The dock description states as requirement for driving two displays: “USB 3.1 Gen2 (10 Gbit/s), DisplayPort Alt Mode and Power Delivery”
In the librem 14 description I am only missing the “Gen2” part which leaves me a bit puzzled.
Also Purism are advertising that it can drive two displays, one via USB-C and one via HDMI, which does not explicitly rule out a chance of success for my plan, but could be implicitly interpreted so
So is anyone already successfully driving two monitors via some dock through USB-C?
All I can find here in the forum is this negative report.
OK, so what you are after is Display Port daisy chain and I am sorry, but no, this is not supported and IIRC the reason is that the Intel SOC does not support it. So if you want more than one external screen you need to take the HDMI port as second screen connector.
The non-working HUB you referred to this is unfortunate but sadly also not uncommon with type-C devices I’m afraid. Type-C is still fairly new and many devices do not 100% comply to the standards - this for sure also includes the Librem 14 and the type-C controller chips themselves. These issues have improve a lot during the past 2 years or so but still are not gone and you can find all kinds of reports of things working or not working on this or that laptop / device. A lot of factors come together. With the Hubs in many cases it has to do with power supply and power consumption. I have a bunch of type-C hubs here and pretty much every single one behaves a bit differently. For example I also have one that produces a similar problem like the one in the referenced blog post. But it starts to work if you first plug it to a really powerful PD source and then to a type-C host - something not so straight up with the power negotiation? Since these devices do not have their own debug output very hard to tell what they actually do and what now.
So my piece of advice for type-C peripherals: Buy them with money back guarantee. Chances are it will not work as expected.
Hi Nicole,
thanks a lot for your reply!
Thank you also for reporting based on actual tests with several docks, that multi-display via a USB-C dock does not work with the Librem 14. This ends my speculation
However I am not sure whether this really involves DP daisy-chain. The mentioned dock has three display outputs, so the two displays I am after would not be connected in a chain but directly to the dock. But I might be misunderstanding you or not know enough about the technological aspects of how the multi-display function is realized in this dock.
Still I am not sure about Intel SOCs not supporting this. The product reviews in a big online store where no one should buy stuff from show reports of this dock actually working with laptops of several manufacturerers, mostly based on Intel processors.
It actually does work with my Librem 14. I can connect two full HD Monitors to my USB-C docking station and they work independently, so MST seams to work with the SOC. I just can’t get a higher resolution than Full HD when using two monitors. Might be because the dock has USB3.0 Ports too.
Oh wow, that is interesting! Then actually I have to admit to have misread the datasheets. What is used for that is the Display Port output of the the Intel CPU package which gets routed through the alt mode pins. So the determining factor here is the Intel chip itself. And so far I was not aware that the 10th gen 10710U was capable of MST on the Display Port lines. Sometimes the Intel documentation is not very helpful…
Good to know, thank you!
So theoretically could we have four screens then? The internal, two via type-.C and a fourth via HDMI?
Last time we tried to figure that out we had some discussions internally and found the available information at least unclear - and indeed were not looking at this web page but at the concrete data sheets of the CPU we us, i.e. the 10710U. The different CPUs indeed support different levels of Display Port and from what we knew from Display Port specs from the Librem5 development it was at least unclear to us if it would really fully be supported, especially being routed through the type-C.
So after the first success and reading a post from you in another thread, stating that depending on the adapter you are able to use 4 lines of the USB-C port for displayport, I researched a bit and ordered another adapter. It just arrived and worked!
Now I was able to increase the output/resolution to two 2560x1440 outputs and one more full HD output simultaneously using only the USB-C port. A higher resolution is not possible, but you can connect the third monitor to HDMI to get a higher resolution. I made three 2560x1440 outputs work that way. That’s actually pretty nice But four monitors are not working. I needed to deactivate the laptop screen for three external screens to work. But I expected this, because on the Intel page it also says the processor only supports three displays in total. But you can run them all through the USB-C Port, you just have limitations in the resolution depending on your adapter
Are you able to output one 4K@60?
From specs it should get even dual 4K @ 120 Hz, but you need a graphics card supporting Display Stream Compression 1.2 (DSC), otherwise you get half refresh rate, as it should be the case for L14.
If so, then I’m interested to a docking station from them (to use together L5 too): https://www.cablematters.com/Cable-Matters-Docking-Station.aspx
Was your previous docking station one of them?
They provide a driver for Linux distributions: https://support.displaylink.com/knowledgebase/articles/679060
I can’t test this, as I don’t have such a display. But one 4K@60HZ should work with this dock, as Purism also states that the USB-C is capable of handling that. But not much more. And it’s not only DCS, it’s also that the Librem 14 is only supporting to DP 1.2 and not DP1.4.
In general there are a two things you need to consider. Docks can decide if they use two or four lanes of the USB-C for Displayport, so if you want 4K@60Hz you need to make sure to buy one with 4 lanes. These docks won’t have USB3 (only USB2) and no Gigabit Ethernet. At least as far as my understanding goes. I’ve also seen some more expensive CableMatters docks, that have a switch for that and let you decide if you want a better resolution or USB3.
The second thing is that the Dock vendors mostly talk about the resolutions you get when you use DP 1.4. So most of the time you will only get around half of that with the Librem 14. All in all you need to be careful reading the specs of the docks.
Also Displaylink is something else. With these USC-C DP alt mode docks you won’t need a special driver, as it’s just a DP output routed through USB-C. Displaylink is kind of like a speparate graphics chip on the dock itself (and therefore will cost you CPU power, but will work with almost any Laptop having at least USB3).
