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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 A more advanced driver could for example check that a HTTP server is
still responding before doing the write call to ping the watchdog. still responding before doing the write call to ping the watchdog.
When the device is closed, the watchdog is disabled. This is not When the device is closed, the watchdog is disabled, unless the "Magic
always such a good idea, since if there is a bug in the watchdog Close" feature is supported (see below). This is not always such a
daemon and it crashes the system will not reboot. Because of this, good idea, since if there is a bug in the watchdog daemon and it
some of the drivers support the configuration option "Disable watchdog crashes the system will not reboot. Because of this, some of the
shutdown on close", CONFIG_WATCHDOG_NOWAYOUT. If it is set to Y when drivers support the configuration option "Disable watchdog shutdown on
compiling the kernel, there is no way of disabling the watchdog once close", CONFIG_WATCHDOG_NOWAYOUT. If it is set to Y when compiling
it has been started. So, if the watchdog daemon crashes, the system the kernel, there is no way of disabling the watchdog once it has been
will reboot after the timeout has passed. Watchdog devices also usually started. So, if the watchdog daemon crashes, the system will reboot
support the nowayout module parameter so that this option can be controlled after the timeout has passed. Watchdog devices also usually support
at runtime. 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' Magic Close feature:
has been sent /dev/watchdog just before closing the file. If the userspace
daemon closes the file without sending this special character, the driver If a driver supports "Magic Close", the driver will not disable the
will assume that the daemon (and userspace in general) died, and will stop watchdog unless a specific magic character 'V' has been sent to
pinging the watchdog without disabling it first. This will then cause a /dev/watchdog just before closing the file. If the userspace daemon
reboot if the watchdog is not re-opened in sufficient time. 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: The ioctl API: