Re: Improper TTBCR for arm 32bit kernel decompression

[Srinivas Ramana]

On 09/09/2016 11:06 PM, Nicolas Pitre wrote:
> On Fri, 9 Sep 2016, Srinivas Ramana wrote:
>
> > Hello,
> >
> > While trying to boot arm-32 bit kernel, I came across a problem where TTBCR is
> > in improper state. If the bootloader uses the long descriptor format and jumps
> > to kernel decompressor code, TTBCR may not be in the right state. So, as soon
> > as the MMU is enabled, execution can not proceed further.
> >
> > Before enabling the MMU, it is required to clear the TTBCR.PD0 field to use
> > TTBR0 for translation table walks. Also, TTBCR.N should be reset to '0' to
> > indicate the correct base address width. The 'commit dbece45894d3a ("ARM:
> > 7501/1: decompressor: reset ttbcr for VMSA ARMv7 cores")' does the reset of
> > TTBCR.N, but doesn't consider all the bits for the size of TTBCR.N.
> >
> > when i tried the below change where i explicitly clear TTBCR.PD0 and use
> > correct mask for TTBCR.N, I see proper memory after MMU is enabled and
> > decompression succeeds.
> >
> > Request your comments on the change below. If it looks good, I can submit a
> > patch for inclusion.
> >
> > ---------------------8<----------------------------------
> > diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
> > index af11c2f..5769f1f 100644
> > --- a/arch/arm/boot/compressed/head.S
> > +++ b/arch/arm/boot/compressed/head.S
> > @@ -779,7 +779,8 @@ __armv7_mmu_cache_on:
> >                  orrne   r0, r0, #1              @ MMU enabled
> >                  movne   r1, #0xfffffffd         @ domain 0 = client
> >                  bic     r6, r6, #1 << 31        @ 32-bit translation system
> > -               bic     r6, r6, #3 << 0         @ use only ttbr0
> > +               bic     r6, r6, #7 << 0         @ width of base address field
> > +               bic     r6, r6, #1 << 4         @ use only ttbr0
>
> You could combine those instructions like this:
>
> 		bic	r6, r6, #(7 << 0) | (1 << 4) @ use only ttbr0

Sure, I can do that. Thanks for the suggestion. Will send out a patch 
for review.

Thanks,
-- Srinivas R

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