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

[prakashk75@gmail.com (Prakash K.B.)]

Merci mate. :-)

On Thu, Mar 31, 2011 at 3:59 AM, Dave Hylands <dhylands@gmail.com> wrote:

> Hi Prakash,
>
> On Wed, Mar 30, 2011 at 2:35 PM, Dave Hylands <dhylands@gmail.com> wrote:
> > Hi Prakash,
> >
> > On Wed, Mar 30, 2011 at 8:19 AM, Prakash K.B. <prakashk75@gmail.com>
> wrote:
> >> Hello.
> >>
> >> Please do not hesitate to let me know if this must be posted elsewhere.
> >>
> >> I have been trying to understand the code that sets up the MMU. I do
> have a
> >> fair understanding of the way MMU is meant to be setup, but something in
> the
> >> kernel code is tripping me.
>
> Some further explanation is due.
>
> When the kernel starts, the MMU is off, and ther ARM is running with
> an implicit identity mapping (i.e. each virtual address maps to the
> same physical address).
>
[Prakash] Aha...So what I ignored as a routine code comment had a deeper
meaning.. :-)

>
> If your physical memory starts at 0x80000000, then the PC will be
> 0x800xxxxx.
>
[Prakash] Agreed.

>
> 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.

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?

>
> So this mapping of 0x800xxxxx to 0x800xxxxx is the "identity" portion
> and is needed while switching the MMU on. The 0xc00xxxxx to 0x800xxxxx
> mapping is what's used while the kernel is running.
>
> --
> Dave Hylands
> Shuswap, BC, Canada
> http://www.davehylands.com
>

-Prakash
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