Comment # 19 on bug 1188954 from Felix Miata

One only. On the fresh TW20210810 installation on host asa88 from which comment
#17 resulted, no DM other than XDM is or was installed.

I found another AMD host with TW20210810/TDM that requires this workaround,
ara88, CPU/APU: AMD PRO A8-8650B R7.

As deano noted in the forum thread, on
https://adamsdesk.com/blog/2021/02/15/gdm-no-longer-starts-automatically/ is
explained this had been surfacing apparently more than 6 months ago on Arch and
Fedora. There it points out the likelihood of this happening was discovered
over 9 years ago: https://gitlab.gnome.org/GNOME/gdm/-/issues/103.

In the adamsdesk article one workaround (out of several) was suggested, and
repeated in my forum thread, which I initially found not to work on asa88 with
TDM: Wants=dev-dri-card0.device & After=dev-dri-card0.device instead of
After=systemd-udev-settle.service & Requires=systemd-udev-settle.service. I
later discovered that I hadn't coupled the dev-dri-card0.device method with a
required /etc/udev/rules.d/99-make-udev-drm-aware.rules containing
'SUBSYSTEM=="drm", TAG+="systemd"'. With the udev rule, dev-dri-card0.device
also works for hosts asa88, ab250 and gb250, but not host ara88.

I need to note to that systemd-udev-settle.service as shipped, which ends with
"ExecStart=udevadm settle", does not work. In order to work it needs the
version shipped in 15.2, which ends with "ExecStart=/usr/bin/udevadm settle".