System: A8-5500B with Radeon HD7560D with VGA and DP ports on an HP Compaq Pro 6305 MT desktop.
Monitor: Westinghouse LTV27 hooked up by DP-to-DVI adapter or VGA
Dual-boots windows 10 and Ubuntu 18.04 from separate hard drives.
Up-to-date AMD drivers are installed under W10 with Catalyst v2015.0804.21.41908; not under Ubuntu.
Problem: TV stopped detecting any signal from the DP several days ago during POST screens and Windows boot. The GRUB screen on the ubuntu drive was also not detected.
However, during Ubuntu boot, signal was detected, but at 1024x768, while the tv's max rez is 1280x720. Ubuntu's display manager listed no higher resolution than 1024x768. After noticing in the kernel log that the OS was declaring the tv's EDID invalid, I followed instructions to generate code in terminal using the xrandr utility and add it to the xorg startup scripts, and now Ubuntu displays perfectly in 1280x720 via DP, but the BIOS and Windows screens are still blank. Everything still works normally via VGA.
The fact that this was easily resolved in Ubuntu rules out hardware faults IMO. The fact that it occurs during POST also rules out a Windows software problem being at fault. The BIOS provides no option of switching the DP on or off or any other display option than choosing between integrated video or pci-e card. So I am fairly certain this is an issue with the Radeon adapter. I'm still trying to figure out how to duplicate the linux workaround in W10; there, under VGA with the DP connected also, Catalyst "detect display" finds only the one VGA display, while Microsoft's instructions for overriding EDID are absurdly complex.
The hardware and software configurations were consistent before and after the DP went out. A potentially significant event was hooking up a laptop to the TV's DVI port via hdmi-to-dvi cable, after which the desktop DP problem started. The laptop still displays perfectly.
I suspect this is a so-called "handshake" problem, maybe involving the dp-to-dvi adaptation scheme, but the fact that it was working before should mean it can work again.