Re: [RFC] BOOKE watchdog and kexec

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)

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