What I find very annoying is that a very large number of small computers and laptops have both DisplayPort and HDMI, but they have full-size HDMI connectors and only USB Type C DisplayPort.

Using Type C for DisplayPort instead of the good full-size DisplayPort connectors is less reliable (easy to disconnect accidentally) and it permits only shorter video cables.

More importantly, this blocks the Type C connector, which I need for other purposes, e.g. an external SSD. I do not want to carry a Type C dock, so I end using HDMI, even if I do not need HDMI and I do not want HDMI and even if in almost all cases the devices had enough free space for a full-size DisplayPort connector.

Even replacing the HDMI connector with a DisplayPort connector (so that the devices would have only full-size and Type C DisplayPort) is always a better solution, because there are a lot of cheap adapters from DisplayPort to HDMI, which do not need a separate power supply and they can even be incorporated inside the video cable. The reverse adapters, from HDMI to DisplayPort, are much more expensive and much bulkier, so usually they are not acceptable.

> 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.

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.

It seems the issue with this open-source driver is supporting some of the highest-performance modes of HDMI (like 4K @ 120Hz). Would that even work in a DisplayPort-to-HDMI adapter?

While there are no fundamental reasons for any video mode to not work, most of the DisplayPort to HDMI adapters that are currently on the market do not support the latest standard versions of DisplayPort and/or HDMI, so when a very high performance mode is desired, it might not work.

However, the main use of adapters is when you travel and you find in your temporary office an HDMI-only monitor, or when you must use a meeting room projector. Such monitors or projectors seldom support high performance video modes.

A problem is also that many video cards that do support HDMI 2.1 only support display port DisplayPort 1.4 which has less bandwith. This makes the sitution with the open source AMD drivers even more annoying because even with an active adapter that supports all the required features (which most don't) you can't get the full HDMI 2.1 resolutions/refresh rates that way.

FYI: USB Type-C ports are DP ports on most modern laptops. You just need the correct cable (or a display with Type-C connector.) I have one of these https://www.club-3d.com/en/quick-view/2470/

(actually works both directions, e.g. if you have a portable display with only Type-C connectors like this https://www.hp.com/us-en/shop/pdp/hp-e14-g4-portable-monitor — BUT it can't power the display, you need to use another connection on the display for that.)

There is no HDMI over Type-C (there was an attempt at it, but it died. Probably for the better of not having even more Type-C confusion and interoperability issues.)

The "relevant devices" is surely referring to the displays here. I would love to go DP for everything but the best I can seem to find is computer monitors with 1 DP input and usually 2 or more HDMI. For living room type displays you won't find DP at all.

For PC displays on geizhals.eu, out of 3327 products:

   157 (4.7%) have  2 DP inputs
  2446 (74%)  have  1 DP input
   724 (22%)  have no DP input

Including USB-C ports,

   949 (29%)  have ≥2 DP inputs
  1806 (54%)  have  1 DP input
   572 (17%)  have no DP input

Compare HDMI:

  1350 (41%)  have ≥2 HDMI inputs
  1791 (54%)  have  1 HDMI input
   186 (5.6%) have no HDMI input

I agree it could be better but I don't think it's as bad as you make it out to be. Looking through the devices that have no DP input at all, 488 of the 572 have VGA inputs, which I'd say indicates an older generation of devices.

"Consumer" electronics (i.e. TVs) is a problem though, I'll agree.

Well, yes. The only relevant device with HDMI that I have is a raspberry pi.

You don't own any monitor or TV?

Every of them runs on DP