port-evbarm/54026: simplefb fails to sync with 4K monitor

[bsiegert%gmail.com@localhost]

>Number:         54026
>Category:       port-evbarm
>Synopsis:       simplefb fails to sync with 4K monitor
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-evbarm-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Feb 28 21:05:00 +0000 2019
>Originator:     Benny Siegert
>Release:        NetBSD 8.99.34 (as of 2019-02-26)
>Organization:
The NetBSD Foundation
>Environment:
NetBSD arm64 8.99.34 NetBSD 8.99.34 (GENERIC) #0: Tue Feb 26 15:55:33 UTC 2019  mkrepro%mkrepro.NetBSD.org@localhost:/user/src/sys/arch/evbarm/compile/GENERIC64 evbarm
>Description:
I had this with two machines, an Orange Pi Plus 2E (32 bits) and a Pine A64-LTS (64 bits). The monitor is a Dell P2415Q (4K), connected via HDMI 1.4, so its preferred resolution is 3840x2160 at 30Hz.

During U-Boot (and NetBSD boot), the picture appears OK, but as soon as the NetBSD kernel takes over, the screen loses sync. The picture re-appears for less than a second, then a black screen again. After a while, the monitor displays an error ("current timing is unsupported") and switches off.

If you boot with "nomodeset" on the kernel command line, you get the desired resolution with no flicker.
>How-To-Repeat:
Boot NetBSD/evbarm on a 4K screen connected via HDMI.
>Fix:
?