[Bug 105962] [KBL] "enable_rc6" parameter deprecation brings back freezing

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=105962

--- Comment #53 from George McCollister <george.mccollister at gmail.com> ---
I'm able to reproduce this issue on five Atom E3845 based embedded systems in a
lab.
With intel_idle.max_cstate=0 processor.max_cstate=0 I can get all systems to
restart due to watchdog reset overnight.

Sometimes, but not always I have observed errors such as these on the serial
console immediately prior to the system lockup/reboot:
[144039.363431] [drm:vlv_set_power_well [i915]] *ERROR* timeout setting power
well state 00000c00 (000ffc00)
[144039.476432] [drm:vlv_set_power_well [i915]] *ERROR* timeout setting power
well state 000000c0 (000ffcc0)
[144039.589165] [drm:vlv_set_power_well [i915]] *ERROR* timeout setting power
well state 00000000 (000fccc0)
[144039.702190] [drm:vlv_set_power_well [i915]] *ERROR* timeout setting power
well state 00000000 (000f0cc0)
[144039.814669] [drm:vlv_set_power_well [i915]] *ERROR* timeout setting power
well state 00000000 (000c0cc0)
[144039.925749] [drm:vlv_set_power_well [i915]] *ERROR* timeout setting power
well state 00000000 (00000cc0)
[144040.120485] [drm:vlv_set_power_well [i915]] *ERROR* timeout setting power
well state 00000c00 (00000cc0)
[144040.233084] [drm:vlv_set_power_well [i915]] *ERROR* timeout setting power
well state 000000c0 (00000cc0)
[144040.388315] [drm:vlv_set_power_well [i915]] *ERROR* timeout setting power
well state 000000c0 (00000cc0)
[144040.607674] [drm:vlv_set_power_well [i915]] *ERROR* timeout setting power
well state 000000c0 (00000cc0)

Most commonly, only one of the above error messages (or none) is printed to the
serial console.

On 4.14.x, adding i915.enable_rc6=0 allows the systems to run 3+ days (until I
stop the test).

Hoping it would fix the problem I checked out and built kernel commit
a75d035fedbdecf83f86767aa2e4d05c8c4ffd95. All systems still rebooted overnight.

I've since found that using i915.disable_power_well=0 also prevents the problem
from occurring on all tested kernel versions. Is this setting less disruptive
to the operation than i915.enable_rc6=0? Is there also value in testing
"enable_dc=0"?

If any of the developers are working on this and think they have a fix, give me
the URI of a git repo and the commit to use and I can build, test it in the
lab. Also specify any kernel config settings and kernel command line arguments
you want me to use.

Since someone might ask I'm using "intel_idle.max_cstate=0
processor.max_cstate=0" since these systems require minimal scheduling latency.
I also noticed they can prevent other i915 lockup issues. I can remove them for
testing purposes upon request.

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