Can someone with an iMac 5K check if Linux works at 5K?

[tornado99]

I am thinking of buying a second hand 5K Intel iMac but I need to make sure Linux will run at the full 5K resolution, so I can keep on using it after OS X support expires.

Background
The 2015-2019 5K iMacs used a special hack in the GPU drivers to tile together 2 half-screens to make 5K - AFAIK this only works on Windows (with Bootcamp drivers) and of course OS X. But perhaps modern Linux can cope with this.

The 2020 5K iMac no longer uses the tiled hack because the display chip is updated, so perhaps 5K works on Linux just like any other monitor would.

Can someone who owns a 5K iMac, especially a 2020 model, check whether 5K works in Linux. You would need to have OCLP installed first, and then boot up any modern Linux distro from a USB stick (no need to install)

Expectation

Linux will probably boot up fine, but display the desktop in 4K instead of 5K, except for the 2020 iMac where it will show 5K. But maybe I'm wrong!

balenaEtcher - Flash OS images to SD cards & USB drives

A cross-platform tool to flash OS images onto SD cards and USB drives safely and easily. Free and open source for makers around the world.

etcher.balena.io

GitHub - dortania/OpenCore-Legacy-Patcher: Experience macOS just like before

Experience macOS just like before. Contribute to dortania/OpenCore-Legacy-Patcher development by creating an account on GitHub.

github.com

https://ubuntu.com/download/desktop (25.04)

 

[gymrat2k]

On iMac 2020 on an Ubuntu "trial from USB" I see 3840x2160 as the largest resolution.
In Windows and macOS on the same machine the reported max resolution is 5120x2880.

Why would I need OCLP?

 

[tornado99]

Ok thanks very much. It is odd though - because I thought the chip driving the display on an iMac 2020 was updated to displayport 1.4, so Apple wouldn't need to do the "stitch together 2 smaller displays" hack.

I'm surprised that the Mac bootloader lets you boot so easily into Ubuntu. One of the OCLP features is that other OSs can be booted too - but maybe I'm misunderstanding something there.

Also - this means pretty much every 5K iMac is headed for the trash in 2028 when security updates for OS X expire. Unless people want to "slum it" in 4K(!)

 

[gymrat2k]

I've set the Secure Boot Settings to No Security (Full, Medium, No Security) so that I can boot from external drives.

About Startup Security Utility on a Mac with the Apple T2 Security Chip - Apple Support

Use Startup Security Utility to make sure that your Mac always starts up from your designated startup disk, and always from a legitimate, trusted operating system.

support.apple.com

"If you're using a Mac with the Apple T2 Security Chip, Startup Security Utility offers three features to help secure your Mac against unauthorized access: Firmware password protection, Secure Boot, and the ability to set allowed boot media."

 

[gymrat2k]

Also see "5K iMac and UEFI: Fixing the dreaded output limitation": https://khronokernel.com/macos/2021/12/08/5K-UEFI.html

"Having now worked through all this to finally get 5k output working, it’s a bit frustrating having to use such a hacky method of loading OpenCore with the firmware downgrading the display.

However, the fact we’ve been able to resolve this greatly helps future development of OpenCore Legacy Patcher as well as potentially allowing Linux users to finally experience 5K natively. I’m excited to see whether the Linux community can use this to their advantage."

 

[tornado99]

Also see "5K iMac and UEFI: Fixing the dreaded output limitation": https://khronokernel.com/macos/2021/12/08/5K-UEFI.html

"Having now worked through all this to finally get 5k output working, it’s a bit frustrating having to use such a hacky method of loading OpenCore with the firmware downgrading the display.

However, the fact we’ve been able to resolve this greatly helps future development of OpenCore Legacy Patcher as well as potentially allowing Linux users to finally experience 5K natively. I’m excited to see whether the Linux community can use this to their advantage."

Yes, I've seen it. However it is not directly talking about Linux, it is more about how OCLP got their bootloader, and everything past the bootloader to display in 5K.

