Memory map on custom MPC7400 board

Greg Griffes ggg at
Tue Mar 26 22:35:54 EST 2002


Thanks very much for your advice.  You have saved me a big headache!

> I really recommend using the linuxppc_2_4_devel tree as a starting
> kernel source base...but it's your choice.

I will try using the linuxppc_2_4_devel tree.  Unforeseen problems I don't need.

> Do you really have to map a contiguous 384MB of physical address
> space? If these are typical RTC and PIC parts...why?

The RTC is typical but the PIC is split between a custom ASIC and
another device.  The RTC is at 0xE400_0000phys, one part of the PIC
is at 0xEC00_0000phys and the other part is at 0xF700_0000phys.
Their size is small, but they are spread out.  If I have to map them with
BATs, I would have to map 384Mb (because the BAT base address
must be a multiple of the mapped size.) Once the KVM is up,
I think that I can use ioremap(), or io_block_mapping() to map in
only what I need.  Is that right?

> You are misunderstanding the mapping of kernel RAM using BATs. The
> 16MB mapping is a temporary translation used before MMU_init().
> If you look at arch/ppc/mm/ppc_mmu.c:bat_mapin_ram() you'll see
> that the final mapping is done using BAT2 and BAT3 (the third
> pair of bats is only used if total_lowmem > 256MB).

Does this mean that before MMU_init() I can use BATs 2&3, then in
ppc_md.setup_io_mappings() (called after mapin_ram()) I would use
io_block_mapping() (as is done in chrp_setup.c?)

This helps a lot!  Thanks again for your advice.

Greg Griffes

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