[Bug 107784] [AMD tahiti XT] displayport broken

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=107784

--- Comment #20 from Sylvain BERTRAND <sylvain.bertrand at gmail.com> ---
Found the bug, very probably.

It seems to be an upstream bug: a 32bits multiplication overflow on TSC
frequency introduced in recent TSC cleanup:

--------------------------------------------------------------------------------
commit cf7a63ef4e0203f6f33284c69e8188d91422de83
Author: Pavel Tatashin <pasha.tatashin at oracle.com>
Date:   Thu Jul 19 16:55:38 2018 -0400

    x86/tsc: Calibrate tsc only once

    During boot tsc is calibrated twice: once in tsc_early_delay_calibrate(),
    and the second time in tsc_init().

    Rename tsc_early_delay_calibrate() to tsc_early_init(), and rework it so
    the calibration is done only early, and make tsc_init() to use the values
    already determined in tsc_early_init().

    Sometimes it is not possible to determine tsc early, as the subsystem that
    is required is not yet initialized, in such case try again later in
    tsc_init().
--------------------------------------------------------------------------------

One nasty thing: this commit is broken, it does not compile, hence it's a
bisect "skipped" commit.

Roughly, if you have a cpu with a frequency above 4.2GHz (max unsigned 32bits),
linux time subsystem gets broken leading to the timeouts in displayport
programming. Ofc, my cpu runs at 4.7GHz.

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