linux-kernel - Unexpected screen flicker during i915 initialization

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Date:   Wed, 30 Oct 2019 18:25:14 +0800
From:   Chris Chiu <chiu@...lessm.com>
To:     Jani Nikula <jani.nikula@...el.com>,
        joonas.lahtinen@...ux.intel.com, rodrigo.vivi@...el.com,
        David Airlie <airlied@...ux.ie>, daniel@...ll.ch,
        intel-gfx@...ts.freedesktop.org,
        dri-devel <dri-devel@...ts.freedesktop.org>
Cc:     Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Unexpected screen flicker during i915 initialization

Hi guys,
    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 about the BIT(2) of CDCLK_CTL
which cause this difference. Can anyone suggest what exactly the
problem is and how should we deal with it? Thanks.

Chris