Evil Cable Confusion
DP_PWR / Pin 20 Functionality
When you read up on the DisplayPort connector in the VESA specification, it will show that both the source and the sink are supposed to provide power at 3.3V+-10% and 500mA on the DP_PWR pin, pin 20, of the connector.
DP_PWR is very useful to supply power to dongles that are connected to the source or the sink. Dongles always have a bit of electronics in them, and thanks to DP_PWR, you don't need a separate power supply wire.
Here are a couple examples of dongles:
DP to DVI convertor
In this case, the dongle with a chip that converts from DP to DVI would be plugged into the source.
DP to Ethernet to DP
In this case, you'd have two dongles: on the source side, a dongle would convert from DP to Ethernet. And on the sink side, a different dongle would convert back from Ethernet to DP. In this case, it's clear that both the source and sink should supply power to the DP_PWR pin.
The problem with this arrangment is that one should never ever connect the DP_PWR of the source to the DP_PWR of the sink: it's impossible for both pins to have exactly the same voltage, so connecting them together would effectively result in a short circuit condition.
In addition, if, your GPU is powered off, while your monitor is powered on, you get a problem that's commonly called 'backdrive': a sneaker path that feeds electrical compoonents in the GPU even though nothing is supposed to have power. This can result in your GPU behaving erratically when trying to power up. In some cases, it can even damage the GPU.
This is why regular DP cables should never have a wire between pin 20 of boths sides of the cable.
The official DisplayPort FAQ used to say the following:
Q: Why isn’t there a wire on pin 20 (POWER) on the standard external DisplayPort cables?
A: The reason power isn’t included in standard cables is because both source and sink devices are designed to provide power. Captive, attached cables often include the power wire. If it is desired, for example, that a particular source device utilize the power available from the mating sink device, then that Source device could include an attached or dedicated cable that carries the DisplayPort power signal. Same could be applied to a sink device.
(This explanation has been removed from the FAQ.)
Yet some DP cable manufacturers never bothered to really understand the specification and flooded the market with some cheap cables that do exactly that. This was especially common for miniDP (mDP) to DP cables.
If you happen to have such a cable, your best advice is to throw it away and buy a new one that doesn't have this issue.