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From: Segher Boessenkool <segher@kernel.crashing.org>
Subject: Re: use of BAT before taking over the MMU
Date: Tue, 5 Oct 2010 09:54:35 +0200
To: Albert Cahalan <acahalan@gmail.com>
Cc: linuxppc-dev@lists.ozlabs.org
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>> The PowerPC OF binding requires the firmware to save and restore
>> the BATs on entry to / exit from the firmware.
>
> This sucks, because using the BAT is **much** easier for
> the firmware. In my case, it also means I don't need to worry
> about Linux stomping on anything -- I have nothing in RAM.

It's easy to save and restore the BATs on firmware entry/exit,
where do you see a problem?

> Page tables would need to go in RAM. If Linux wants to
> use that memory...?

Same way as when the client uses *any* memory OF has claimed:
it has to stop calling OF from then on.

> It seems that Linux does tend to ask;
> will it panic if I refuse? Are there addresses Linux won't
> ever ask for?

Linux always "kills" OF pretty early on.

> It also looks like I could just start up Linux w/o the MMU on.

Yes, that's common and works.  You will still need BATs (or
something else) to do I/O from the firmware though.


Segher