[QUESTION] [amd-gfx] iMac 5K resolution

[Максим Мосейчук]

Hello. I try to install Linux (Gentoo) on iMac 27'' with 5700 XT Pro.
And I want to get 5k resolution.
5k uses 2 DP 1.2 links, but Linux detects only one connection. Max
available resolution is 4k.
kernel 5.6 and 5.14.
Linux-firmware latest from git.

Display sends EDID with max 4k resolution. It's standard behavior for
one link connection.

I got 3 EDIDs from macOS:
1) 2.5k from eDP-1 connect with tile section
2) 2.5k form DP-1 connect with tile section
3) 5K (I think it's fake EDID, not works)

And I force enable ports (video=eDP-1:e video=DP-1:e
drm.edid_firmware=eDP-1:edid/edid1.bin,DP-1:edid/edid2.bin).

But I get only 2.5k resolutions with EDID, Xorg detects a second
connect and uses it, but the monitor shows a scaled image from eDP-1
only.

I know that the intel driver works with LG Ultrafine display in native
5k support after 5.9 kernel. It's the same display that in iMac. But
connected over one TB3 (2 DP 1.2 too).
I checked last ubuntu 21.04 and fedora 34. Max 4k too.

I don't know how to check where the problem is. In DRM or amdgpu? Or
does Apple use non-typical controller and need to activate a second
connect over i2c or another bus?

edid1.bin base64
AP///////wAGEDKuFsR5ABYdAQS1PCJ4IghBrlJBsyYOUFQAAAABAQEBAQEBAQEBAQEBAQEBxbwAoKBAUrAwIDoAK1ARAAAaj3oAoIAAQpAwIDoAK1ARAAAaAAAA/ABpTWFjCiAgICAgICAgAAAA/wAwMjA2MDkzMTZDNDM5ASdwE3kDABIAFoIQAAD/CT8LAAAAAABBUFAzrhbEea0DADxM0AAE/w6fAC+AHwBvCD0AAgAEAMyRAAR/DJ8AL4AfAAcHMwACAAQAVV4ABP8JnwAvgB8AnwUoAAIABAB/gRj6EAABAQASdjH8ePv/AhCIYtP6+Pj+//8AAAAAAABykA==

edid2.bin base64
AP///////wAGEDKuFsR5ABYdAQS1PCJ4IghBrlJBsyYOUFQAAAABAQEBAQEBAQEBAQEBAQEBxbwAoKBAUrAwIDoAK1ARAAAaj3oAoIAAQpAwIDoAK1ARAAAaAAAA/ABpTWFjCiAgICAgICAgAAAA/wAwMjA2MDkzMTZDNDM5ASdwE3kDABIAFoAQEAD/CT8LAAAAAABBUFAzrhbEea0AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAACVkA==