https://bugs.freedesktop.org/show_bug.cgi?id=106191
--- Comment #11 from François Jacques francois.jacques@gmail.com ---
The bug is caused by having both the audio signal and image simultaneously, it seems.
After I added "radeon.audio=0" to the kernel argument list and rebooted, the video interface and the monitor concluded their signal handshake successfully (!) and I could switch the monitor to full res and, lo' behol', a 1440p image (!!).
For future reference, should anyone have issues, try radeon.audio=0. If you were relying on the radeon for audio, look for an alternative, or use a DP-to-HDMI adapter and tolerate the lower resolution until the bug is fixed.
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Googling on this specific symptom revealed multiple regressions on this matter before, it could be a new one or it's been lurking for a while. Which means either: - tons of dupes in the DRM bugs list wrt radeon and DP /w audio enabled and users don't know how to disable it. - all radeon users having switched their audio off - most radeon users gave up on their DisplayPort or their radeon-based card altogether (perhaps switching brand in the process, frustration kicking in) - it's a recent regression and I've been lucky to catch it before the kernel goes mainstream in major distros.
I'll still provide a dmesg dump, this time with drm.debug=0x06 (0x06 any good? any prefered value?) by rebooting with audio back on and image gone.