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

[bugzilla-daemon at freedesktop.org]

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

            Bug ID: 92841
           Summary: Linux 4.3 on Broadwell with i915.enable_fdc=0:
                    "Unclaimed register detected after writing to register
                    0x4510c"
           Product: DRI
           Version: unspecified
          Hardware: Other
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: adam_richter2004 at yahoo.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

Created attachment 119429
  --> https://bugs.freedesktop.org/attachment.cgi?id=119429&action=edit
dmesg log of boot on Intel Broadwell with i915.enable_fdc=0.

I am posting this bug with incomplete information because I suspect that
tracking down a bug report about "Unclaimed register detected AFTER [my
emphasis] writing register 0x4510c" may be a lot easier than the more
mysterious bug reports about an unclaimed register access being detected before
some register access event.  I imagine that this a matter of confirming what
register 0x4510c is, confirming that it does not exist on Broadwell and
figuring out what code is incorrectly doing that access.  Anyhow, here is the
bug report in more detail.

Linux 4.3 booted with the command line argument "i915.enable_fbc=0" on an Intel
Broadwell i3-5010U processor emitted the warning message "Unclaimed register
detected after writing to register 0x4510c", along with a call trace showing
that error occurred at ilk_update_wm+0x7da (the stack trace is attached).

I have only observed this problem when I have added some other debug messages
of my own, although I suspect that this just indicates that timing is a factor
in reproducing the problem.  If register 0x4510c exists in Broadwell, then I
would expect that accessing it would never set the unclaimed register flag.

As another warning, I should mention that I suspect that the attached call
trace is at least slightly inaccurate, because 0x7da probably exceeds the
length of the function ilk_update_wm (I guess the "ilk" prefix means "Iron
Lake" graphics), and I I don't think ilk_update_vm calls the i915 register
access functions directly.  However, I am hoping that the register address
might be enough to determine which register that does not exist on Broadwell
was being accessed.

Please note that the attached dmesg log includes some additional call traces,
because I had modified the unclaimed register access code to 

By the way, I passed "i915.enabled_fbc=0" because my attempts to track down a
warning message about "Unclaimed register detected before writing to register
0x..." (where the register number varies), indicated that the previous i915
register access (which did not trigger an unclaimed register warning) was a
read from ilk_fbc_disable.  I will probably file a separate bug report about
that if I can track it down the point where I think I have information that
would help point to the cause.

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