RFC: Deprecating io_block_mapping

Mark A. Greer mgreer at mvista.com
Fri May 27 06:30:04 EST 2005

Benjamin Herrenschmidt wrote:

> - There is _one_ important point to keep in mind, but that has always
>been true: None of this work before MMU_init(), 

This is very true and raises a couple issues that we should fix while 
we're at it:

1) There are progress calls in MMU_init which will try to access the 
uart before its possible to create a mapping to the uart's regs 
(assuming you don't make a hack to map them and that you set up 
ppc_md.progress in your platform_init routine).  We should either get 
rid of those calls in MMU_init, provide an acceptable way to make 
temporary pre-MMU_init mappings, or make sure nobody sets up 
ppc_md.progress until ioremap is working (and also get rid of the calls 
in MMU_init b/c they're never used).

2) Some firmwares don't provide any info on how much memory is in the 
system but MMU_init needs to know that.  So the platform code has to 
read the SPD from the mem sticks via i2c, read the mem ctlr, or read a 
board reg that has the info.  All of those require access to hw regs 
before or during MMU_init.  I should be able to get rid of this one by 
figuring out the amount of memory in the bootwrapper and passing it in 
to the kernel.  I am assuming that all the boards with this problem use 
the bootwrapper.  I think that's a safe assumption but I'll have to verify.

BTW, these are the reasons that I made that set_bat hack that Dan is so 
fond of.  :)  I'll get rid of that hack but I need an answer to 1) first.


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