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[WATCHDOG] clarify watchdog operation in documentation

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It was not clear what the difference is/was between the 
nowayout feature and the Magic Close feature.

Signed-off-by: "Andrew Dyer" <amdyer@gmail.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
This commit is contained in:
Andrew Dyer 2008-01-08 14:40:37 -06:00 committed by Wim Van Sebroeck
parent cde10ba3ba
commit 0d710cba3a
1 changed files with 20 additions and 16 deletions
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36
Documentation/watchdog/watchdog-api.txt
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@ -42,23 +42,27 @@ like this source file: see Documentation/watchdog/src/watchdog-simple.c
A more advanced driver could for example check that a HTTP server is
still responding before doing the write call to ping the watchdog.
When the device is closed, the watchdog is disabled. This is not
always such a good idea, since if there is a bug in the watchdog
daemon and it crashes the system will not reboot. Because of this,
some of the drivers support the configuration option "Disable watchdog
shutdown on close", CONFIG_WATCHDOG_NOWAYOUT. If it is set to Y when
compiling the kernel, there is no way of disabling the watchdog once
it has been started. So, if the watchdog daemon crashes, the system
will reboot after the timeout has passed. Watchdog devices also usually
support the nowayout module parameter so that this option can be controlled
at runtime.
When the device is closed, the watchdog is disabled, unless the "Magic
Close" feature is supported (see below). This is not always such a
good idea, since if there is a bug in the watchdog daemon and it
crashes the system will not reboot. Because of this, some of the
drivers support the configuration option "Disable watchdog shutdown on
close", CONFIG_WATCHDOG_NOWAYOUT. If it is set to Y when compiling
the kernel, there is no way of disabling the watchdog once it has been
started. So, if the watchdog daemon crashes, the system will reboot
after the timeout has passed. Watchdog devices also usually support
the nowayout module parameter so that this option can be controlled at
runtime.
Drivers will not disable the watchdog, unless a specific magic character 'V'
has been sent /dev/watchdog just before closing the file. If the userspace
daemon closes the file without sending this special character, the driver
will assume that the daemon (and userspace in general) died, and will stop
pinging the watchdog without disabling it first. This will then cause a
reboot if the watchdog is not re-opened in sufficient time.
Magic Close feature:
If a driver supports "Magic Close", the driver will not disable the
watchdog unless a specific magic character 'V' has been sent to
/dev/watchdog just before closing the file. If the userspace daemon
closes the file without sending this special character, the driver
will assume that the daemon (and userspace in general) died, and will
stop pinging the watchdog without disabling it first. This will then
cause a reboot if the watchdog is not re-opened in sufficient time.
The ioctl API: