Can't use third monitor, Radeon HD 8830M

[Alex Deucher]

On Tue, Sep 11, 2018 at 12:24 PM Eino Virtanen <eino.vir at gmail.com> wrote:
>
> Hello everyone,
>
> I have three monitors. Each connected with DVI cables to a Radeon HD
> 8830M. Two of them are connected with a DVI-DP adapter. The problem is
> that I can only enable two of the monitors simultaneously, otherwise I
> get this error message: "xrandr: Configure crtc 0 failed"

Are you using active or passive DVI->DP adapters?  Are all of the
displays on a single GPU?  Your GPU only has two PLLs for non-DP
displays, so it can only drive two independent sets if display timing.
If you want to use more than 2 non-DP monitors, you need to use the
exact same modeline on at least two of them.  All of the modes you are
trying to set have different timings, so it's not possible:

  1680x1050 (0x81) 146.250MHz -HSync +VSync +preferred
        h: width  1680 start 1784 end 1960 total 2240 skew    0 clock  65.29KHz
        v: height 1050 start 1053 end 1059 total 1089           clock  59.95Hz

  1920x1200 (0x8c) 154.000MHz +HSync -VSync *current +preferred
        h: width  1920 start 1968 end 2000 total 2080 skew    0 clock  74.04KHz
        v: height 1200 start 1203 end 1209 total 1235           clock  59.95Hz

  1680x1050 (0x8f) 119.000MHz +HSync -VSync *current +preferred
        h: width  1680 start 1728 end 1760 total 1840 skew    0 clock  64.67KHz
        v: height 1050 start 1053 end 1059 total 1080           clock  59.88Hz

Try either selecting a modeline that is common across at least two of
the displays, or use randr to add one of the 1680x1050 modelines to
the other connector and hope that the monitor will support it.

Alex