[Open-graphics] From PCI to VGA registers

["Ing. Daniel Rozsnyó"]

>> a) How do plain VGA registers (lots of documentation talks about
>> things like address 3B4h) get 'mapped' to the correct PCI device? Is
>> address 3B4h in C/BE#'s 'I/O Space', 'Memory Space', or some added
>> fields to the configuration space?
> 
> 3B4h is in I/O space.  And it's not in a BAR-mapped space.  When our
> PCI controller sees that address (and we've enabled VGA), it decodes a
> limited range of "legacy" I/O space addresses.  These are the only I/O
> space addresses we support.  All others are memory space.
> 
> Since these legacy addresses are not allocated to a particular device,
> there are certainly concerns in a PC system about more than one device
> attempting to accept them.  I think the correct thing to do is have
> the first VGA device figure out that it was initialized first (there's
> a certain order in which a PC BIOS initializes devices, with the
> console gfx card being very early) and be the only one to enable the
> decode.  (Enabling the decode would be device-dependent, I would
> imagine.)

The MB's BIOS initializes only one of the graphic cards. As far as I 
remember, the secondary card's boot message appeared only at the time 
when X was started in dualhead mode.

Which one is being initialized at boot depends on bios settings, usually 
you have a choice to init PCIe or PCI card first.

Result: we don't need to handle sharing of cards in any way. Reset = 
turn off everything and if the bios is run, then initialize the card. 
Question is how to detect which card's bios is being executed in case 
you have 2 cards on same bus :)