[Intel-gfx] Enable DisplayPort MST on low cost USB-C docks

Matthias Walther matthias at walther.xyz
Sun Feb 20 14:33:56 UTC 2022

Hm, no ideas on this?

Am 28.01.22 um 20:50 schrieb Matthias Walther:
> Hello,
> there are a lot of quite similar, low cost USB-C docks with multiple 
> display output (usually 2x HDMI + 1x VGA) available on the big online 
> platforms such as Amazon, Ebay, and Aliexpress.
> Internally the display outputs are connected via DisplayPort. If you 
> connect a monitor to one of the ports, it's detected as display port 
> connection in xrandr. Always the same dpX in xrandr, independently of 
> which physical port in use. This suggests that all physical outputs 
> are connected to the same DisplayPort output.
> On Microsoft's Windows these docks support multi headed output, like a 
> different image on all displays (called expand mode in Windows). 
> However the vendor advertises, that on MacOS the adapter can only 
> display the same image on all ports of the adapter. This might be a 
> hint, that the adapter internally uses DisplayPort's Multi-Stream 
> Transport (MST) technology for the second and third display output 
> (2nd HDMI, VGA), as Apple does not support MST while Microsoft does. 
> Linux behaves just like MacOS here and only mirrors the image.
> Linux is supposed to support MST since like around 2014. There are 
> parameters to enable it for i915, e. g. i915.enable_dp_mst={1,2}.
> However unfortunately those USB-C docks do not support multi-headed 
> output on Linux. The second monitor is not detected, there is just a 
> mirrored image of the first monitor on monitor two and three.
> Does Linux support MST over Thunderbolt 3/4? Is there maybe a hidden 
> command that the Windows driver uses to switch MST on in the dock's 
> chipset?
> Any hints on how to debug this would be highly appreciated! Those 
> adapters become more and more popular, they are affordable and it 
> would be awesome to make them fully work with Linux.
> Best,
> Matthias

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