So actually, if you had OCLP installed and booted your Ubuntu stick through OCLP you might get a different result.

 

[gymrat2k]

Yes, I've seen it. However it is not directly talking about Linux, it is more about how OCLP got their bootloader, and everything past the bootloader to display in 5K.

So actually, if you had OCLP installed and booted your Ubuntu stick through OCLP you might get a different result.

Yes, OCLP would be the way forward with testing. But that would have to come from someone with a 2020 5K more willing to experiment, sorry.
A reddit user reported getting the full resolution on another machine after some additions to the kernel command line:

https://www.reddit.com/r/OpenCoreLegacyPatcher/comments/1cxydga/how_to_get_full_resolution_in_linux_on_a_retina

"I have a dual-boot OS-X and Linux 2017 21.5" retina iMac. I get the full 4096x2304 pixels in OS-X, but only 3840x2160 pixels in Linux."

---

"By the way I solved this problem on my 4K iMac by adding video=eDP-1:4096x2304@MR to the kernal command line. Everything works perfectly now!

I suspect this wouldn't work for the 5K iMac as 5K resolution wouldn't fit into the displayport bandwidth ~ 2015 era, so Apple had to do some kind of internal tiling trick - hence the need for the Bootcamp driver.

Happily, 4096x2304 fits (just about) into the bandwidth of earlier displayport versions - so the solution is simply setting the mode timings correctly."

---

It could be possible. Adding Display Stream Compression (DSC) meant that the 2020 iMac 5K got dual-6K external display support – with a normally massive bandwidth demand. DSC and it's "visually lossless" bandwidth savings is done by compressing video data by up to 3:1. Then 5K on Linux would be inside DisplayPort bandwidth specifications.

 

[tornado99]

5K/8 bit color at 60Hz is within Displayport 1.4 bandwidth without compression. 5k/10 bit color at 60Hz would need DSC. Linux has support for DSC in the AMD drivers that would include the Navi GPUs in iMac 2020.

So the above leads me to believe that 5K on a 2020 iMac is technically possible without any unusual tricks.

A kernel mode line parameter of

video=eDP-1:5120x2880@MR-24

might give a 5K desktop at 8 bit color.

Looks like this is possible to try even with a live USB

How to modify kernel boot parameters

The real-time performance-tuning features compiled into the kernel can be controlled by various means. The approach used here is a common one: passing command line parameters via a bootloader. The ...

documentation.ubuntu.com

And for a 10 bit color desktop, I think there is a chance this would happen automatically when booting Linux via OCLP. There are no special commands to turn on DSC in Linux, it simply works if it is told a 5K monitor is connected and the GPU driver has DSC support.

 

[gymrat2k]

5K/8 bit color at 60Hz is within Displayport 1.4 bandwidth without compression. 5k/10 bit color at 60Hz would need DSC. Linux has support for DSC in the AMD drivers that would include the Navi GPUs in iMac 2020.

So the above leads me to believe that 5K on a 2020 iMac is technically possible without any unusual tricks.

A kernel mode line parameter of

video=eDP-1:5120x2880@MR-24

might give a 5K desktop at 8 bit color.

Looks like this is possible to try even with a live USB

How to modify kernel boot parameters

The real-time performance-tuning features compiled into the kernel can be controlled by various means. The approach used here is a common one: passing command line parameters via a bootloader. The ...

documentation.ubuntu.com

And for a 10 bit color desktop, I think there is a chance this would happen automatically when booting Linux via OCLP. There are no special commands to turn on DSC in Linux, it simply works if it is told a 5K monitor is connected and the GPU driver has DSC support.

Tried adding video=eDP-1:5120x2880@MR-24 by itself in GRUB edit mode:

setparams "Try or Install Ubuntu'
set gfxpayload-keep
linux / casper/vmlinuz --- quiet splash
initrd /casper/initrd
video-eDP-1:5120x28800MR-24

But that didn't load. Is that how you add a kernel mode line parameter?

