[Bug 93281] Kernel modesetting causes the kernel to lock up during boot on a late 2011 MacBook Pro

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=93281

Roland Scheidegger <rscheidegger at gmx.ch> changed:

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                 CC|                            |rscheidegger at gmx.ch

--- Comment #11 from Roland Scheidegger <rscheidegger at gmx.ch> ---
FWIW I've got the same notebook and I can confirm the gmux is problematic (the
apple-gmux driver should support it in theory). Though I don't think it ever
hanged for me, but you can easily lose the display on bootup depending on how
you boot - there's essentially 3 different methods:
1) via CSM bios emulation. In this case you will get working AMD graphics, with
no ability to switch to the intel one (it does not even appear in the pci
device list).
2) via direct efi boot (refind or similar). This will get you working AMD
graphics like the above, with the theoretical ability to switch to the intel
one with vgaswitcheroo as it is now properly enumerated and the apple-gmux
driver used, however switching does not actually work (it seems the intel
graphics only works if it was active on boot in my experiments). (*)
3) via grub-efi (looks like this is what you do?). Unfortunately this will not
work without extra grub parameters, because the radeon graphics bios cannot be
retrieved (though last I looked there was some discussion about fixing this in
grub, so it may work with a newer grub version). The grub parameters are to
force the mux to switch to intel graphics, in which case you get working intel
(but not amd) graphics, though you won't see initial bootup messages before kms
is loaded. (Note that after suspend/resume, the AMD chip will be powered on but
unused again for some needlessly higher (IIRC over 5W) power consumption,
though you can write a hack app to force it off again.)
This page has some information on this - in particular the 4 outb commands for
the grub config:
http://blog.tkassembled.com/364/intel-graphics-on-a-2011-macbook-pro-in-linux/

(*) I actually got switching graphics to work as a proof-of-concept by using
direct efi boot and forcing the gmux to intel by using the efi console, though
halfway through you will lose the screen in there and have to remember what
keystrokes are needed to select the right boot entry in the refind menu...

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