[eqvinox]

> The reverse adapters, from HDMI to DisplayPort, are much more expensive and much bulkier, so usually they are not acceptable.

That's because those are active converters — contrast DisplayPort has "DP++" which means the source port is electrically capable of transmitting either DP or HDMI signals; the graphics card can switch modes. The adapter is a tiny IC to signal doing that switchover and just wires the data lanes through. HDMI has no such thing, you need an active protocol converter IC to get DisplayPort.

(NB: there are also active DP→HDMI converters, they have a bit longer range than the passive ones. I had to use one of them for my home projector, it's on a 10m HDMI cable which only worked on a blue moon with a passive DP++ adapter. Funnily enough it doesn't work on my native HDMI port either, only the active converter gets it running reliably… might be a poor 10m cable ;D)

DP++ wasn't part of the original DP spec, but I don't believe any DP source hardware that doesn't support DP++ is being manufactured at this point.

[adrian_b]

The DisplayPort connector includes a supply voltage. While it is weaker than in USB, it is strong enough to provide power to an active DisplayPort to HDMI converter, which can have the appearance of a video cable that can connect a DisplayPort source to an HDMI sink.

On one of the HDMI pins there is a DC voltage, but it has other purposes and it is too weak to provide power for a video converter.

This is why an HDMI to DisplayPort converter always requires an additional external power supply.