[Bug 111179] [Regression]Multiple monitors via DP TB dock detection/resolution/layout failure after Skylake->Coffeelake upgrade

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=111179

--- Comment #11 from Arcadiy Ivanov <arcadiy at ivanov.biz> ---
Thanks for looking at this.

Both Dell Precision 7510 (SKL) and Dell Precision 5540 (CFL) have the same TB
capabilities, TB3 ***HBR2***. It's true, with HBR2 they don't support
2x4k at 60Hz(**), but the point of filing this bug was somewhat different - there
was a change in behavior in monitor detection and resolution/refresh rate
setting.

SKL was supporting 2x4k with auto-detection, which, given hardware limitations,
means that it was settling on 2x4k at 30Hz mode all by itself. When an upgrade was
made to CFL, this auto-detection stopped working properly - as you can see it
tried to configure 2x4K at 60Hz and failed, without fallback to 2x4K at 30Hz.

The change in behavior cannot be blamed on changes in Xorg, xrandr etc - the
entire SSD was moved. Dock is similarly blameless - it was the same dock. The
only change was laptop hardware. Given that monitors could be detected and EDID
was read the expected fallback should've been onto the 2x4K at 30Hz on CFL same as
on SKL. It wasn't.

All firmware is up-to-date.

=====
** there is a way with WD19TB for 2x4k at 60Hz with DP pass-through, but this
wasn't the setup for this bug

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