Each device in libinput is assigned to one seat. A seat has two identifiers, the physical name and the logical name. The physical name is summarized as the list of devices a process on the same physical seat has access to. The logical seat name is the seat name for a logical group of devices. A compositor may use that to create additional seats as independent device sets. Alternatively, a compositor may limit itself to a single logical seat, leaving a second compositor to manage devices on the other logical seats.
Below is an illustration of how physical seats and logical seats interact:
The devices “Foo”, “Bar” and “Spam” share the same physical seat and are thus available in the same libinput context. Only “Foo” and “Bar” share the same logical seat. The device “Egg” is not available in the libinput context associated with the physical seat 0.
The above graph is for illustration purposes only. In libinput, a struct libinput_seat comprises both physical seat and logical seat. From a caller’s point-of-view the above device layout is presented as:
Thus, devices “Foo” and “Bar” both reference the same struct libinput_seat, all other devices reference their own respective seats.
The effect of seat assignment
A logical set is interpreted as a group of devices that usually belong to a single user that interacts with a computer. Thus, the devices are semantically related. This means for devices within the same logical seat:
if the same button is pressed on different devices, the button should only be considered logically pressed once.
if the same button is released on one device, the button should be considered logically down if still down on another device.
if two different buttons or keys are pressed on different devices, the logical state is that of both buttons/keys down.
if a button is pressed on one device and another device moves, this should count as dragging.
if two touches are down on different devices, the logical state is that of two touches down.
libinput provides functions to aid with the above: libinput_event_pointer_get_seat_button_count(), libinput_event_keyboard_get_seat_key_count(), and libinput_event_touch_get_seat_slot().
Internally, libinput counts devices within the same logical seat as related. Cross-device features only activate if all required devices are in the same logical seat. For example, libinput will only activate the top software buttons (see Lenovo *40 series touchpad support) if both trackstick and touchpad are assigned to the same logical seat.
A device may change the logical seat it is assigned to at runtime with libinput_device_set_seat_logical_name(). The physical seat is immutable and may not be changed.
Changing the logical seat for a device is equivalent to unplugging the device and plugging it back in with the new logical seat. No device state carries over across a logical seat change.