[Bug 101290] Radeon R9 390X regression - No output (CVT) or corruption (CVT-R) at 4096x2160 at 60Hz over DisplayPort

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=101290

            Bug ID: 101290
           Summary: Radeon R9 390X regression - No output (CVT) or
                    corruption (CVT-R) at 4096x2160 at 60Hz over DisplayPort
           Product: DRI
           Version: unspecified
          Hardware: Other
                OS: All
            Status: NEW
          Severity: major
          Priority: medium
         Component: DRM/Radeon
          Assignee: dri-devel at lists.freedesktop.org
          Reporter: freedesktop at nuclearsunshine.com

I had thought I had filed this bug a long time ago on here, but I can't find
it...

Graphics card: Radeon R9 390X (Saphhire 11241-04-20G)
Display: LG 31MU97 (via DisplayPort)

With Linux 4.4.x, 4096 x 2160 @ 60 Hz progressive output was working with
standard CVT timing.

With Linux 4.5.x onwards, standard CVT timing at that resolution fails (no
signal) with the following dmesg output:

kernel: [drm:radeon_dp_link_train [radeon]] *ERROR* channel eq failed
kernel: [drm:radeon_dp_link_train [radeon]] *ERROR* channel eq failed: 5 tries

CVT-R timing results in output, but with a flickering/corrupted full-height
band down the right hand side (it approximates the intended output, but pixels
seem be to be corrupt in single-pixel horizontal bands and/or flicker in
horizontal single-pixel bands) It could be complete coincidence, but the width
of the corrupt band seems to be the difference in width vs. 3840x2160, i.e.
aobut 256 pixels. With this timing there are no messages in dmesg.

There's a downstream Fedora bug report here:
https://bugzilla.redhat.com/show_bug.cgi?id=1353341

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