[Bug 70687] vgaswitcheroo issues on Linux 3.12

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=70687

--- Comment #13 from Armin K <krejzi at email.com> ---
I seem to have discovered the root of the issue.

I've just built 3.13-rc5 kernel which has the dynamic powering of the discrete
gpu and all hell broke loose.

I've narrowed the error down to the pci hotplug driver. My machine loads shpchp
pci hotplug driver from what I can see in lsmod output. But the trick is, that
there is another pci hotplug driver, acpi pci hotplug one, which seems to break
all hell loose here. Disabling it seems to fix everything for me, at least on
kernel 3.13.

# CONFIG_HOTPLUG_PCI_ACPI is not set

This kernel config option is the culprit for this, and that also can be seen
from my backtrace:

[   22.731998]  [<ffffffff81343cb1>] ? acpiphp_check_bridge+0x72/0x88

So the trick behind this is that acpi pci hotplug driver conflicts with shpchp
one that my machine uses. And since it is a builtin driver, and can't be built
as module it is always loaded. The other possibility is that this machine
doesn't support acpi hotplug, but does support shpc pci hotplug. We need a
kernel workarround so that acpi pci hotplug is disabled and out of the way when
shpc pci hotplug is enabled.

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