timeout is set using a module parameter "margin" which
345 defaults to 60 seconds. The timeout can also be set using
346 SETTIMEOUT and read using GETTIMEOUT.
347
348 Supports a module parameter "nowayout" that is initialized
349 with the value of CONFIG_WATCHDOG_NOWAYOUT. Also supports the
350 magic character 'V' handling.
351
352shwdt.c -- SuperH 3/4 processors
353
354 [FIXME -- I'm unable to tell what the timeout is]
355
356 Supports CONFIG_WATCHDOG_NOWAYOUT
357
358 GETSUPPORT returns WDIOF_KEEPALIVEPING, and the GETSTATUS call
359 returns if the device is open or not. [FIXME -- silliness
360 again?]
361
362softdog.c -- Software watchdog
363
364 The timeout is set with the module parameter "soft_margin"
365 which defaults to 60 seconds, the timeout is also settable
366 using the SETTIMEOUT ioctl.
367
368 Supports CONFIG_WATCHDOG_NOWAYOUT
369
370 WDIOF_SETTIMEOUT bit set in GETSUPPORT
371
372w83877f_wdt.c -- W83877F Computer
373
374 Hardcoded timeout of 30 seconds
375
376 Does not support CONFIG_WATCHDOG_NOWAYOUT, but has the magic
377 character 'V' close handling.
378
379 No bits set in GETSUPPORT
380
381w83627hf_wdt.c -- w83627hf watchdog
382
383 Timeout that defaults to 60 seconds, supports SETTIMEOUT.
384
385 Supports CONFIG_WATCHDOG_NOWAYOUT
386
387 GETSUPPORT returns WDIOF_KEEPALIVEPING and WDIOF_SETTIMEOUT.
388 The GETSTATUS call returns if the device is open or not.
389
390wdt.c -- ICS WDT500/501 ISA and
391wdt_pci.c -- ICS WDT500/501 PCI
392
393 Default timeout of 60 seconds. The timeout is also settable
394 using the SETTIMEOUT ioctl.
395
396 Supports CONFIG_WATCHDOG_NOWAYOUT
397
398 GETSUPPORT returns with bits set depending on the actual
399 card. The WDT501 supports a lot of external monitoring, the
400 WDT500 much less.
401
402wdt285.c -- Footbridge watchdog
403
404 The timeout is set with the module parameter "soft_margin"
405 which defaults to 60 seconds. The timeout is also settable
406 using the SETTIMEOUT ioctl.
407
408 Does not support CONFIG_WATCHDOG_NOWAYOUT
409
410 WDIOF_SETTIMEOUT bit set in GETSUPPORT
411
412wdt977.c -- Netwinder W83977AF chip
413
414 Hardcoded timeout of 3 minutes
415
416 Supports CONFIG_WATCHDOG_NOWAYOUT
417
418 Does not support any ioctls at all.
419