VGA arbitration: API proposal

[Egbert Eich]

Alan Cox writes:
 > On Sad, 2005-03-05 at 04:46, Jon Smirl wrote:
 > > mark all drivers as VGA or not
 > > only allow a single card in the system that is marked as requiring VGA
 > > require fbdev/fbcon on the non-VGA cards
 > 
 > Why ?
 > 
 > > if there is a second VGA required card, print a big message and refuse to boot
 > > discourage any further writing of code that uses VGA
 > 
 > Doesn't work in the real world.
 > 
 > There is a fairly easy vga API for the kernel and I proposed pretty much
 > that with the vga class driver stuff I sent folks to play with.
 > 
 > All you actually need is
 > 
 > 	/* Caller must ensure devices are VGA */
 > 	vga_conflicts(struct pci_dev *card1, struct pci_dev *card2)
 > 	vga_set_routing(struct pci_dev *card)
 > 
 > That is sufficient to encapsulate the low level wonders of the system.
 > PC
 > class devices have a vga_conflicts of "return 1" while the SGI's can do
 > smarter stuff. For most platforms vga_set_routing() becomes a pci
 > library call to shared code.

That's not unsimilar to the sematic we have in X. Just that
we do a
resource_conflicts(resource_type, state, card) to see if 'card' has any
resource conflict for a resource of 'type' during 'setup' or 'operating'
state. 
Furthermore we can do a vga_set_routing(NULL) to disable VGA routing 
to any bus.

 > 
 > At the higher level there are some small complexities I can find no way
 > to hide, one of which is that if one VGA device is locked in use you
 > must be very very careful if and where you sleep waiting for VGA
 > control. The practicalities of multiple kernel side vga consoles for
 > example are unpleasant.

Right. 

Egbert.