Hi,
I’ve got a small problem. Last week I obtained an “old” iMac 21.5 inch. I installed Debian 10 on it. At first login, I noticed I just get a black screen and a non blinking cursor(*). Strange, so I moved to a tty and noticed that ligthdm was in a failed state and X wasn’t running. After some fiddling around installing some addition packages (firmware-amd-graphics and firmware-linux-nonfree) lightdm no longer failed and X acutally no longer “failed”. But still a black screen. Also if I ran xinit firefox, I can see a running firefox process (in top on another tty) but a black screen. The funny thing is that I can SSH into the iMac and run firefox in the SSH session with X-forwarding.
Also, I installed gdm3 which weirdly enough seems to work. Only if I want to launch my desktop environment, I get a black screen again (even the backlight goes off). It 's like the computer kernel panics and seems to be turned off.
When I switch to a TTY, and go back to the gui, I’m back at the login screen of gdm3.
I get the feeling that it might be confused somehow because it’s got the Iris graphics and an AMD GPU. Somehow I think it’s actually showing something on a non-existent screen or so.
I also attached some relevant log files/output of commands.
Does someone maybe have an idea as to what’s going on here and what else I can do to debug this problem?
Thanks in advance 
/var/log/messages
Nov 1 06:16:40 imac gnome-session[761]: gnome-session-binary[761]: WARNING: Lost name on bus: org.gnome.SessionManager
Nov 1 06:16:40 imac gsd-housekeepin[899]: Error releasing name org.gnome.SettingsDaemon.Housekeeping: The connection is closed
Nov 1 06:16:40 imac gnome-shell[786]: Connection to xwayland lost
Nov 1 06:16:40 imac ibus-daemon[1067]: GChildWatchSource: Exit status of a child process was requested but ECHILD was received by waitpid(). See the documentation of g_child_watch_source_new() for possible causes.
Nov 1 06:16:40 imac ibus-daemon[1067]: GChildWatchSource: Exit status of a child process was requested but ECHILD was received by waitpid(). See the documentation of g_child_watch_source_new() for possible causes.
Nov 1 06:16:40 imac gdm3: Gdm: Child process -750 was already dead.
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output of journalctl -x -p err
Nov 01 05:41:14 imac kernel: ACPI Error: Evaluating _BCM failed (20200717/video-357)
Nov 01 05:41:14 imac kernel: sd 7:0:0:0: [sdc] No Caching mode page found
Nov 01 05:41:14 imac kernel: sd 7:0:0:0: [sdc] Assuming drive cache: write through
Nov 01 05:41:23 imac bluetoothd[586]: profiles/sap/server.c:sap_server_register() Sap driver initialization failed.
Nov 01 05:41:23 imac bluetoothd[586]: sap-server: Operation not permitted (1)
Nov 01 05:47:02 imac gdm-password][1180]: pam_unix(gdm-password:auth): conversation failed
Nov 01 05:47:02 imac gdm-password][1180]: pam_unix(gdm-password:auth): auth could not identify password for [wannes]
Nov 01 06:00:54 imac bluetoothd[586]: src/profile.c:ext_start_servers() RFCOMM server failed for Headset Voice gateway: rfcomm_bind: Address already in use>
Nov 01 06:00:54 imac bluetoothd[586]: src/profile.c:ext_start_servers() RFCOMM server failed for Headset unit: rfcomm_bind: Address already in use (98)
Nov 01 06:01:46 imac bluetoothd[586]: src/profile.c:ext_start_servers() RFCOMM server failed for Headset Voice gateway: rfcomm_bind: Address already in use>
Nov 01 06:01:46 imac bluetoothd[586]: src/profile.c:ext_start_servers() RFCOMM server failed for Headset unit: rfcomm_bind: Address already in use (98)
Nov 01 06:16:30 imac gdm-password][1984]: gkr-pam: unable to locate daemon control file
Nov 01 06:16:40 imac gdm-wayland-session[750]: GLib: Source ID 2 was not found when attempting to remove it
Nov 01 06:16:40 imac gdm-launch-environment][704]: pam_systemd(gdm-launch-environment:session): Failed to release session: Interrupted system call
Nov 01 06:19:38 imac gdm-x-session[1998]: GLib: Source ID 2 was not found when attempting to remove it
Nov 01 06:19:38 imac gdm-wayland-session[2386]: GLib: Source ID 2 was not found when attempting to remove it
kernel ring buffer
[ 1191.555870] snd_hda_codec_hdmi hdaudioC1D0: HDMI ATI/AMD: no speaker allocation for ELD
[ 1244.410905] snd_hda_codec_hdmi hdaudioC1D0: HDMI ATI/AMD: no speaker allocation for ELD
[ 2074.290927] rfkill: input handler disabled
[ 2128.553965] rfkill: input handler enabled
[ 2130.893163] show_signal: 8 callbacks suppressed
[ 2130.893165] traps: light-locker[2186] trap int3 ip:7f32ee63d9d7 sp:7ffecaf7ce10 error:0 in libglib-2.0.so.0.6600.1[7f32ee601000+88000]
[ 2134.500584] ISO 9660 Extensions: Microsoft Joliet Level 3
[ 2134.503407] ISO 9660 Extensions: Microsoft Joliet Level 3
[ 2134.517540] ISO 9660 Extensions: RRIP_1991A
[ 2260.533459] snd_hda_codec_hdmi hdaudioC1D0: HDMI ATI/AMD: no speaker allocation for ELD
[ 2260.715568] radeon_dp_aux_transfer_native: 116 callbacks suppressed
[ 2262.698859] rfkill: input handler disabled
[ 2302.637460] rfkill: input handler enabled
[ 2316.961472] rfkill: input handler disabled
[ 2316.994857] rfkill: input handler enabled
[ 2317.266599] snd_hda_codec_hdmi hdaudioC1D0: HDMI ATI/AMD: no speaker allocation for ELD
root@imac:~#
output of lshw -class video
root@imac:/var/log/gdm3# lshw -class video
*-display
description: VGA compatible controller
product: Whistler [Radeon HD 6630M/6650M/6750M/7670M/7690M]
vendor: Advanced Micro Devices, Inc. [AMD/ATI]
physical id: 0
bus info: pci@0000:01:00.0
version: 00
width: 64 bits
clock: 33MHz
capabilities: pm pciexpress msi vga_controller bus_master cap_list rom
configuration: driver=radeon latency=0
resources: irq:50 memory:90000000-9fffffff memory:a8800000-a881ffff ioport:2000(size=256) memory:a8820000-a883ffff
*-display
description: Display controller
product: 2nd Generation Core Processor Family Integrated Graphics Controller
vendor: Intel Corporation
physical id: 2
bus info: pci@0000:00:02.0
version: 09
width: 64 bits
clock: 33MHz
capabilities: msi pm bus_master cap_list
configuration: driver=i915 latency=0
resources: irq:49 memory:a8000000-a83fffff memory:a0000000-a7ffffff ioport:3000(size=64)
root@imac:/var/log/gdm3#
(*): edit it was blinking after all