add RHEL 8 desktop documentation

_toc.yml

@@ -32,5 +32,6 @@ parts:
       sections:
         - file: rhel8/installation
         - file: rhel8/packages
+        - file: rhel8/desktop
         - file: rhel8/vendor_documentation
 

rhel8/desktop.md

@@ -0,0 +1,57 @@
+# Desktop on RHEL 8
+
+## Many Servers and Managers
+
+Following software is involved in getting the desktop on Linux up and running.
+
+- **Display Server** paints the image onto the screen
+  - **Xorg**: good ol' Unix X Server with network redirection
+  - **Wayland**: new and modern
+- **Display Manager** shows up at startup to authenticate the user and then start the desktop session
+  - **gdm** Gnome Display Manager is default on RHEL 8, but NoMachine NX cannot start Gnome with it
+  - **lightdm** is very flexible, but automatic Gnome screen lock does not work with it, manual locking would be needed (`dm_tool lock`)
+  - **sddm** the Simple Desktop Display Manager from the KDE world fails due to a kernel bug on RHEL 8.6
+- **Greeter**: user interface part of the display manager, e.g. for `lightdm` it is exchangable
+- **Accounts Service** (accounts-daemon) used by `gdm` to know about user information (last desktop session, profile image, etc)
+- **Session Manager** starts the actual desktop. The installed options are found in `/usr/share/wayland-sessions/` for `Wayland` and `/usr/share/xsessions/` for `Xorg`. 
+  - **gnome-session** normal Gnome starter (for `Xorg` and `Wayland`)
+  - **gnome-session-custom-session** to select a specific saved Gnome session
+  - **icewm-session** IceWM starter, `Xorg` only
+  - **startxfce4**  XFCE starter, `Xorg` only
+  - **startplasma-x11** KDE Plasma starter for `Xorg`
+  - **startplasma-wayland** KDE Plasma starter for `Wayland`
+
+Out of the box RHEL 8 starts Gnome using `gdm` and `Wayland`. `Xorg` is also supported. Others can be installed from EPEL, but there is no support from Red Hat.
+
+## PSI Specific Desktop Settings
+Per default Puppet starts `gdm` which then starts Gnome with `Xorg` using `/usr/share/xsessions/gnome-xorg.desktop`.
+
+Normally the Display Managers offer the user to select one of the available Desktop Sessions (`*.desktop` files in `/usr/share/wayland-sessions/` and `/usr/share/xsessions/`). This has been disabled as normally at the PSI this is more set per system and not per user.
+
+In Hiera the actual Desktop Session to be started can be selected/overriden by setting the `desktop::session_manager` to one of the `.desktop` files in above listed directories. Set it e.g. to `gnome-wayland` to test `Wayland`.
+
+Note when changing the default Session Manager, previous users will still get the one they used before. To reset that, you need to delete
+- `/var/lib/AccountsService/users/*` (for `gdm`)
+- `/var/cache/lightdm/dmrc/*.dmrc` (for `lightdm`)
+- `/var/lib/lightdm/.cache/lightdm-gtk-greeter/state` (for `lightdm` with `lightdm-gtk-greeter`)
+
+### NoMachine NX
+As NoMachine requires `lightdm` to start Gnome ([bug](https://kb.nomachine.com/TR08Q09375)), `lightdm` is used on these systems.
+It uses a custom patched `lightdm-gtk-greeter` to allow to select only the configured keyboard layouts and to have a custom (PS: the patches have been merged upstream very fast, yeah!).
+
+### XFCE
+XFCE is installed when `base::enable_xfce: true` is set in Hiera.
+It then is also used by default with `base::xfce_default: true` or `desktop::session_manager: xfce`.
+
+### IceWM
+IceWM is installed when `base::enable_icewm: true` is set in Hiera.
+It then is also used by default with `desktop::session_manager: icewm-session`.
+
+### Using a different Desktop (e.g. KDE)
+The respective Destop needs to be installed, e.g. manually or through Puppet. 
+The respective Session Manager can be set as system default in Hiera with  `desktop::session_manager`.
+
+If a different Display Manager is needed, or `lightdm` on other occasions, then changes in our Puppet code are required.
+
+
+