[Bug 94852] New: rc6_residency_ms unreliable

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=94852

            Bug ID: 94852
           Summary: rc6_residency_ms unreliable
           Product: DRI
           Version: unspecified
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: lenb at kernel.org
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

Kernel: Linux-4.6.0-rc1

Hardware: SKL, HSW

Description:
Value read from /sys/class/drm/card0/power/rc6_residency_ms
resets before it reaches 5500 seconds.

This make it "somewhat awkward" to calculate residence from this counter,
since it can effectively go backwards - or worse, appear to increment
extremely slowly when it should really be incrementing quickly.

turbostat has a workaround for former, but there is no workaround
for the later.  powertop does not have a workaround, it will just
occasionally display garbage.

Debug steps taken:
booted both the SKL and HSW with cmdline "i915.enable_dc=0"
but it made no difference.

To Reproduce:
while /bin/true; do cat /sys/class/drm/card0/power/rc6_residency_ms; sleep 1;
done
...
5491367
5492352
5493334
5494326
5495307
5496299
5497280
703
1685
2677
3658
4650
5631
6612

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