Re: Backlight control does not work on an Apple dual-GPU (intel/nvidia) system using nouveau

[Lukas Wunner <lukas@wunner.de>]

Hi Michael,

On Sat, Feb 13, 2016 at 10:36:47PM -0800, Michael Marineau wrote:
> On Feb 13, 2016 9:38 PM, "Matthew Garrett" <mjg59@srcf.ucam.org> wrote:
> >
> > On Sun, Feb 14, 2016 at 12:39:12AM +0100, Lukas Wunner wrote:
> > > @Bjorn Helgaas: Do you think this masquerade patch would be eligible
> > > for inclusion in the kernel or is this not in your department?
> > > https://www.marc.info/?l=grub-deavel&m=141586614924917&w=2
> >
> > I think this is an entirely appropriate piece of code to add to the EFI
> > boot code in the kernel.
> 
> One note on that code: making the Intel card visible exposes a number of
> other bugs such as power management with nouveau not working any more and
> despite vga switcheroo becoming available it isn't possible to switch to
> the Intel gpu because, at least on my machine, neither the Intel nor
> nouveau drivers can figure out how to bring up the eDP link or some such.

That is correct. GPU switching for pre-retinas was finally queued for
4.6 this week, retinas are my next target. To overcome the link training
issues I had experimented with AUX proxying last year until I realized in
December that the DP spec has a link training exemption for eDP. Basically
what we need to do is wait until the active GPU has set up its eDP output,
then communicate voltage swing, pre-emphasis, lane count etc to the
inactive GPU's driver (via vga_switcheroo) and use these pre-calibrated
values to set up the output. I've hacked together an initial implementation
of the i915 and vga_switcheroo side of things in early January, then got
side-tracked while working on the nouveau part. However I'll get back to
this pretty soon.

Best regards,

Lukas