On the Plasma workspaces Display Power Management Signaling (DPMS) is handled by the power management daemon (powerdevil). After a configurable idle time it signals the X-Server through the X11 DPMS Extension. The X-Server handles the timeout, switching to a different DPMS level and restoring to enabled after an input event all by itself.
The X11 extension is a one-way channel. One can tell the X-Server to go to DPMS, but it neither notifies when it has done so nor when it restored. This means powerdevil doesn’t know when the screens are off. Granted, there is a call to query the current state, but we cannot perform active polling to figure out whether the state changed. That would be rather bad from a power management perspective.
So overall it’s kind of a black box. Our power management doesn’t know when the screens turn off, Neither does any other application including KWin. For KWin it would be rather important to know about it, because it’s the compositor and there is no point in running the compositor to render for something which will never be visible. The same is obviously true for any other application: it will happily continue to render updated states although it won’t be visible. For a power management feature this is rather disappointing.
Now let’s switch to Wayland – in this case kwin_wayland. Last week I started to implement support for DPMS in KWin. Given that KWin is responsible for DRM, we need to add support for it in KWin. Of course we want to leverage the existing code in powerdevil, so we don’t add the actual power management functionality, but rather an easy to use interface to tell KWin to enter a DPMS state on a given output.
Of course we tried to not repeat the mistake from X11 and thus the interface is bi-directional: it notifies when a given state is entered. Although in a Wayland world that’s not so important any more: KWin enables DPMS and knows that DPMS is enabled and can make internal use of this. So whenever the outputs are put into standby the compositor loop is stopped and doesn’t try to repaint any more. If the compositor is stopped windows are not being repainted and aren’t signaled that the last frame got painted. So all applications properly supporting frame callbacks stop rendering automatically, too. This means on Wayland we are able to properly put the rendering of all applications into an off state when the connected outputs are in standby mode. A huge improvement for power management.