Scribbles for my memory

I run a lid-closed laptop with two external monitors as a remote-access host. Remote desktop into it works perfectly, but the physical monitors sitting on my desk were pitch black. Cables fine, monitors powered, nothing in the logs.

The screens weren't broken — they were just DPMS-blanked, and nothing I did remotely could wake them:

$ sudo XAUTHORITY=/var/run/lightdm/root/:0 DISPLAY=:0 xset q
DPMS (Display Power Management Signaling):
  Standby: 600    Suspend: 0    Off: 900
  DPMS is Enabled
  Monitor is Off

One command brings them back:

sudo XAUTHORITY=/var/run/lightdm/root/:0 DISPLAY=:0 xset dpms force on

The reason it happened at all is the part worth remembering: xrdp gives you a second X server. The console session is Xorg :0; your RDP session is Xorg :10. They're separate processes with separate idle timers. Every keystroke and mouse move you make remotely resets the idle timer on :10 — and :0 never hears about any of it. So :0 sits there accumulating idle time forever and blanks on schedule, no matter how busy you look on the other display.

If the host is supposed to stay lit, disable blanking in the console user's session, not yours. On XFCE:

xfconf-query -c xfce4-power-manager -p /xfce4-power-manager/dpms-enabled -s false
for k in blank-on-ac blank-on-battery dpms-on-ac-sleep dpms-on-ac-off \
         dpms-on-battery-sleep dpms-on-battery-off; do
  xfconf-query -c xfce4-power-manager -p /xfce4-power-manager/$k -s 0
done

Those only take effect when xfce4-power-manager restarts, so log the console user out and back in. Note the settings persist to ~/.config/xfce4/xfconf/xfce-perchannel-xml/, not ~/.config/xfconf/ — I looked in the wrong place first.

One trap after all that: I logged back in, confirmed DPMS is Disabled, and declared victory too early. xset q still read timeout: 600. That's X's own built-in screensaver, a completely separate mechanism that the power manager's blank-on-ac=0 never touched. DPMS controls whether the monitor gets powered down; the X screensaver just paints over the screen while it stays on. From a chair three feet away the two are indistinguishable, and remote input can't dismiss either one. Kill both, from the console user's autostart:

xset s off s noblank -dpms

Check for timeout: 0 and DPMS is Disabled before you believe it.