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

[Matthias Walther]

Thanks for the reply! Posted and cross link:
https://gitlab.freedesktop.org/drm/intel/-/issues/5331

Am 21.02.22 um 12:26 schrieb 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
>>>