[Bug 60606] vga_switcheroo / new Radeon DPM code mess up fbcon

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=60606

--- Comment #2 from Sebastien Fievet <sebastien.fievet@free.fr> ---
After a *lot* of googling effort, I managed to narrow down and find a
workaround to this bug.
- First, my mistake was to have an xorg.conf.d directory defining the video
drivers. 
  Letting X auto detect the driver was the necessary first step.

- Second I applied the "drm/i915: do not disable backlight on vgaswitcheroo
switch off" patch referenced in
https://bugs.freedesktop.org/show_bug.cgi?id=59785
  It made switching from IGD to discrete functional, but switching away from
IGD with discrete OFF was still screwing up the display.

- Finally, I found
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1077675, which was
reporting the exact same bug as I was experiencing.
  Same hardware, same issue. Same workaround : echoing MIGD to
/sys/kernel/debug/vgaswitcheroo/switch restores display when discrete is
  switched back ON

- I am now on 3.11.0-rc4 + drm/i915 patch and everything works fine (with the
workaround)

Conclusion :
  1. DPM is not involved at all.
  2. Switcheroo doesn't fully support Acer 3820TG hybrid (muxed) laptop out of
the box, but workaround exists.

If anyone wants to investigate further, I'll be glad to provide any relevant
information.
Thanks,
Seb.

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