[Bug 92841] Linux 4.3 on Broadwell with i915.enable_fbc=0: "Unclaimed register detected after writing to register 0x4510c"

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=92841

--- Comment #3 from Adam J. Richter <adam_richter2004 at yahoo.com> ---
Hi, Jani.

Thank you for responding to my bug.  It is useful for me to know that someone
else has read it and thought about it for a moment.  However, I think you may
be missing the point.

It is not my end goal to disable frame buffer compression (enable_fbc=0),
although I don't understand what you mean by "it might lead users astray".  I
am just providing that argument to document how I produced the problem.

The big news in this bug report is the word "after" in the unclaimed register
error.  That means that the register number in the errror message probably
really is the invalid register number (in this case, 0x4510c = WM2_LP_ILK). 
Since this happens to be a register that is not used in very many places, it is
probable that there is very little left to do figure out and resolve this
problem for someone who knows more than I do about which when hardware supports
which registers.

Here is a non-exhaustive list of possible responses that I was hoping for:

1) "It turns out that WM2_LP_ILK does not exist in Broadwell, so we need to
change function _________ to test for Broadwell and do _____________ in that
case."

2) "Actually WM2_LP_ILK does exist in Broadwell, so you are probably observing
some race with another core touching a nonexistent register while we're doing a
polling loop on WM2_LP_ILK."

3) "Thanks for the report, but I have determined that your bug really is
specific to enabled_fbc=0 and not indicative of any problem that could effect
enabled_fbc=1.  Supporting enable_fbc=0 is very low priority to me (and
everyone else, as far as I know), but please feel free to track it down on your
own update this bug report with a proposed fix."

I'm not being sarcastic here.  If you meant something other than example #3 by 
your response, I would be interested in knowing.  Thank you, in any case, for
your response.  Just getting an indication that enable_fbc=0 is rarely used was
helpful.

Thanks again for your response.

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