check MMU state

Benjamin Herrenschmidt benh at
Sun Apr 6 01:24:06 EST 2003

On Sat, 2003-04-05 at 18:15, Thomas Gleixner wrote:
> On Saturday 05 April 2003 16:49, Benjamin Herrenschmidt wrote:
> > > Is there a clean way to check, if the MMU is enabled on kernel entry ? I
> > > have to port Linux to a PMC-module, where the bootloader code should not
> > > be changed.
> >
> > The kernel entry can cope with both cases, provided that early_init
> > properly returns the physical address where the kernel is loaded,
> > regardless of the translations setup by the bootloader. Also, the
> > kernel has to be contiguous in physical memory.
> I don't reach early_init. The CPU is a 860 and I'm using 2.4.20-ben8. I get
> lost in initial_mmu. If I turn off the mmu before "bl initial_mmu" everything
> is fine.

Ah well, I was talking about the "common" CPU case (CONFIG_6xx), I don't
know about 8xx but since the first thing the code  in head_8xx.S does is
"tlbia", I think you are in a bad situation running that with
translation enabled and no vectors around to handle the TLB miss...

For machines without BATs, I tend to think that a bootloader loading
you with MMU on shall be shot. What I'd suggest is that you hack a piggy
back loader in the zImage wrapper that shuts MMU off before entering the
kernel. If you _have_ to run vmlinux as is, then stuff a bl to some code
to disable the MMU before reaching initial_mmu in head_8xx.S


** Sent via the linuxppc-dev mail list. See

More information about the Linuxppc-dev mailing list