linuxppc_2_4_devel Memory map on PPC / MPC5200

Sylvain Munaut tnt at
Mon Mar 8 22:50:43 EST 2004


I've got more general question about the memory mapping on the MPC5200.

Gerhard Jaeger wrote:

>>My questions currently are :
>> - Apparently 0xf0000000 is mapped to somewhere ( the MBAR ) but I don't
>>see where it's done ... And how do I tell it that it's IO and should not
>>be cached. Apparently on 2.4 it works but I don't see where is the piece
>>of code that just does that ...
>checkout kernel/mpc5xxx_common.c
Thanks, I've seen this but that code is executed way later. I'm talking
about the mapping that exists at near the kernel entry point, in head.S.
Maybe it's just a residual mapping of U-Boot, I should check that.

But talking about this, I see in mpc5xxx_common

void __init
        io_block_mapping(0x40000000, 0x40000000, 0x10000000, _PAGE_IO);
        io_block_mapping(0x50000000, 0x50000000, 0x01000000, _PAGE_IO);
        io_block_mapping(0x80000000, 0x80000000, 0x10000000, _PAGE_IO);
        io_block_mapping(0xf0000000, 0xf0000000, 0x10000000, _PAGE_IO);

I've read the relevant part of the MPC5200 manual and still don't
understand ...

0x80000000 is the default address of the MBAR. But AFAIK, MBAR is
changed by U-Boot to remap the registers to 0xf0000000 ( physical
address space, not remapped using the mmu ).
So I see the last line as a 1:1 mapping of the internal mpc5200
registers. And looking at all the consts, it seems correct.
But then, what are the 3 other mappings ?

Also, to be sure of what I understood. When the 0xf0000000 area is 1:1
mapped via the BAT, I suppose it's just to get serial debug during early
init and the map_io call. Because the BAT2 is gonna be overwritten when
the kernel maps all the physical memory. I'm not sure whether the call
to map_io occurs after or before that. I suppose it's after, else the
same mapping would exists in two different ways and that's discribed as
a bad situation in the user manual.

Finally ( I know, I have a lot of questions ), the kernel doesn't use
the HIGH_BATS features ? I mean there is 8 BAT on this processor and I
don' t see where the 5 6 7 8 are gonna be used ? io_block_mapping could
be slightly modified to use them as well or am I wrong ?


    Sylvain Munaut

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