Snowfall — YetiOS Login Manager

A minimal, DRM-native graphical login manager for YetiOS.

Architecture

Snowfall is a framebuffer login manager that renders directly to DRM/KMS (no X11, no Wayland compositor required). It authenticates users via PAM, then execs the selected session as the authenticated user. When no session files are installed, snowfall falls back to a built-in default — on YetiOS, CrystallineLattice (glacier).

snowfall (DRM master)
  ├── drm.c          KMS/DRM framebuffer setup
  ├── input.c        libinput keyboard handling
  ├── renderer.c     Cairo-based UI drawing
  ├── auth.c         PAM authentication
  ├── session.c      Session discovery + launch
  ├── ui_state.c     UI state machine (field focus, navigation)
  └── main.c         Lifecycle: init → render loop → auth → exec

When snowfall starts, it grabs DRM master (which causes snowcone to exit). After successful authentication it drops DRM master, switches to the user's VT, and execs the compositor.

The compositor (glacier, sway, …) acquires the GPU and input devices through libseat, which talks to the seatd daemon. snowfall does not start a seat manager itself: seatd runs as its own OpenRC service that snowfall needs, and snowfall sets LIBSEAT_BACKEND=seatd in the session environment. See Seat management.

Dependencies

• libdrm
• libinput + libudev
• cairo
• libpam
• libxkbcommon (keymap handling)
• seatd (runtime — seat/GPU/input management for the launched compositor)

Alpine / YetiOS

apk add libdrm-dev libinput-dev cairo-dev linux-pam-dev libxkbcommon-dev eudev-dev

Build

make
sudo make install

OpenRC

sudo rc-update add snowfall default

The service depends on udev (for libinput) and seatd (for the compositor's seat), and starts after snowcone (the boot splash). When snowfall grabs DRM master, snowcone detects the loss and exits.

Seat management

The login manager renders directly to DRM/KMS, but the compositor it hands off to (glacier, sway, …) needs a seat manager to be given the GPU and input devices. snowfall uses seatd for this — not seatd-launch, which is upstream's workaround for systems without a running seatd.

Two things must be in place:

1. The seatd service is enabled. snowfall needs it, so installing the seatd package (which ships /etc/init.d/seatd) is enough for OpenRC to start it on demand. To run it independently:

   sudo rc-update add seatd boot

2. Login users are in the seat group. libseat connects to /run/seatd.sock, which is owned root:seat. snowfall applies the user's groups (initgroups) before launching the session, so simply:

   sudo addgroup <user> seat

snowfall sets LIBSEAT_BACKEND=seatd for the session so libseat targets seatd directly instead of probing for a (nonexistent on YetiOS) logind. A missing/stopped seatd shows up in ~/.snowfall-session.log as:

[ERR ] libseat_open_seat: No such file or directory (is seatd running, user in 'seat'?)

Session files

Snowfall reads .desktop files from /usr/share/wayland-sessions/. Each file must have an Exec= line. Example:

[Desktop Entry]
Name=Sway
Exec=sway
Type=Application

If the directory is empty or absent, snowfall offers a built-in default session that launches CrystallineLattice (glacier), YetiOS's from-scratch DRM/KMS layer. Override it by dropping a .desktop file here, or change SF_DEFAULT_EXEC in src/session.c.

License

MIT License - YetiOS Project