[Bug 112178] New: Unexpected screen flicker during i915 initialization

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=112178

            Bug ID: 112178
           Summary: Unexpected screen flicker during i915 initialization
           Product: DRI
           Version: XOrg git
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: not set
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: chiu at endlessm.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

We have 2 laptops, ASUS Z406MA and Acer TravelMate B118, both powered by the
same Intel N5000 GemniLake CPU. On the Acer laptop, the panel will blink once
during boot which never happens on the ASUS laptop. It caught my attention and
I find the difference between them but I need help for more information,

    The major difference happens in bxt_sanitize_cdclk() on the following
condition check.
        if (cdctl == expected)
                /* All well; nothing to sanitize */
                return;

    On the problematic Acer laptop, the value of cdctl is 0x27a while the same
cdctl is 0x278 on ASUS machine. Due to the 0x27a is not equal to the expected
value 0x278 so it needs to be sanitized by assigning -1 to 
dev_priv->cdclk.hw.vco. Then the consequent bxt_set_cdclk() will force the full
PLL disable and enable. And that's the flicker (blink) we observed during boot.

    Although I can't find the definition of the BIT(2) of CDCLK_CTL which
causes this difference. I reproduced this bug on latest 5.3 mainline and also
kernel built from https://github.com/freedesktop/drm-intel. But I think it's
not really about i915 driver if the content from register CDCLK_CTL is
unexpected. Can anyone suggest what exactly the problem is and how should we
deal with it? Thanks.

Please ref. https://lkml.org/lkml/2019/10/30/240 mail thread here.

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