[Bug 97573] [IVB/SNB/BYT/HSW/BDW] GuC boot kernel command lines are causing regressions

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=97573

david.s.gordon at intel.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |NOTABUG
             Status|NEW                         |RESOLVED

--- Comment #3 from david.s.gordon at intel.com ---
> Steps:
> ------
> 1. Boot with kernel boot command lines: 
i915.enable_guc_loading=2 i915.enable_guc_submission=2

Please note that these variables are marked as UNSAFE.
That means that:
(a) you shouldn't set them unless you know what you're doing, and what they
mean, and
(b) the kernel is *tainted* when you do so, which in turn means,
(c) whatever happens, it's not a bug, because you asked for it.

Secondly, the number "2" for these variables means that you are telling the
driver that loading and use of the GuC is *mandatory*, and the driver *must*
fail startup (leaving the GPU wedged) if *for any reason* those mandatory
requirements cannot be fulfilled.

Good reasons to fail here include
(a) you don't have the (correct) GuC firmware installed, or
(b) the driver doesn't know what GuC firmware would be appropriate, or
(c) you don't have a GuC!

Therefore, this part of the bug report is wrong
> Expected result:
> ---------------
> 2. Trying to load GuC has no impact

The expected result of telling the driver that it *must* use the GuC on a
platform that doesn't have one, is that it will develop paranoid schizophrenia
^W^W^W log an error and leave the GPU wedged. Which, according to the attached
log, is exactly what it is doing :)

Note that setting either or both of these variables to "1" is much more
sensible. That means "try to load/use the GuC, but continue in execlist mode if
you can't". Or even "-1", meaning "try to use the GuC *if it's supported on
this platform*, but fall back to execlist mode if it isn't supported, or the
firmware is missing, or there's any other reason it can't be used".

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