Re: 21:9 monitor resolution incorrect since 4.14 kernel

[Neil Mayhew <neil_mayhew-Rn4VEauK+AKRv+LV9MX5uipxlwaOVQ5f@public.gmane.org>]

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I enabled kms debugging with drm.debug=4 on the kernel command line.
This enabled me to see that the relevant modes are being rejected with
error 11 which is DC_EXCEED_DONGLE_CAP. Sure enough, I see that there's
a dongle detected that's rated at 165MHz and my preferred modes are all
above that.

However, if I switch the monitors around everything is fine, because I
can use a lower resolution monitor with the dongle and get the 21:9
resolution on another output without a dongle.

The dongle is actually a passive DP to HDMI cable, and I need it because
one of my outputs is DP and I don't have any monitors with DP inputs. I
was told somewhere that I'd need to use the DP output to get the clock
speeds needed for 21:9, but apparently that's not true. Clearly the
cable can go above 165MHz since the 4.14 kernel can drive the display at
the preferred resolution using it, but thankfully I don't need to
concern myself with that now.

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