m8xx hang in reboot

Dan Malek dan at embeddededge.com
Thu Aug 26 13:29:00 EST 2004

On Aug 25, 2004, at 4:22 PM, Mark S. Mathews wrote:

> I've got an mpc850 based setup running 2.4.27 that, under certain
> circumstances, hangs in the the m8xx_restart() function (instead of
> rebooting) when I issue the usermode reboot cmd.


> I've traced all through the path and confirmed that yes, it is hanging
> in
> this function.  ;-)

Does it get as far at the printk that indicates the checkstop failed?

> I'm still looking over the databook and what this code does.  Just
> figured
> I'd ask around and see if anyone else has seen this behavior.

It's supposed to enable a "checkstop" reset, such that on a bad
bus cycle that times out the processor will be reset.  Then, it
forces a bad bus cycle.  Since it works in most cases, I suspect
the problem is outside of the processor.

This type of reset will cause the processor reset, but it will
not reset the rest of the hardware on the board.  It seems there is
something external on the board that must be in a state that
prevents the bus from operating properly.  The processor
resets, but can't fetch code properly.

This reset function is generic to all 8xx processors, in that it
will reset the processor.  Done properly, a system design will
have some external control register that can be written to
reset the entire board, just like a power up or pressing a
reset switch.  Without such complete system reset, this
is the best we can do, but it certainly is not the same as
a full reset.

	-- Dan

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