The new monitor I added (an Asus VS278Q-P) is connected via DisplayPort. My video card is an AMD Radeon HD 5830 card which has DVI, HDMI, and DisplayPort connections using Catalyst 15.7 driver version 15.20.1046
This is a video card, video card driver, or operating system power management issue. The monitor DP (DisplayPort) is passive. It simply waits for the signal from the video card to awake. By powering the monitor off/on, you are forcing the operating system and/or video card to reinitiate the DP handshake. The Radeon HD 7790 has eight power management states through its PowerTune technology. My guess is somewhere in that software is the ability to tell it to adjust what the card is doing as far as power management. (source: -forums/desktop/f/3515/t/19520244 )
In spite of the original intentions behind DP (a single electric hop from the south bridge to the display matrix driver), the monitor does contain an "AD board", giving it the ability to select among multiple video inputs (VGA, HDMI, DP). It's the "AD board" inside the monitor that drives the HPD signal - and it's not a plain pullup.
I've noticed the ULPS keyword in this debate and elsewhere, typically in the context of AMD graphics. My graphics adaptor is an Intel IGP (3rd gen = Ivy Bridge in this case). A rare note or two about ULPS in the context of Intel graphics can be found in some open hardware documentation, intended for open-source driver writers. Not much use in a Windows environment. Also, intel's IGP driver config utilities used to be more decent than they are now - especially the IEGD was an excellent tweakable driver package, but now we have to live with what's available. I've tried prolonging the DelayedDetectionForDP in the registry, which had no effect. And, in the config util and VGA driver properties there's no way to "force the port up". (Nor is there an option to disable dependence on DDC, but the DDC/EDID availability seems to be a separate problem, different from the HPD input or VGA load impedance measurement.)
Ultimately, I resorted to soldering on the AD board (inside the monitor). Long story short, fortunately there was a neat PCB trace going from pin 18 at the back of the DP socket. I found a 10-Ohm resistor in series with the gate output driving the HPD signal - so I removed that. And, I attached a 1k pull-up to a nearby capacitor (MLCC) blocking the +3.3V standby voltage rail. Now the HPD line is always pulled high, as long as the display is plugged into the wall. Apparently the theoretical possibility of monitor-to-PC interrupts is not a requirement for the monitor to function properly. I'm attaching a photo merely for illustration. No I'm not gonna mention the monitor or AD board makers. A word of caution: you cannot just short the HPD trace to +3.3V and be done with it - in my case, the gate output (HPD line driver) measured as 30 Ohms against GND when low. A short to +3.3V would blow something (you'd be lucky to fry just the gate output). This kind of a hack takes some precautions and "know how" that belong to electronics.stackexchange.com. Not to mention some basic tools: a soldering pen, a multimeter with sharp probes, and a strong magnifying glass. (And something to cleanly desolder the poppy seed sized resistor... some would use a thin stream of hot air, I may prefer a vintage soldering gun with a custom double-tipped loop made of AWG24 wire.) 2b1af7f3a8