[coreboot] Fwd: SimNOW VGA int 1a

[Carl-Daniel Hailfinger]

On 16.10.2008 21:56, ron minnich wrote:
> I did not realize we had gone private. see below.
>
> Basically, vga bios tries to size a 16 meg register and at an
> intermediate point the vga hardware ends up decoding the top 16 mb of
> memory.
>
> I welcome a fix. It's just not obvious to me.
>   

Please help me understand this better. Is this a bug of real hardware, a
bug of an emulator or an intended design feature?

Depending on what it is, we could employ various ways to fix it.

Regards,
Carl-Daniel

> ---------- Forwarded message ----------
> From: ron minnich <rminnich at gmail.com>
> Date: Thu, Oct 16, 2008 at 12:15 PM
> Subject: Re: [coreboot] SimNOW VGA int 1a
> To: Marc Jones <marc.jones at amd.com>
> Cc: Myles Watson <mylesgw at gmail.com>, Jordan Crouse <jordan.crouse at amd.com>
>
>
> On Thu, Oct 16, 2008 at 12:08 PM, Marc Jones <marc.jones at amd.com> wrote:
>   
>> ron minnich wrote:
>>     
>
>   
>>> well, hang on.
>>>
>>> I write ffffffff to BAR 10.
>>> Then what is left in BAR 10 is ff000000. That decodes to the top 16 MB
>>> of address space. At that point, all the memory goes bye bye.
>>> So do we really want the device enabled?
>>>
>>> Is this maybe a bug in the vga bios? This won't be an issue for code
>>> not running in the top 16 MB
>>>
>>>       
>> Yes it goes away but nothing should access it then. Put the BAR back and it
>> should be fine.
>>     
>
> but the EIP is accessing it then. We're running code at ffffxxxx.
>
> Here is the sequence:
>
> we're running at ffffxxxx. We write ffffffff at request of vgabios to
> the BAR 10. At that point vga is decoding ffxxxxxx.
>
> We can no longer fetch code from ffffxxxx. We go bye bye.
>
> Fix is to NOT compile the pcibios into stage0, or to also decode it into stage2.
>
> ron
>
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>   


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