I am doing it on Linux Mint live USB, since for me Ubuntu live USB only can be started once, then I have to reflash the OS image for some reason. It just stops loading after booting live once.

Running videoinfo on Linux Mint command line in GRUB menu gives:

List of supported video modes:
Legend: mask/position=red/green/blue/reserved

Adapter Bochs PCI Video Driver':
No info available

Adapter 'Cirrus CLGD 5446 PCI Video Driver':
No info available

Adapter 'EFI GOP driver":
0x000 3840 x 2160 x 32 (15360) Direct color, mask: 8/8/8/8 pos: 16/8/0/24

 

[tornado99]

No. You need to add it before 'quiet splash' in the line that starts with linux.... There should be a space before the parameter you insert, and no line breaks.

linux / casper/vmlinuz --- video=eDP-1:5120x2880@MR-24 quiet splash

You added an extra 0 in the 5120x2880, could also try without the -24.

The videoinfo code seems to indicate the Apple is only advertising 4K 10 bit resolution. But maybe the kernel mode line parameter will override that.

 

[gymrat2k]

You added an extra 0 in the 5120x2880, could also try without the -24.

The text recognition software (I took a photo of the screen) interpreted the @-sign as an extra 0.

I'm still on 3840x2160 as the max resolution in Ubuntu. Tried both variants:

linux /casper/vmlinuz --- video=eDP-1:5120x2880@MR-24 quiet splash

and

linux /casper/vmlinuz --- video=eDP-1:5120x2880@MR quiet splash

 

[gymrat2k]

Don't know if it's any help, but here's system info from Mint and Ubuntu:

lspci
00:00.0 Host bridge: Intel Corporation 10th Gen Core Processor Host Bridge/DRAM Registers (rev 05)
00:01.0 PCI bridge: Intel Corporation 6th-10th Gen Core Processor PCIe Controller (x16) (rev 05)
00:12.0 Signal processing controller: Intel Corporation Comet Lake PCH Thermal Controller
00:14.0 USB controller: Intel Corporation Comet Lake USB 3.1 xHCI Host Controller
00:14.2 RAM memory: Intel Corporation Comet Lake PCH Shared SRAM
00:16.0 Communication controller: Intel Corporation Comet Lake HECI Controller
00:1b.0 PCI bridge: Intel Corporation Comet Lake PCI Express Root Port #17 (rev f0)
00:1c.0 PCI bridge: Intel Corporation Comet Lake PCIe Root Port #1 (rev f0)
00:1c.4 PCI bridge: Intel Corporation Device 06bc (rev f0)
00:1d.0 PCI bridge: Intel Corporation Device 06b4 (rev f0)
00:1e.0 Communication controller: Intel Corporation Comet Lake PCH Serial IO UART Host Controller #0
00:1e.1 Communication controller: Intel Corporation Comet Lake PCH Serial IO UART Host Controller #1
00:1f.0 ISA bridge: Intel Corporation Z490 Chipset LPC/eSPI Controller
00:1f.3 Audio device: Intel Corporation Comet Lake PCH cAVS
00:1f.4 SMBus: Intel Corporation Comet Lake PCH SMBus Controller
00:1f.5 Serial bus controller: Intel Corporation Comet Lake PCH SPI Controller
01:00.0 PCI bridge: Advanced Micro Devices, Inc. [AMD/ATI] Navi 10 XL Upstream Port of PCI Express Switch (rev 40)
02:00.0 PCI bridge: Advanced Micro Devices, Inc. [AMD/ATI] Navi 10 XL Downstream Port of PCI Express Switch
03:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Navi 10 [Radeon Pro 5700 XT] (rev 40)
03:00.1 Audio device: Advanced Micro Devices, Inc. [AMD/ATI] Navi 10 HDMI Audio
04:00.0 Mass storage controller: Apple Inc. ANS2 NVMe Controller (rev 01)
04:00.1 Non-VGA unclassified device: Apple Inc. T2 Bridge Controller (rev 01)
04:00.2 Non-VGA unclassified device: Apple Inc. T2 Secure Enclave Processor (rev 01)
04:00.3 Multimedia audio controller: Apple Inc. Apple Audio Device (rev 01)
05:00.0 Network controller: Broadcom Inc. and subsidiaries BCM4364 802.11ac Wireless Network Adapter (rev 04)
06:00.0 Ethernet controller: Aquantia Corp. AQtion AQC107 NBase-T/IEEE 802.3an Ethernet Controller [Atlantic 10G] (rev 02)
07:00.0 PCI bridge: Intel Corporation JHL7540 Thunderbolt 3 Bridge [Titan Ridge 4C 2018] (rev 06)
08:00.0 PCI bridge: Intel Corporation JHL7540 Thunderbolt 3 Bridge [Titan Ridge 4C 2018] (rev 06)
08:01.0 PCI bridge: Intel Corporation JHL7540 Thunderbolt 3 Bridge [Titan Ridge 4C 2018] (rev 06)
08:02.0 PCI bridge: Intel Corporation JHL7540 Thunderbolt 3 Bridge [Titan Ridge 4C 2018] (rev 06)
08:04.0 PCI bridge: Intel Corporation JHL7540 Thunderbolt 3 Bridge [Titan Ridge 4C 2018] (rev 06)
09:00.0 System peripheral: Intel Corporation JHL7540 Thunderbolt 3 NHI [Titan Ridge 4C 2018] (rev 06)
0a:00.0 USB controller: Intel Corporation JHL7540 Thunderbolt 3 USB Controller [Titan Ridge 4C 2018] (rev 06)

