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

Wolfgang Denk
2009-08-31T17:46:10Z

Dear Haavard Skinnemoen,

In message <20090831155327.62b58f8f@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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