Finally I found the courage to try commit bisecting the kernel. This is the result:
4eebd5a4e72697aac25a8a57d3f888a9d5f80370 is the first bad commit commit 4eebd5a4e72697aac25a8a57d3f888a9d5f80370 Author: Bruno Prémont <[email protected]> Date: Wed Mar 11 22:34:45 2015 +0100 apple-gmux: lock iGP IO to protect from vgaarb changes As GMUX depends on IO for iGP to be enabled and active, lock the IO at vgaarb level. This should prevent GPU driver for dGPU to disable IO for iGP while it tries to own legacy VGA IO. This fixes usage of backlight control combined with closed nvidia driver on some Apple dual-GPU (intel/nvidia) systems. On those systems loading nvidia driver disables intel IO decoding, disabling the gmux backlight controls as a side effect. Prior to commits moving boot_vga from (optional) efifb to less optional vgaarb this mis-behavior could be avoided by using right kernel config (efifb enabled but vgaarb disabled). This patch explicitly does not try to trigger vgaarb changes in order to avoid confusing already running graphics drivers. If IO has been mis-configured by vgaarb gmux will thus fail to probe. It is expected to load/probe gmux prior to graphics drivers. Fixes: ce027dac592c0ada241ce0f95ae65856828ac450 # nvidia interaction Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=86121 Reported-by: Petri Hodju <[email protected]> Tested-by: Petri Hodju <[email protected]> Cc: Bjorn Helgaas <[email protected]> Cc: Matthew Garrett <[email protected]> Signed-off-by: Bruno Prémont <[email protected]> Signed-off-by: Darren Hart <[email protected]> :040000 040000 b9fa60f95e23f0d9916f92df51b95e4b73c80ecf d842064bce0c54557a3b1dd9dcd357f0e204dde1 M drivers ** Bug watch added: Linux Kernel Bug Tracker #86121 http://bugzilla.kernel.org/show_bug.cgi?id=86121 ** Attachment added: "log.txt" https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1512691/+attachment/4567749/+files/log.txt -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1512691 Title: [MacBookPro11,3] Backlight control does not work, but there are entries in /sys/class/backlight To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1512691/+subscriptions -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
