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Demystifying Dell WD19TB supported display resolutions

A while ago I wrote a “Demystifying USB-C and Thunderbolt” thread here, which addressed how USB-C and Thunderbolt worked, including in docking station scenarios, and how that related to supported display setups.  Since then, Dell has released the WD19 dock family that includes support for the newer HBR3 standard, and the WD19TB dock in particular has some limitations on maximum resolutions with various display output combinations that might seem strange.  So I decided to write this thread for anyone who was simply curious from a technical perspective about why those exist.

First of all, it’s important to note that although the WD19 dock family can take advantage of HBR3 support (DisplayPort 1.3 or 1.4) if the system has it available through its USB-C/TB3 port, the vast majority of systems on the market here in May 2019 still only support HBR2, for the simple reason that Intel GPUs today still only support HBR2 (DisplayPort 1.2). Even among systems that also have discrete NVIDIA/AMD GPUs, the USB-C/TB3 port is often still physically wired to the Intel GPU and is therefore subject to its limitations -- so at the moment, the only systems that have HBR3 support on USB-C/TB3 are those that have those ports driven directly by a discrete GPU.  However, Intel’s upcoming “Ice Lake” family of CPUs will incorporate a new GPU that supports DisplayPort 1.4 and therefore HBR3.  Those CPUs are slated to begin arriving in late 2019, as of this writing.

The main focus of this thread, however, is that the WD19TB has altered how it allocates display bandwidth to its various outputs compared to the TB16 that it replaces. That’s why if you look at the manual’s Display Resolution Table for a Thunderbolt system, you’ll find some limitations that might seem unintuitive or arbitrary. For example, when using an HBR2 system, running dual 4K 60 Hz displays requires that one of them be connected to the dock’s “downstream” Thunderbolt port, a limitation that didn’t exist on the older TB16 dock. But on an HBR3 system, that same Thunderbolt 3 port is limited to just QHD resolution whenever any other output is also in use. So what’s going on here?

There are two underlying causes for these limitations. The simple one is that the WD19 simply doesn’t support using its HDMI port and USB-C port for video output at the same time (although using the latter for a data device while using HDMI for video seems to be fine.) The second and much less obvious reason is that the WD19 family only allocates 4 of the incoming HBR lanes from the system to be shared across all of its “core” display outputs, i.e. all outputs except the Thunderbolt 3 port built into the removable attachment module. Any remaining HBR lanes coming from the system are only available to that Thunderbolt 3 port, regardless of whether it’s actually being used.  This ends up accounting for both of the unintuitive and seemingly contradictory limitations relating to the Thunderbolt 3 port I mentioned earlier.

For the HBR2 system scenario, on a system that has two GPU outputs wired to its Thunderbolt 3 port (which to my knowledge all Dell systems have), an HBR2 connection over TB3 includes 8 HBR lanes, since a full DisplayPort link has always been defined as 4 HBR lanes, even before USB-C/TB3 arrived.  But since the “core” display outputs only have access to half of those, which is equivalent to the bandwidth of a single full DisplayPort 1.2 link, you can only use those ports for display setups that fall within those bandwidth limits.  That’s why even though the system is providing enough total bandwidth for dual 4K 60 Hz displays, for example, you’re limited to QHD if you want both displays on “core” outputs.  However, if you instead connect only one display to a “core” port and the other to the Thunderbolt 3 port where the other 4 lanes are available, you can run dual 4K 60 Hz just fine.

For the HBR3 system scenario, there are at most 5 lanes coming from the system. The reason for this is that two full DisplayPort connections (i.e. 8 lanes) at HBR3 would require 64.8 Gbps of bandwidth, which is well beyond the 40 Gbps of Thunderbolt 3, and that’s before even considering any non-display data you might want to send across your Thunderbolt 3 connection to the dock, such as USB data for external hard drives, Ethernet data, etc. (If you're wondering, Thunderbolt 3 always prioritizes display traffic and throttles everything else when there isn't enough bandwidth to run everything at max performance. However, Thunderbolt 3 supports 40 Gbps in each direction simultaneously, and display traffic only ever runs one way, so depending on what else you're doing, high-bandwidth display setups might not bottleneck you.)  In an HBR3 scenario where only 5 lanes are available, the first 4 get allocated to the “core” outputs, and then the Thunderbolt 3 port only gets access to that single remaining HBR3 lane – which is why it’s limited to QHD.  The only exception seems to be if the Thunderbolt 3 port is the only one being used for display traffic, in which case it gets access to all 5 lanes, since the manual specifies that a single 8K 30 Hz display can be used from that port, just like all other ports.

