Re: Thunderbolt display chaining

[Mika Westerberg <mika.westerberg@linux.intel.com>]

Hi,

On Mon, Apr 21, 2025 at 06:35:49PM +0200, Peter Zijlstra wrote:
> Hi guys,
> 
> I got two Dell U2725QE screens, that can do 4k@120Hz. I'm trying to
> daisy chain them on thunderbolt 4.
> 
> Using a windows laptop (Thinkpad X1) it all 'just' works, and both
> screens are running 4k@60Hz, all good.
> 
> Using a Linux laptop (Thinkpad X13 Gen2i -- tigerlake) it sorta works.
> It drives the first at 4k@120Hz and the second at 4k@30Hz, which is
> kinda terrible. Trying to manually set the first screen to 60Hz works,
> but doesn't appear to enable me to then manually set the second screen
> to 60, it's stuck at 30 :-(
> 
> Using some Tigerlake NUC, I only get a single screen at 4k120Hz and
> nothing at all on the second screen.
> 
> I tried having a quick look at tb.c, and it has this bandwidth
> calculation in tb_available_bandwidth(), but I'm not quite sure where to
> start poking.

If this is Tigerlake then that file is never used - DP tunneling is all
firmware. So what you have is:

  TGL <-> M <-> M

The USB4/TB link is 40G with 10% reserved so you get 36G for DP for the
first link (since both go through that one). 4k @ 120Hz takes 25.82G which
leaves 36 - 28.82 = ~7G for the second monitor that's exactly the 4k at
30Hz you see.

Bandwidth allocation mode can help here but it is not supported until MTL
and I suspect this is what the Windows laptop is using.

You can remedy this by not daisy chaining if you have multiple ports:

  TGL <-> M
   ^
   |
   v
   M

Let me know if you need more information.