[systemd-devel] [WIP PATCH 2/4] ACPI: button: remove the LID input node when the state is unknown

[Peter Hutterer]

On Tue, Jun 06, 2017 at 12:22:09PM +0200, Benjamin Tissoires wrote:
[...]
> > This is because the aml table puts many Notify(LID, 0x80) in various control methods.
> > And not one of them but multiple of them will be invoked by various OS drivers during suspend/resume period.
> > I think this is not an isolated platform that will invoke multiple redundant "Notify(lid)".
> > 
> > Fortunately, the lid state for the multiple notify(lid) should be same as the first "Notify(lid)".
> > I suppose this is why SW_LID is invented, as it can really filter such redundant events.
> > And user space finally can only see 1 "close" event.
> > 
> > But unconditionally prepending "open" before all "close" events surely can break the logic by
> > delivering multiple "close" events to the user space.
> 
> That doesn't matter. What matters is the state of the switch, not the
> event. So if user space receives (in case we marked the switch as not
> reliable) several close events, all user space will do is realize that
> the state is still closed and will act accordingly.

[...]

> Again, we don't care if the "event" comes from ACPI, the driver itself or
> user space (libinput). All that matters is the current state of the
> input node switch, that needs to match the physical world at any time.

as extra comment on those two: you cannot guarantee when e.g. libinput
checks the state. it may start up after the kernel has updated the EV_SW
state, it may close and re-open a device without notice (disabling a
device in X has that effect for example). So the EV_SW/SW_LID events are
nice to have, but we really rely on the *state* of the switch more than the
events.

Cheers,
   Peter