One question not addressed by the manual is whether the dock supports DisplayPort DSC, i.e. Display Stream Compression. That’s part of the DisplayPort 1.4 spec, but I don’t know if it’s mandatory. But if the system and dock both support it, then higher-end display setups than indicated in the manual would be possible -- OR a given display setup would require less bandwidth, which would especially benefit Thunderbolt 3 connections because that would open up more bandwidth for other traffic.  (On regular USB-C, currently half of the high speed lanes are allocated to video and half are allocated to USB, so reducing display bandwidth consumption doesn't benefit USB traffic -- although USB4 will be changing that to allow dynamic bandwidth allocation.) The higher-end display setup option could potentially even be achievable if the displays themselves didn’t support DSC as long the WD19 could “decompress” the DSC signal from the system and output a standard DisplayPort 1.4 signal to the attached display(s). And if the attached displays DID support DSC, then assuming all of the aforementioned support was still in place, even the maximum per-display resolution would increase, because at that point even the "normal" constraints on the DisplayPort 1.4 link between the dock and display could be exceeded.  Hopefully we’ll find out through some testing once suitable systems and displays are more widely available.

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Good to know. Disappointing but at least I can stop banging my head against the wall Leaving ‘dongle-town’ on the next plane out. 

I have a Precision 3551 that supports DP 1.4 over Thunderbolt. Is there any reason why I cannot support 4 displays running 1920x1080 or is this a software/driver limitation in the WD19TB?

@gentoo9ball  I’m not 100% sure here, but I’m PRETTY sure the Precision 3551 has its USB-C/TB3 output wired to the Intel GPU, not the NVIDIA GPU.  And Intel GPUs as of this writing are still limited to 3 simultaneous independent displays, regardless of resolution or available bandwidth.

@gentoo9ball  By the way, where are you seeing that your Precision 3551 supports DP 1.4 over TB3? The Intel GPU built into the CPUs used on that system are definitely DP 1.2. If you’re certain that the system has its TB3 video output wired to the NVIDIA GPU to support DP 1.4, then the NVIDIA GPU should also support 4 displays. In that case, the next question is whether the NVIDIA GPU also runs the built-in display or if it only runs the external displays connectors. If it does run the internal display, then if you’re running Windows, you can run 4 external displays. If you’re on Linux, which I mention based on your username, then you can only run 4 total displays and the built-in display always counts as one of them because Linux doesn’t allow the built-in display to be completely disabled as is possible on Windows. If on the other hand you verify that the NVIDIA GPU controls the TB3 output but NOT the built-in display, then yes 4 external displays should be possible as long as you connect them within the options specified in the WD19TB User Guide for a DP 1.4/HBR3 system.

@jphughan Thank you!! This is a brand new 3551 that I just got in the mail. It specifically states that it has DP 1.4 over Thunderbolt. However I have not seen reference anywhere stating which video card it connects to. I am running Windows 10 on it currently. It has a NVIDIA Quadro P620 in addition to the Intel UHD that comes with the i9-10885H.

I appreciate the education and help!

@gentoo9ball  Interesting, the Setup and Specifications Guide for that system on does indeed mention DP 1.4.  If that's accurate, then the USB-C port would have to be wired to the NVIDIA GPU.  If you want to verify that, open NVIDIA Control Panel and go to the Surround/PhysX section.  In there you'll see a diagram showing which GPU controls each active display -- so you'll need a display connected through the USB-C/TB3 port to test this.  If it shows as connected to the NVIDIA GPU, then it's DP 1.4.  If it shows as connected to the Intel GPU, then as far as I know, it would still be DP 1.2 because to my knowledge the only current Intel GPUs that support DP 1.4 are those built into the CPUs that use the Ice Lake and Tiger Lake architectures -- but this system doesn't use those.  In that case, the DP 1.4 mention may be a typo, and that wouldn't be unprecedented.  The XPS 15 9500's specs indicate that the USB-C port on the right of the system supports DP 1.4 even though it's been confirmed that it's wired to the Intel GPU and only supports DP 1.2.  Meanwhile, the XPS 17 9700's specs say that it supports DP 1.2, even though certain configurations of that system include a BIOS option that allows the display outputs to be controlled by the NVIDIA GPU and therefore to support DP 1.4.  So while it's nice to see Dell including the actual DisplayPort revision in its specs rather than just saying that the port supports "DisplayPort" as most system specs did, it seems that the additional information may not always be accurate.

