Hi guys!
I bought a 4K UHD screen as a display for my Magic Mirror project. I was able to make my Raspi 3B output a HDMI signal at 3840 x 2160 (15Hz), but it only works in Landscape Mode. When I switch to portrait mode (by adding display_rotate=1 to /boot/config.txt) the Raspberry Pi would not boot up anymore (display rotation however worked at 1920x1080).

I have seen other people struggle as well, but some have succeeded by making changes to the X Framebuffer. Unfortunately, that has not worked for me.

So: can anyone share a working configuration for 4K Portrait Mode with a Raspi 3B on Raspbian 9 Stretch?

Thanks for your help!
Kevin

P.S. So far, my /boot/config.txt contains the following video-related commands that I had put together from various sources:

disable_overscan=1

framebuffer_width=3840
framebuffer_height=2160
max_framebuffer_width=3840
max_framebuffer_height=2160

hdmi_group=2
hdmi_mode=87

hdmi_ignore_edid=0xa5000080
hdmi_cvt=3840 2160 15

gpu_mem=256