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Comment by mikepurvis | Hacker News Reader

I'm pretty surprised that works for display passthru, especially for multiple displays. My impression had been that most TB/USB-C docks provide their display interfaces by internally being a PCIe graphics card.

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Many USB-C docks use DisplayLink to create a virtual graphics card over USB, it is implemented as software that renders using your computers/laptops graphics card that then sends the output over USB to the DisplayLink controller.

Thunderbolt docks (and some USB-C docks) instead will use Alternate Mode to pass-through the DisplayPort directly from the computer/laptop to the attached monitor. There is no additional graphics card or PCIe device involved in the matter, and no virtual graphics rendering that is sending frames across USB.

DisplayLink is not great, especially on macOS, even on Windows it would cause random slow downs and in general when using it with multiple monitors it would cause random tearing and other issues. It was fine for spreadsheets and the like, but for video it broke down quickly.

Whereas alternate mode just passes through your graphics card output, so there is none of that, and instead you get the full rendering power.

There are also non-Thunderbolt USB-C docks that support DP Alt mode. E.g. I had a Lenovo USB-C Dock Gen2, which uses 2 (superspeed) lanes for DisplayPort with HBR3. If a machine supports it, it can pass through 4k@60Hz. It uses the other two lanes for USB 3.1 Gen 2 10Gbit.

Though, I would always go for a Thunderbolt Dock. The extra bandwidth is nice (e.g. most Thunderbolt Docks support two 4k@60Hz screens). Also, Thunderbolt at least offers the option of using PCIe NICs, offering alternatives to the crappy Realtek USB ethernet controllers. Be cautious - even many expensive Thunderbolt hubs use USB Realtek. However, there are also models that have an Intel I210 or I225 on PCIe.

(The story changes a bit with USB4, since it can also optionally tunnel PCIe, but I wouldn't expect cheap USB-C adapters to do that.)

Right, there are some USB-C docks that are not Thunderbolt that do DP alt mode, but many of the cheaper ones seems to come with DisplayLink technology built-in.

DisplayLink is the only way to stop my MacBook from overheating when working with 2 external monitors

I have had multiple MacBook/MacBook Pro's, never had an issue with overheating while using external monitors. At one point I had 3 hooked up to my MacBook Pro.

The key thing is to not use it in clamshell mode.

Also DisplayLink wouldn't really help much with running cooler, since its drawing a virtual frame buffer using your existing graphics card anyway, although I don't know if macOS would recognize it and switch to the discrete graphics when in use vs continuing to use the iGPU.

I have two big monitors, I do not have space on my desk to keep laptop lid open (and that would be waste, because I would never even look at built-in screen anyway).

As for running cooler it really did change for me after buying DisplayLink docking station - before I would hear the fan noise constantly, now the laptop just stays quiet.

I mean, the thing is it’s not really a “dock”. It’s just a tb4 controller in a box.

This one, apparently https://ark.intel.com/content/www/us/en/ark/products/189982/...

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