 

[gymrat2k]

sudo lshw -c display

  *-display                 
       description: VGA compatible controller
       product: Navi 10 [Radeon Pro 5700 XT]
       vendor: Advanced Micro Devices, Inc. [AMD/ATI]
       physical id: 0
       bus info: pci@0000:03:00.0
       logical name: /dev/fb0
       version: 40
       width: 64 bits
       clock: 33MHz
       capabilities: pm pciexpress msi vga_controller bus_master cap_list rom fb
       configuration: depth=32 driver=amdgpu latency=0 mode=3840x2160 resolution=3840,2160 visual=truecolor xres=3840 yres=2160
       resources: iomemory:440-43f iomemory:420-41f irq:79 memory:4400000000-47ffffffff memory:4200000000-42001fffff ioport:3000(size=256) memory:81e00000-81e7ffff memory:81e80000-81e9ffff

 

[tornado99]

I tend to stay away from Linux Mint as their GPU drivers/kernel versions are always about 1 year old. Fedora and Manjaro are more up to date. However in this case I doubt it would make much difference.

Titan Ridge tells us that the 2020 iMac supports Displayport 1.4 which allows 5K without tiling. The 5700 XT has built in support for DSC in Linux since 2019 via the open source AMD drivers.

So all the hardware is there. It seems to be Apple is still telling 3rd party OSs that the iMac is a 4K display, and there isn't a way to override that. The only thing left would be see if OCLP can boot Linux in 5K*. There is no harm in installing OCLP on machines still supported by Apple (https://dortania.github.io/OpenCore-Legacy-Patcher/MODELS.html) but as you said that's for someone else to try.

* OCLP seems to recognise Fedora and Ubuntu best and will directly boot those, but pass over booting for some other distros.

 

[hamacoi]

Hi all, I just joined this forum to update on this topic. I have an iMac 5K 2020 as the OP asked. I use OCLP bootloader to boot into my ArchLinux (installed on an external SSD). It booted successfully. But now I got an interesting screen resolution: 1/2 of 5K. Here is the information I got from xrandr:

