Re: [PATCH][RFC] AMD Elan patch

["H. Peter Anvin" <hpa@zytor.com>]

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.)

	-hpa