[U-Boot] [PATCH] atngw100: Use virtual address in CONFIG_ENV_ADDR

[Wolfgang Denk]

Dear Haavard Skinnemoen,

In message <20090831155327.62b58f8f at hskinnemoen-d830> you wrote:
>
> Possibly. But it means even more effort and even larger code, so I'm
> not exactly happy about it.

Really? Sorry if I'm asking dumb questions - I don't know  AVR32:  is
it  true  that  stting  up a non-1:1 mapping for the NOR flash (i. e.
what you are doing now) is easier (less code) than setting up  a  1:1
mapping? What exactly are the reasons for this?

> > Indeed I am, and intentionally, because this is a different topic. If
> > your system requires to set up the MMU to enable  caching,  then  you
> > are  supposed  to  do  it  in  a  way compatible with the rest of the
> > software design, i. e. as transparently  as  possible.  None  of  the
> > architectures  I  know resonably well have problems setting up a 1: 1
> > address mapping even when the MMU is on (but I  have  to  admit  that
> > AVR32 is not among these architectures).
> 
> I suspect quite a few other architectures run with caches disabled
> because it's not safe to run with caches enabled with the current
> software design.

Well, usually we run with IC on and  with  DC  off,  usually  because
quite  a  number  of  drivers  and  other  code do not use proper I/O
accessors yet, and/or because it's easier and  nobody  really  cares.
For  example  on  PowerPC,  adding support for the data cache usually
gives only a minimal performance boost.  This  may  be  different  on
other architectures.

> > Cache should not be relevant  at  all  when  defining  a  physcal  or
> > virtual memory map.
> 
> Physical, no, but it's very common that the MMU defines caching
> properties (enabled/disabled, writeback/writethrough, etc.)

Agreed. But it should be not so difficult to use the MMU to set up  a
1:1  mapping  if  you have to set up the MMU at all - or is there any
problems with that which I'm not aware of?

> > Heh. If more  platforms had broken this rule we would probably have
> > become aware of these violations earlier, and stopped them doing such
> > naughty things ;-)
> 
> Seems like you think it's more important to follow arbitrary rules than
> writing code that works well.

Keeping the code as simple as possible is not exactly an arbitrary
rule. At least not for me.


Best regards,

Wolfgang Denk

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