Screen 0: minimum 16 x 16, current 2560 x 2880, maximum 32767 x 32767
eDP-1 connected 2560x2880+0+0 (normal left inverted right x axis y axis) 600mm x 340mm
2560x2880 59.96*+
2048x1536 59.95
1920x1440 59.90
1600x1200 59.87
1440x1080 59.87
1400x1050 59.86
1280x1024 59.89
1280x960 59.94
1152x864 59.78
1024x768 59.92
800x600 59.86
640x480 59.38
320x240 59.29
2560x1600 59.94
1920x1200 59.88
1680x1050 59.95
1440x900 59.89
1280x800 59.81
1152x720 59.75
960x600 59.63
928x580 59.88
800x500 59.50
768x480 59.90
720x480 59.71
640x400 59.20
320x200 58.14
2560x1440 59.91
2048x1152 59.90
1920x1080 59.96
1600x900 59.95
1368x768 59.88
1280x720 59.86
1024x576 59.90
864x486 59.92
720x400 59.27
640x350 59.28

If I boot without OCLP, as you already know, I will have the 4K resolution. Maybe I don't have a proper drivers installed.

03:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Navi 14 [Radeon RX 5500/5500M / Pro 5500M] (rev 41)
Subsystem: Apple Inc. Device 0218
Kernel driver in use: amdgpu
Kernel modules: amdgpu

I don't have another USB to try the live Fedora or other distros. Any suggestions?

 

[tornado99]

Oh wow, very interesting! Also, it seems Apple may have retained the 5K tiled method for the 2020 model even though the Displayport chip supports single-cable 5K. I think the problem now is on the Linux side rather than Apple, which means it should be fixable with some detective work.

AFAIK the open-source amdgpu is the best, most actively developed, driver.

You could try this: https://bbs.archlinux.org/viewtopic.php?pid=1944161#p1944161

Personally I use Manjaro Linux (Arch based) with Wayland as Manjaro is very up to date. Maybe a Wayland session will have better support for 5K than X11.

You could also try adding the kernel parameter I mentioned above. On Manjaro you would type:

sudo nano /etc/default/grub

add video=eDP-1:5120x2880@MR-24 to the kernel line (normally contains things like quiet and splash)

save and exit

sudo update grub

and reboot (or try @MR-32, or @MR, or nothing after the 5120x2880)

Also I found this which could be relevant

Making sure you're not a bot!

the studio display actually has a 5K non-tiled mode, its shown in the edid. KDE Plasma defaults to the non-tiled mode whereas Gnome defaults to the tiled mode with no way of selecting the non-tiled mode.

What is the output of drm_info on your Mac?

Basically look for anyone using an Apple Studio Display, an LG Ultrafine 5K, or a Dell UP2715K with Linux and they might provide some clues for this puzzle!

 

[hamacoi]

I can't try https://bbs.archlinux.org/viewtopic.php?pid=1944161#p1944161 because xrandr report only 1 monitor (eDP-1).
Tried adding kernel parameter -> not working

Decoded EDID for eDP-1:

Block 0, Base EDID:
  EDID Structure Version & Revision: 1.4
  Vendor & Product Identification:
    Manufacturer: APP
    Model: 44594
    Serial Number: 6714457 (0x00667459)
    Made in: week 22 of 2019
  Basic Display Parameters & Features:
    Digital display
    Bits per primary color channel: 10
    DisplayPort interface
    Maximum image size: 60 cm x 34 cm
    Gamma: 2.20
    DPMS levels: Off
    Supported color formats: RGB 4:4:4
    First detailed timing includes the native pixel format and preferred refresh rate
  Color Characteristics:
    Red  : 0.6796, 0.3203
    Green: 0.2548, 0.6982
    Blue : 0.1484, 0.0566
    White: 0.3125, 0.3291
  Established Timings I & II: none
  Standard Timings: none
  Detailed Timing Descriptors:
    DTD 1:  2560x2880   59.981580 Hz   8:9    177.665 kHz    483.250000 MHz (299 mm x 336 mm)
                 Hfront   48 Hsync  32 Hback   80 Hpol P
                 Vfront    3 Vsync  10 Vback   69 Vpol N
    DTD 2:  2048x2304   59.956506 Hz   8:9    142.097 kHz    313.750000 MHz (299 mm x 336 mm)
                 Hfront   48 Hsync  32 Hback   80 Hpol P
                 Vfront    3 Vsync  10 Vback   53 Vpol N
    Display Product Name: 'iMac'
    Display Product Serial Number: '9B0835B597426'
  Extension blocks: 1
Checksum: 0x49

