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

[Imre Deak]

Hi Matthias,

could you open a ticket at gitlab, providing a dmesg as described at
https://gitlab.freedesktop.org/drm/intel/-/wikis/How-to-file-i915-bugs

Thanks,
Imre

On Sun, Feb 20, 2022 at 03:33:56PM +0100, Matthias Walther wrote:
> 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
> > 
>