[Bug 94689] Two monitor DisplayPort MST DaisyChain (Second/Last Monitor In Chain Black Screen)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=94689

            Bug ID: 94689
           Summary: Two monitor DisplayPort MST DaisyChain (Second/Last
                    Monitor In Chain Black Screen)
           Product: DRI
           Version: XOrg git
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: major
          Priority: medium
         Component: DRM/Radeon
          Assignee: dri-devel at lists.freedesktop.org
          Reporter: mike.ennen at gmail.com

As per our chat in #radeon (I am brcolow).

Here is the dmesg with dri.debug=6:

https://gist.github.com/brcolow/e27a5e46b175dd0966af

Here is /sys/kernel/debug/dri/0/radeon_mst_info contents:

    mst: ffff88061cedfb40, 0
    port: 1: ddps: 1 ldps: 0, sdp: 1/1, ffff8800bb193000, conn:
ffff880622b27000
    port: 8: ddps: 1 ldps: 0, sdp: 1/1, ffff8800bb192800, conn:
ffff880622049000
    port: 0: ddps: 1 ldps: 0, sdp: 0/0, ffff8800bb192000, conn:          
(null)
vcpi: 7 3
vcpi 0: 8 1 22
vcpi 1: 1 2 22
vcpi 2:unsed
vcpi 3:unsed
vcpi 4:unsed
vcpi 5:unsed
payload 0: 2, 1, 22
payload 1: 2, 23, 22
payload 2: 0, 45, 0
payload 3: 0, 45, 0
payload 4: 0, 45, 0
payload 5: 0, 45, 0
dpcd: 12 14 c4 01 01 01 01 81 02 00 06 00 00 00 00 
faux/mst: 00 01 
mst ctrl: 07 
branch oui: 00e04c devid: Dp1.2 revision: hw: 0.0 sw: 0.0
payload table: 07 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01 01
01 01 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 02 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
attrib 0: 4 22
attrib 1: 2 22

The two monitors that are being daisy-chained are Dell UltraSharp U2715H's. GPU
is Radeon R9 290.

I have tried doing:

xrandr --output DisplayPort-0-1 --rate X

where X = 59.9, 60, 49.9, 50, 39.9, 40 etc but it doesn't seem to change
anything (and this is the extent of my debugging abilities).

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