Hi, On Tue, May 31, 2022 at 09:45:55PM +0200, Stefan Hoffmeister wrote: > What could be reasons that the second tunnel is not established on the > Dell? I read somewhere that Intel hands off the firmware to vendors > (Dell) who then customize it for their systems? Could the vendor have > made bad customizations / configurations of that package while > integrating it? Probably not a firmware issue. > I would imagine that plugging in a DisplayPort cable makes the dock > (firmware) signal something to the notebook (TB firmware) and a > negotiation will take place. That negotiation fails, otherwise the > tunnel would be established, and remain established? Is there a means > to trace the negotiation? It is all done in firmware but when you plug in DisplayPort cable to the dock, it generates a hotplug event for that DP OUT adapter and this will then be handled by the firmware connection manager by establishing a DP tunnel (if it finds resources). > FWIW, I have read the phrase "insufficent provision of GPU Interfaces > to the TB port" (sic, on Reddit), and a lengthy related post at > https://www.dell.com/community/XPS/Understanding-Thunderbolt-docks-GPU-bandwidth-and-GPU-interfaces/td-p/7678776 > which I will not pretend to understand. > > What I wonder about is whether the "GPU interfaces" situation would be > reliably discoverable by inspecting ... something ... anything? > > Anyway, my impression, from a layering point of view, is that on the > stack (my imagination!) > > * notebook hardware > * firmware (BIOS, Thunderbolt firmware / connection manager, ...) > * Linux thunderbolt driver > * Linux graphics drivers: drm / kms (i915 / nvidia / nouveau) > > the graphics drivers are not involved when it comes to building / > maintaining the Thunderbolt(!) tunnel? Correct. > I am also reading "Thunderbolt Alternate Mode encapsulates DisplayPort > Alternate Mode". To my ears this sounds like "wrap the raw DisplayPort > Alternate Mode bitstream", just with more bandwidth. Pure "DisplayPort > Alternate Mode" I can force with success by way of disabling > Thunderbolt in the BIOS (at the expense of bandwidth -> bad refresh > rate). And "DisplayPort Alternate Mode" gives me _both_ screens, > apparently very much scraping along at the max protocol bandwidth, > with the 4K screen going black (out of sync?) every once in a while. > > Sorry for my rambling, this is an area where I have no expertise. > > Anyway, if those graphics drivers are involved for _Thunderbolt_, > please do tell me, and I'll venture over to dri-devel. In case of firmware based connection manager, the Thunderbolt driver does not do much. Pretty much just the PCIe tunnel authorization and power management things (and P2P). IIRC this non-working system had a discrete (NVIDIA?) GPU? It may be that routing it to the DP IN adapters in the Thunderbolt host router requires something we don't implement in Linux side yet. > And given what I see above, is that still "Thunderbolt 4 Certified" > ("Two 4K displays") in the case of the Dell Inspiron 7610? This I don't know I would expect Dell testing this, at least with their own dock.
