[Bug 92874] New: FBC on HD4600 causes screen flickering on certain screen content

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=92874

            Bug ID: 92874
           Summary: FBC on HD4600 causes screen flickering on certain
                    screen content
           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: brain at jikos.cz
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

Machine: Asus "Zenbook" UX501
CPU: i7 4720HQ
GPU: Intel HD4600
Kernel 4.3.2
OS: Kubuntu 15.10

After enabling FBC with kernel option "i915.enable_fbc=1" the screen flickers
with specific video content. Typically Firefox browser with some page. As an
example the "https://01.org/linuxgraphics/community/kernel" at the default zoom
level.

Please note that my notebook has 4k display. I have seen the issue only on the
built-in panel, not on an external screen.

The flickering typically looks like the picture is switching between correct
look and the same picture shifted down. The Y offset varies depending on the
content. So does the flickering frequency. In the above example the offset is
about half of the screen height.

If the content changes, e.g. change the zoom level in Firefox (on the above
example it's sufficient to scroll down to the text so the picture disappears),
the flickering changes or disappears.

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