Re: Stupid NVIDIA 3D vgaarb.c patch

From: Pasi Kärkkäinen
Date: Sat Sep 27 2014 - 07:11:44 EST

On Mon, Sep 22, 2014 at 06:10:50PM -0400, Alex Deucher wrote:
> >
> > With dual VGA controllers, we can change VGA routing in the chipset so
> > that we can address one device or the other using the VGA address space.
> > This lets things like Xorg switch between cards to initialize a card via
> > the VGA BIOS or execute runtime VGA BIOS calls. If one of those devices
> > reports itself as a 3D controller, then we have no reason to believe
> > that the device actually responds to VGA accesses or that the PCI ROM
> > payload includes a VGA BIOS.
> >
> > Maybe that's how the arbiter device registration should work, if the
> > class code is VGA add it, if it's 3D controller, switch VGA routing and
> > see if it responds to VGA. There are still a couple problems that I've
> > been beating my head against though. First, i915 lies when it opts-out
> > of VGA arbitration, so you might think you've found a VGA device
> > elsewhere, but you're actually still talking to IGD. Second is the Xorg
> > DRI problem where it's going to disable DRI support if suddenly a second
> > arbitration participant appears. This is what happened when I tried to
> > fix the i915 problem and suddenly anybody with IGD + plugin graphics
> > lost DRI and the fix was reverted. The whole thing is pretty broken.
> >
> > In this case, if your laptop actually supports disabling and hiding the
> > IGD device, I'd expect the Nvidia device to switch to reporting itself
> > as VGA. It seems like often the Optimus graphics aren't even directly
> > connected to an output device, which makes me curious that you can
> > actually pick one or the other. Thanks,
> AFAIK, on most recent (like in the last 2-3 years) hybrid laptops
> don't really have the option to boot to the dGPU or even disable the
> integrated graphics (I'm not sure that was ever possibly, even on the
> old muxed ones). Depending on the OEM, the dGPU may be wired to one
> or more of the external ports, but I don't think those are usable
> until after the drivers have loaded. Newer radeon PowerXpress laptops
> mark the dGPU as PCI_CLASS_DISPLAY_OTHER and I don't think there is
> any expectation that they should be messing with vga.

I had Lenovo T430 hybrid graphics laptop earlier, which is in "last 2-3 years",
and it allowed me to choose "IGD only", "Optimus", or "Nvidia only" in BIOS..

-- Pasi

> Alex

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at