Re: 21:9 monitor resolution incorrect since 4.14 kernel

[Neil Mayhew <neil_mayhew@xxxxxxxxxxxxxxxxxxxxx>]

    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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