Crate drmkit_session 

Holding devices across a VT switch.

A compositor that owns the display cannot simply open /dev/dri/card0 and keep it: switching to another virtual terminal has to take the display away and give it back. A seat – logind, seatd, or a bare-root fallback – is what arbitrates that, handing out descriptors it can revoke.

What arrives from the seat is two events. Paused: stop touching every descriptor immediately, because the ioctls will start failing. Resumed: here are fresh descriptors, and every per-descriptor kernel object you had – framebuffers, property blobs, client caps, GEM handles – is gone. drmkit_cursor::Paused is the shape that pairs with this on the other side.

Why the backend is a trait

Because a seat cannot be opened on a machine that has no session. Over SSH, in a container, on a CI runner, logind says the session is busy, seatd is not running, and the builtin backend wants root and a TTY. That is most places this code will be built.

So the sequencing – which devices are held, what happens to them across a pause, when the pause is acknowledged, what a caller is allowed to do while inactive – lives in Session over a Backend, and is tested against a fake. The libseat binding is a thin adapter behind the libseat feature, and libseat’s own diagnostics arrive through drmkit-log, tagged [libseat], at error level.

Not here yet

Two things the reference has and this does not.

TakeDeviceOpts::preserve_fd_across_resume asks the seat to hand back the same descriptor rather than a fresh one, on the grounds that the capability-revoking backends leave it valid. Plausible, and it saves a reopen per device per switch, but it is an optimization over a path that is already correct – and getting it wrong means scanning out through a descriptor the kernel has stopped honoring, on a machine none of the CI lanes resemble. It waits for a device to be proved on.

Seat::input_opener routes libinput’s privileged device opens through the same seat, so input descriptors get the revocable lifetime the DRM one has.

Both halves it needs now exist: this crate opens devices through a seat, and drmkit_input::Seat::open_with takes any LibinputInterface the caller supplies. What is missing is the piece between them — an implementation of that trait backed by a Session, so an input descriptor is revoked and reissued on a VT switch the way the DRM one is. Until it exists a caller passing their own opener gets input descriptors with an ordinary lifetime, which works and is not what the seat is for.

Structs

DeviceId

A seat’s handle for one open device.

LibseatBackendlibseat

A session backed by libseat.

Session

Devices held across VT switches.

TakenDevice

A device this session is holding.

Enums

RawEvent

What a seat says, before the session has made sense of it.

SessionError

What can go wrong holding a session.

SessionEvent

What a caller needs to act on.

Constants

DEFAULT_ENABLE_TIMEOUTlibseat

How long LibseatBackend::open waits for the seat to activate.

Traits

Backend

What a seat has to be able to do.