[RFC] BOOKE watchdog and kexec

[Michael Ellerman]

On Tue, 2007-05-22 at 16:53 -0700, Dave Jiang wrote:
> What would be the appropriate way to deal with the BOOKE watchdog in order to
> properly kexec? The BOOKE watchdog cannot be disabled. With the current
> implementation, a watchdog daemon in userland is required to poke the
> /dev/watchdog continously in order to keep it from going off. In the kexec
> situation, the watchdog daemon in userland goes away when the new kernel is
> executed. It is very possible that the new kernel can potentially timeout on a
> certain hardware device initialization (i.e. SCSI discovery/timeout) and causes
> the watchdog to go off and reset the hardware. The reset is of course not
> wanted in this situation.
> 
> Several solutions comes into mind:
> 1. Have the kernel timer poke the watchdog. This would ensure situation
> described above would never happen. I think x86 does this with NMI watchdog.
> 
> 2. Have the watchdog driver spawn a kernel thread to poke the watchdog at a
> periodic time. Or perhaps use the delayed-work mechanism to do that.
> 
> 3. Set the highest bit of the watchdog register so that it does not expire for
> 2^32 ticks.

#3 sounds the easiest. You'd set it in machine_kexec_prepare() and then
have the second kernel restore a sane value. I assume 2^32 ticks is long
enough to boot?

cheers

-- 
Michael Ellerman
OzLabs, IBM Australia Development Lab

wwweb: http://michael.ellerman.id.au
phone: +61 2 6212 1183 (tie line 70 21183)

We do not inherit the earth from our ancestors,
we borrow it from our children. - S.M.A.R.T Person
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