Rotate Plymouth-based UC22 splash to portrait

[tobias]

Continuing the discussion from Psplash for custom pi gadget device as a new topic since my follow-up question does not involve psplash:

@tobias

How do I rotate the Plymouth splash? I found this discussion pointing me to the kernel command line video=HDMI-A-1:panel_orientation=right_side_up (for portrait orientation), but that does not seem to do the trick. Tested on an RPi 4, the intermittent blinking cursor was displayed upright. My cmdline addition is in my gadget’s cmdline.extra file. Maybe these options are not passed to the initramfs-based Plymouth splash?

Users of my appliance image may attach a wide variety of off-the-shelf HDMI displays in both upright and normal (landscape) positions, so I cannot rely on EDID to provide the current orientation. ubuntu-frame gets the proper display layout via the gadget snap, i.e. only the splash screen has the wrong orientation.