Re: [PATCH][RFC] AMD Elan patch

From: H. Peter Anvin (
Date: Tue Jan 01 2002 - 19:48:31 EST

Christer Weinigel wrote:

> Alternate Gate A20 Control Register (Port 00EEh) A special 8-bit
> read/write control register provides a fast and reliable way to
> control the CPU A20 signal. A dummy read of this register returns
> a value of FFh and forces the CPU A20 to propagate to the core
> logic, while a dummy write to this register will cause the CPU A20
> signal to be forced Low as long as no other A20 gate control
> sources are forcing the CPU A20 signal to propagate.
> I think it's safe to assume that it takes effect immediately.

What leads you to that conclusion? There is nothing in the above
paragraph that would lead you to believe that.

>>I'm also very uncomfortable with putting this where you do; I think it
>>should be put before a20_kbc instead. If the BIOS is implemented
>>correctly, it should be used.
> I disagree, the Elan SC410 is an embedded CPU, it's used in systems
> that might not even have a BIOS (such as the Ericsson eBox that I've
> been working with). Since there has to be a config option for this
> CPU (the clock frequency selection) and a SC410-enabled kernel won't
> work properly on a normal PC anyway, why not modify the boot sequence
> so that it _always_ works on this CPU.

If used sans BIOS you need *massive* modifications to the setup anyway,
and you're talking a completely custom kernel. The reason to use the
BIOS first is to give the platform vendor a hook to deal with
platform-specific issues, and God knows there are plenty of those when
it comes to A20. (The fact that the BIOSes on the boards we have
discussed haven't provided such a hook is a bug in itself.)


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