[Bug 93578] Intel HD 520 failed to train DP when connected with a external monitor

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=93578

--- Comment #17 from PJ <foobar at pjmodos.net> ---
I think there are several things wrong, this is also related to
https://bugzilla.kernel.org/show_bug.cgi?id=113121 afaics. I have similar
problems, I played with various versions of kernel from 4.4.1 to
intel-drm-nightly and for older kernels it often manifests as "ERROR 5.4Gbps
rate without HBR2/TPS3 support" which is the closed bug
https://bugs.freedesktop.org/show_bug.cgi?id=92932 where it was changed to just
debug, which masks the issue but does not really solve it. On some kernel
versions it shows as DP training failed, on some other it just blanks the
display. I think there is something wrong in the way the i915 detects/setsup
the DP link.

The DA200 is detected as not supporting TB3 but supporting the 5.4Ghz lane
speed. What I find curious though is that i915 uses the 5.4Ghz link for my
monitor at 1920x1200 at 60Hz (which kinda makes sense on single lane given the
154Mhz pixel clock) and so wants to use HBR2 but on windows and mac with same
dongle I get the monitor running at 1920x1200 at 60Hz even when I disable HBR2 on
the monitor. So I wondered if the DA200 actually has two 2.7Ghz lanes or
something and tried to play with forcing the kernel module to use various
bandwidths and number of lanes but without success. For now I can also only get
resolutions that fit into single lane at 2.7Ghz (which is 1080i for example or
1080 at 30Hz) as reported by others.

I am attaching dmesg with KMS and DRIVER debug on from the drm-nightly as of
27th Feb. One thing to note about the latest nightly is that once I set the
dispaly to some resolution which works and then try to set it to something
which doesn't it freezes my computer (and there is no log that is not garbled
so can't get the info about what happened much). With older versions it always
just blanked the monitor when I was switching multiple times just like it does
the first time with nigtly.

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