SDDM

Simple Desktop Display Manager (SDDM) is Article description::a modern [[display manager that supports both the X11 server and the Wayland protocol.]]

Emerge
Install :

If there are performance issues, it might help to add the sddm user to the video group:

Files
SDDM has two configuration files: the package installed and  which is used to override specific options. The second is not created by the package. KDE Plasma writes user changed options to. Both files have the same format. See comments in the file and for details on available options.

Prior to 0.18.0
Prior to 0.18.0 the configuration was only in. Some of the files may have had to be generated manually. If installing 0.18.0, skip ahead to Service section.

Keymap
To select the correct keymap on the login screen, add following lines to the file:

This file is not created automatically when the package is installed so it may need to be created.

Next create the directory

and the file.

the first country code is the default. Finally set execute permissions on the file.

With display-manager
If is not present, emerge it with:

The configuration file should be modified to use SDDM:

To start the chosen display manager on boot, add the display-manager to the system's default runlevel:

To start the display-manager immediately, run:

With the deprecated xdm init script
Set SDDM as the default display manager:

To start SDDM on boot, add xdm to the default runlevel:

To start SDDM now:

systemd
To start SDDM on boot:

To start SDDM now:

Plasma
Graphical configuration is integrated in Plasma 5 system settings by installing :

Long load time before SDDM shows the greeter
A low entropy pool can cause long SDDM load time - see upstream bug report. If using systemd, the graphical target is reached and then everything appears to hang. Moving the mouse or using the keyboard will make the SDDM greeter launch (faster).

Solve the problem by using, for example, the package to increase the entropy pool or by enabling RANDOM_TRUST_CPU kernel config option with a recent-enough CPU.

Permission denied errors in Xorg.log
The X server will not start and permission denied errors (such as the following) are present in the log file:

It is likely the sddm user has not been added to the video group. Running the following command should fix the problem:

Missing system buttons
SDDM only displays buttons if the functionality evaluates to being available. This can depend on several factors. For OpenRC systems using elogind, this can be caused by the service not running when the display manager is initialized. To make sure it is running, just add elogind to the boot run level.

Missing users
If the login screen is missing some user(s) to choose from, this might be caused by - standard   is 1000 and some existing users may have lower uids.

SDDM service starts but yields a black screen
On NVIDIA cards, SDDM appears to require DRM enabled and operational according to this closed bug report.

Ensure Direct Rendering Manager is enabled in the kernel. If it isn't, enable it, rebuild the kernel, and re-emerge nvidia-drivers.

Ensure the nvidia-drm module is configured to load at startup:

Finally, ensure nvidia-drm has modesetting enabled:

Alternatively, modesetting can be enabled on the kernel command line with nvidia-drm.modeset=1. This can be set in the bootloader or embedded in the kernel configuration.