v6 software reset fails on 1176

[Will Deacon]

On Tue, Aug 23, 2011 at 05:32:47PM +0100, Jamie Iles wrote:
> Hi Will,

Hi Jamie,

> I'm trying to use the cpu_v6_reset that you added in "ARM: proc: add 
> definition of cpu_reset for ARMv6 and ARMv7 cores", but I've found that 
> on my 1176 platform, it never gets to the branch to the reset vector.  

Ok. How are you calling cpu_v6_reset? If you call it via arch_reset from
arm_machine_restart then there should be an identity mapping in place, so
you need to ensure that the reset code is called via this mapping in your
implementation of arch_reset.

Unfortunately, the current flat mapping only covers userspace, so it relies
on the physical address of the reset code not aliasing with the kernel virtual
addresses.

I have some (experimental) patches to fix this in my kexec branch:

http://www.linux-arm.org/git?p=linux-2.6-wd.git;a=shortlog;h=refs/heads/kexec-mmu-off

> Removing the ISB allows the branch instruction to be in the pipeline by 
> the time the MMU is disabled, but I'm not sure if this is the correct 
> fix.  Having said that, I don't see how this can work with an ISB in 
> there.

With modern CPUs, you can't rely on characteristics of the pipeline to play
tricks like this. Instead, you need to ensure that the reset code is
executed with a 1:1 mapping.

Will