ARM : Kernel : Setting up of MMU in head.S

[Prakash K.B.]

Hi Dave.

On Thu, Mar 31, 2011 at 7:29 PM, Dave Hylands <dhylands at gmail.com> wrote:

> Hi Prakash,
>
> On Wed, Mar 30, 2011 at 11:01 PM, Prakash K.B. <prakashk75 at gmail.com>
> wrote:
> > Merci mate. :-)
> >
> > On Thu, Mar 31, 2011 at 3:59 AM, Dave Hylands <dhylands at gmail.com>
> wrote:
> >>
> >> Hi Prakash,
> >>
> >> On Wed, Mar 30, 2011 at 2:35 PM, Dave Hylands <dhylands at gmail.com>
> wrote:
> ...snip...
> >> When the MMU table is turned on, the PC is still at 0x800xxxx, so even
> >> though the kernel has 0xc00xxxxx mapped to 0x800xxxxx it also has to
> >> have 0x800xxxxx mapped to 0x800xxxxx.
> >
> > [Prakash] I think you meant to say "So even though the kernel intends to
> map
> > 0xc00XXXX to 0x800XXX in the future, it has currently mapped 0x800xxx to
> > 0x800xxx.
>
> No. the create_page_table function creates a page table which has
> 0x800xxxxx and 0xc00xxxxx both mapped to 0x800xxxxx. The one store
> intrustion saves the identity mapping (for 1 Mb) and the loop sets up
> the 0xc00xxxxx to 0x800xxxxx mapping.
>
> The identity portion is used as we discussed, and a few instructions
> after turning on the MMU, the CPU then does a jump from the 0x800xxxxx
> space to the 0xc00xxxxx space. After that, the identity mapping is no
> longer needed. Shortly after this, head.S calls into the start_kernel
> function (from init/main.c) and the paging_init function reinitializes
> the MMU removing the identity mapping.
>

[Prakash] I confirm I now see this exactly as you describe above and below.
Thanks a bunch Dave.

>
> >
> > Now that I know this identity mapping is done on purpose, I hope to make
> > good progress with the succeeding sequence.
> >
> > Do you confirm that only one entry is written into this L1 table because
> > both mmu_enable and enable_mmu_end are on the same section?
>
> Yeah - essentially, that one mapping entry covers 1Mb of code space,
> which is sufficient to cover all of head.S, which is always at the
> front of the kernel image.
>
> --
> Dave Hylands
> Shuswap, BC, Canada
> http://www.davehylands.com
>

-Prakash
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