@jphughan You are quite correct. The NVIDIA Control Panel shows all the monitors connected to the UHD. Pretty sad, if I can't make this happen. The laptop is great however DP 1.4 was a major sticking point in buying it and the WD19TB. This screenshot was taken with one monitor plugged into the laptop HDMI Port, another into the USB-C on the WD19TB, and two more in the full size Display Ports on the Dock. 



@gentoo9ball  Well that’s certainly disappointing. Based on that setup with all displays driven by the Intel GPU, you wouldn’t be able to run quad displays even if the Intel GPU actually DID support DP 1.4, because Intel GPUs are still limited to 3 simultaneous independent displays, even the new ones that support DP 1.4 — and that limitation always applies regardless of how they’re connected, how much bandwidth is available, how much bandwidth the displays require, etc. In your case, the only way to run more than 3 total displays would be to run at least one of them through “indirect display” technology like DisplayLink — not to be confused with DisplayPort. There are single display USB dongles that use DisplayLink and docking stations that support multiple displays driven via DisplayLink — like the Dell D6000. But DisplayLink has some drawbacks that can be significant in some use cases. I wrote about those in the post marked as the answer in this thread.

If those aren’t acceptable, then unfortunately you’ve got the wrong laptop for your purposes. Dell doesn’t make very many laptops that have display outputs wired to the NVIDIA GPU. The Precision 7000 Series systems have a BIOS option allowing you to choose which GPU controls the built-in display as well as the display outputs, but they’re of course larger, heavier, and more expensive. The XPS 17 9700 and I believe its sister system the Precision 5750 have this type of option only if you order it with an NVIDIA RTX GPU, not a GTX (or non-RTX Quadro) GPU. And some gaming-oriented systems have at least one display output connector wired to the NVIDIA GPU to support certain features that don’t work when running through an Intel GPU, like VR or G-Sync. Other than that, this capability seems to be more commonly found on Lenovo systems. I myself have an X1 Extreme Gen 2, which is the only real non-Apple competitor to the XPS 15 (at least before the XPS 15 went purely USB-C). On that system, the built-in display is controlled by the Intel GPU, but a BIOS option allows it to be controlled by the NVIDIA GPU — but all of the display outputs are always controlled by the NVIDIA GPU. In the default mode with the Intel GPU still running the built-in display, I can actually run 5 total displays, i.e. the built-in display and 4 external displays (the max for the NVIDIA GPU). It’s not officially supported, probably because there’s a pretty narrow set of ways that this setup can be achieved due to considerations around bandwidth, ports, and GPU interfaces allocated to those ports, but I managed it with the help of a Thunderbolt dock. I had two external QHD displays, two external FHD displays, and the built-in FHD display all running simultaneously.

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@jphughan  Funny you mention Lenovo, I was heavily debating a Lenovo P series against this laptop. I believe I looked up the UHD 630 that comes with this processor and saw that it was DP1.2. So I thought they must have the thunderbolt wired to the NVIDIA card instead. Oh well. I'll talk to the advanced support line tomorrow and see if they can correct all the documentation regarding this system.

As for DisplayLink, I'm not using all these monitors for gaming, it's mostly coding, comms, and browsing. I wonder if a USB-C to 2x HDMI adapter might work though the "USB 3.1 Gen1/Gen2 Type-C port with DisplayPort 1.4" or another USB-C port on this dock. We'll find out as soon as amazon delivers it, too cheap to not try.

All that being said this laptop is awesome, just wish their product documentation had been a little more polished.

@gentoo9ball  Well the sister system of the ThinkPad X1 Extreme is the P1. But if you don’t specifically need the capabilities of a Quadro GPU, there isn’t really a benefit to going that route. Same for XPS vs. their sister Precision systems.

I commend you for wanting to work with support about getting documentation updated, although I wouldn’t hold your breath. I doubt most support reps would even know the difference between these DisplayPort revs, never mind knowing enough about the capabilities of that Intel GPU to know that it’s DP 1.2 — particularly as Intel doesn’t even make that clear on their own specs pages that they publish on (and that in fairness are, generally speaking, very helpful and detailed).

A USB-C to dual HDMI adapter will typically be an MST hub that will rely on the USB-C port’s DisplayPort Alt Mode capability, which is the GPU output wired to the port — and in that case any displays you attach to that device will be driven directly the GPU and will therefore count towards its display maximum. But if you can find an adapter that specifically mentions DisplayLink rather than DP Alt Mode, then it would able to sidestep that limitation.

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