[Bug 117131] vga_switcheroo does not switch IGP -> DIS ( IGP == i915 , DIS == radeon )

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https://bugzilla.kernel.org/show_bug.cgi?id=117131

--- Comment #9 from Alex Deucher <alexdeucher at gmail.com> ---
(In reply to Jason Vas Dias from comment #8)
> As stated on the web-page quoted above , 
> ( https://wiki.archlinux.org/index.php/PRIME )
> an example xorg.conf to use the discrete card
> as primary GPU is:
> 

This is only possible if there are displays physically connected to the dGPU
(e.g., laptop panel is connected to IGP and HDMI is connected to dGPU).  I'd
need to see your full dmesg output, but I suspect there are no displays
physically connected to the dGPU on your system.  As such, there is no way to
make it the primary.

> 
> 
> My xorg.conf is basically the same with the string 'nouveau' 
> replaced by 'ati' .
> 
> It started out as simple as this, but then grew as I tried 
> every conceivable method to specify the modes ,  but still
> the X-server exits with a 'No modes.' error in every case .
> 
> The reason the X-server cannot determine the modes is because
> of this Linux bug - the radeon DRM driver makes incorrect ACPI
> calls.

No.  There are no displays or display connectors actually wired to the dGPU. 
This is what is called a mux-less laptop.  mux-less in that there is no mux to
switch the display connections between the IGP and the dGPU.

> 
> Also I think that if vga_switcheroo does not work or is 
> incapable of working , as it provably is in the above 
> example, it should return EIO on the write to the 
> debugfs control 'switch' file , and not proceed to
> display spurious data in that file.

vag-switcheroo should only be used on old muxed laptops where there was a mux
to switch the display routing.

As I said, the only way to use the dGPU on your system is to follow the "PRIME
GPU offloading" section of that page.

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