----------------

Block 1, DisplayID Extension Block:
  Version: 1.3
  Extension Count: 0
  Display Product Type: Standalone display device
  Tiled Display Topology Data Block (0x12):
    Capabilities:
      Behavior if it is the only tile: Image is scaled to fit the entire tiled display
      Behavior if more than one tile and fewer than total number of tiles: Undefined
    Tiled display consists of a single physical display enclosure
    Num horizontal tiles: 2 Num vertical tiles: 1
    Tile location: 0, 0
    Tile resolution: 2560x2880
    Tiled Display Manufacturer/Vendor ID: APP
    Tiled Display Product ID Code: 44595
    Tiled Display Serial Number: 3446043737
  Video Timing Modes Type 1 - Detailed Timings Data Block:
    DTD:  3840x2160   59.996625 Hz  16:9    133.312 kHz    533.250000 MHz (aspect 16:9, no 3D stereo)
               Hfront   48 Hsync  32 Hback   80 Hpol P
               Vfront    3 Vsync   5 Vback   54 Vpol N
    DTD:  3200x1800   59.981809 Hz  16:9    111.086 kHz    373.250000 MHz (aspect 16:9, no 3D stereo)
               Hfront   48 Hsync  32 Hback   80 Hpol P
               Vfront    3 Vsync   5 Vback   44 Vpol N
    DTD:  2560x1440   59.950550 Hz  16:9     88.787 kHz    241.500000 MHz (aspect 16:9, no 3D stereo)
               Hfront   48 Hsync  32 Hback   80 Hpol P
               Vfront    3 Vsync   5 Vback   33 Vpol N
  Vendor-Specific Data Block (0x7f) (Apple), OUI 00-10-FA:
    Type: BLC Info/Corrections, Version: 1
    00 12 76 31 fc 78 fb ff 02 10 88 62 d3 fa f8 f8 '..v1.x.....b....'
    fe ff ff                                        '...'
  Checksum: 0x72
Checksum: 0x90

I attached drm_info output

 

[Nermal]

it seems Apple may have retained the 5K tiled method for the 2020 model even though the Displayport chip supports single-cable 5K.

I think you might be right: in early versions of the OS for this model there was a bug where sometimes True Tone would only apply for half of the display at a time. I can't think of why that would happen if it wasn't still using two displays internally.

 

[hamacoi]

Just bought a new USB drive so I can try other Linux Distro.

Boot with OCLP, run videoinfo from Grub command line giving me 5120x2880x32 (as in the attachment). I added video=eDP-1:5120x2880@MR-32 kernel parameter but it's not working. Booted to Ubuntu and got the same 1/2 5K resolution as in the attached image.

 

[hamacoi]

Boot without OCLP, running `edid-decode /sys/call/drm/card1-eDP-1/edid`, I got this:

Block 0, Base EDID:
  EDID Structure Version & Revision: 1.4
  Vendor & Product Identification:
    Manufacturer: APP
    Model: 44593
    Serial Number: 6714457 (0x00667459)
    Made in: week 22 of 2019
  Basic Display Parameters & Features:
    Digital display
    Bits per primary color channel: 10
    DisplayPort interface
    Maximum image size: 60 cm x 34 cm
    Gamma: 2.20
    DPMS levels: Off
    Supported color formats: RGB 4:4:4
    First detailed timing does not include the native pixel format and preferred refresh rate
  Color Characteristics:
    Red  : 0.6796, 0.3203
    Green: 0.2548, 0.6982
    Blue : 0.1484, 0.0566
    White: 0.3125, 0.3291
  Established Timings I & II: none
  Standard Timings: none
  Detailed Timing Descriptors:
    DTD 1:  3840x2160   59.996625 Hz  16:9    133.312 kHz    533.250000 MHz (597 mm x 336 mm)
                 Hfront   48 Hsync  32 Hback   80 Hpol P
                 Vfront    3 Vsync   5 Vback   54 Vpol N
    DTD 2:  2560x1440   59.950550 Hz  16:9     88.787 kHz    241.500000 MHz (597 mm x 336 mm)
                 Hfront   48 Hsync  32 Hback   80 Hpol P
                 Vfront    3 Vsync   5 Vback   33 Vpol N
    Display Product Name: 'iMac'
    Display Product Serial Number: '9B0835B597426'
  Extension blocks: 1
Checksum: 0xf8

