Display Corrupt / Unusable Multiple Monitors (AMD7750)

[Warren Block]

On Wed, 5 Oct 2016, Brandon Falk wrote:

> Hello,
>
> I have a 6 monitor setup using mini displayport off of a single AMD7750
> card. There are no adapters in this setup, and all displayport outputs go
> to displayport capable monitors.
>
> I have fooled around with different xorg.conf settings, as well as omitting
> the xorg.conf file entirely.
>
> When I do a 'startx' command, I get 4 monitors covered in vertical various
> colored lines. One is black. And the main one is working as expected.
> xrandr correctly identifies the screens and their capabilities. This
> 'corruption' persists even after X is exited, and I go back to the normal
> VT terminal (but the terminal is in graphical mode due to the switch to x).
>
> What it looks like visually: https://i.imgur.com/sVUsHeM.jpg
>
> I can move the cursor off the main monitor and it seems the other
> monitors/workspaces work (I can blindly open an xterm and touch a file, and
> verify it exists) however I cannot see anything that is occurring on them.
>
> I've tried unplugging all but 2 monitors from the card, rebooting, and when
> it comes up I get one usable monitor and the other is black (no cursor, no
> nothing). Similar behavior as before, I can move the cursor off the main
> monitor, other workspaces seem to exist but are not visible.
>
> This is not new hardware, previously it was running Windows and had no
> problems using all monitors on this card. I have verified that FreeBSD can
> display to each one individually (if only one is plugged in).
>
> Disabling individual screens via xrandr has the screens go into an off
> state, so it seems like xrandr is able to both detect and modify screens.
>
> I can also modify the resolutions of the screens (including the corrupt
> ones), and they stay corrupt but do seem to change resolutions.
>
> Any idea what is occurring here?

It's not clear, but are you daisy-chaining any of the cables through a 
monitor?  Some newer Dell monitors have the passthrough but need a 
secret hidden menu entry enabled to allow pass-through.  Or DisplayPort 
1.2.  Or maybe just proper pass-through.

I have the details carefully saved somewhere, so carefully that a quick 
search could not find them.  But a more detailed search might locate 
that.

In general, I've found DisplayPort to be unforgiving and difficult with 
X.  Turn off the monitor power, turn it back on, and the monitor never 
detects input again until after a computer reset.  Maybe this is due to 
the old Radeon HD5750, or possibly due to a 25-foot DisplayPort cable.