----------------
Block 1, CTA-861 Extension Block:
  Revision: 3
  Underscans IT Video Formats by default
  Native detailed modes: 0
  Vendor-Specific Data Block (Apple), OUI 00-10-FA:
    00 12 76 31 fc 78 fb ff 02 10 88 62 d3          '..v1.x.....b.'
  Vendor-Specific Data Block (Apple), OUI 00-10-FA:
    fa f8 f8 fe ff ff                               '......'
  Detailed Timing Descriptors:
    DTD 3:  3200x1800   59.981809 Hz  16:9    111.086 kHz    373.250000 MHz (597 mm x 336 mm)
                 Hfront   48 Hsync  32 Hback   80 Hpol P
                 Vfront    3 Vsync   5 Vback   44 Vpol N
Checksum: 0x44  Unused space in Extension Block: 78 bytes

So with OLCP, the monitor information correctly collected by the amdgpu graphic driver (in my previous message), it's just not able to deal with the tiled display as the bootcamp driver for Windows.

 

[tornado99]

Drminfo shows that Apple is advertising both tiles to the system. It's just that Linux is picking only one tile to display. Seems likely there are commands to switch tile, and also hopefully display both tiles together.

You could try changing the desktop environment and/or windowing environment (e.g. Manjaro KDE). Gnome+X11, Gnome+Wayland, KDE+X11, KDE+Wayland all handle tiled displays in different ways. As a test, one of the 4 combinations might work. There is normally a Wayland/X11 toggle in the bottom left/right corner of the login screen.

I bet that if you connected an external monitor to the iMac, linux might try and put 1 tile on each display (but obviously that doesn't really solve the problem).

 

[hamacoi]

I connected to an 4k external monitor, it works normally as a second desktop/workspace with 4k resolution.

I also tried to use a modified EDID with 5K resolution via kernel parameter drm.edid_firmware but it didn't work, just shown a black screen.

I think we need an updated amdgpu driver to support the dual-tile configuration to support 5K resolution: https://wiki.archlinux.org/title/Laptop/Apple#iMac_2020

 

[antibolo]

I’ve been thinking of moving my old iMac 5K 15,1 to Linux, I already knew about the 5K limitation but tried again today, and much to my dismay this issue is still not fixed in 2025? Ugh.

I’m already running the OCLP bootloader (since I’m currently running Sequoia through it), so I just assumed their boot hack to force 5K at boot time would make it available under Linux as well, but I guess khronokernel’s comment about “whether the Linux community can use this to their advantage” turned out to be a resounding “nope”? So disappointing!

Anyway, I’ll be following this thread closely.

 

[tornado99]

You could try changing the desktop environment and/or windowing environment (e.g. Manjaro KDE). Gnome+X11, Gnome+Wayland, KDE+X11, KDE+Wayland all handle tiled displays in different ways. As a test, one of the 4 combinations might work. There is normally a Wayland/X11 toggle in the bottom left/right corner of the login screen.

I'm not clear if someone has tried this (apologies I don't have a 5K iMac yet to test myself)? I read a report somewhere that the Dell 32" 8K tiling display has been fixed, but only in X11 so far. It would involve 2 different usb sticks e.g. Manjaro KDE and Manjaro Gnome, and then using the toggles on the login screens to switch from Wayland to X11.