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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.


Replies (161)

@M4L00  Yes, you would need one of these options:

 

  • WD19/WD19S to WD19TB/WD19TBS kit with a 180-watt AC Adapter
  • WD19/WD19S/WD19DC/WD19DCS/WD19TB/WD19TBS to WD22TB4 kit without an AC adapter (180-watt AC Adapter required)

The first option is the simplest, since it gets you what you need to run your display setup and includes the 180W power adapter that you will need.

The second option upgrades you to the latest generation of Thunderbolt dock (WD22TB4), which when paired with a system like yours that supports DisplayPort DSC can support some even higher-end display setups than the WD19TB/TBS.  But you don't need that to run your current display setup.  And that second option does NOT come with a 180W adapter, so if your WD19S doesn't already have a 180W adapter, then you would need to buy that separately.  (The WD19 and WD19S can both be ordered with either a 130W or 180W power adapter, which allows the dock to provide up to 90W or up to 130W to the attached system.  Make sure you don't confuse the power adapter wattage with the system Power Delivery wattage if you need to check which one you have.)

 


Thanks, I was referring not to the cable, but to the upgrade kit. Would it be okay? 

Upgrading the WD19/WD19S series dock with the Cable Module Customer Kit | Dell US
WD19/19S to WD19TB/TBS

@M4L00  Since you're not even using the same dock model, you should really create your own topic rather than posting in an existing thread that isn't even about the same issue.  But the issue in your case is that you're using a regular WD19S, which does not support Thunderbolt.  Just getting a Thunderbolt cable won't change anything.  You need an actual Thunderbolt dock.  The WD19S when paired with your system is only rated for dual 2560x1440.  You don't have enough bandwidth available for 3440x1440 + 2560x1440, which is why your second display is running at a lower resolution.

So either get a Thunderbolt dock or connect one of the displays directly to the system using its HDMI output or the other USB-C/TB port.  In that setup, that display won't be running through the dock, which means you won't have a bandwidth limitation problem.


2 Bronze
2 Bronze

Great topic. 

What's the problem in my case: 

Latitude 7420 + WD19S

2 iiyamas displays: 
32" and 34", both connected to DP ports 

34" achieves 3440x1440 

but 32" can't get it's maximum 2560x1440. Only 1920x1080 is possible. 

 

Is it a matter of not having thunderbolt cable? 

best regards, 

Maciej 

Regarding an upgrading the TB-Module to the WD22TB4 module:

I do not see how this would change lane allocation, because I own pure Thunderbolt 4 docks that allocate DP-lanes exactly like the WD19TB. But since we have not found out, whether the TB NVM firmware has a say in this or not, it could make a difference, but why would it. This would only take bandwidth away from main display outputs the dock has. It is the total Thunderbolt bandwidth that becomes a limitation with HBR3 hosts.

You will get the additional TB-out in case one was not enough (although you still will only be able to use one of those as DP-out, so this does not support more / other displays).

Secondly, the communication with the dock will switch to TB4 instead of TB3 for your notebook. In practice this should only change whether the dock can establish further TB4 connections on the TB-outs (not relevant for now, all current TB4 devices are TB3 backward compatible).

Also waking up non-dell hosts from keyboard/mouse interactions is only possible with TB4. But we do not know whether the main dock part supports this as dell has a custom solution designed for its own devices). As part of this change, all USB traffic through the dock would then be technically slower, because all USB-ports on the dock would share a single USB 10G connection to the host. But this will be barely noticeable in most situations, since only very specific port combinations can exceed 10G of USB bandwidth on the WD19TB (TB-out + 1 10G USB can in sum achieve slightly above 10G, same as both TB-outs of each side of your notebook).

The change to TB4 could also nominally increase security, because with TB4 the dock would no longer need access to PCIe, which is used for some supposedly fixed attack vectors. But an attacker exploiting this would require modifying the components inside the dock in the first place, and if you can upgrade the TB controller it probably can also be downgraded again. And windows will automatically give docks like the WD19TB PCIe access on up-to-date drivers, so there is only a benefit to the dock no longer needing this, if you actually forbid the host from using PCIe through Thunderbolt in the BIOS, if that is even exposed as an option.

A couple corrections:

The Intel 11800H is new enough to support HBR3, 4 monitors simultaneously and also DSC (although only version 1.1 not 1.2a as Nvidia does. I have not found out whether this changes available bandwidth or anything else).

Also, there is no dynamic allocation as to which GPU outputs to the dock. Only the "Graphics Controller Mode" in BIOS changes this. Right now, Nvidia's Advanced Optimus only works for the integrated displays and only on explicitly supported (gaming) notebooks. So as long as not explicitly switched to dGPU in BIOS, it should always be the Intel GPU who outputs to the displays and therefore controls bandwidth allocation through MST. Which GPU "renders" which program us see is entirely different. Here the Nvidia GPU will be much faster. Also, HDR processing, especially of video takes quite a chunk of performance on iGPU.

 

HDR should only require more bandwidth than SDR, when 10 Bit Color Depth is employed. But HDR also works with 8 bits (Windows shows this as "8 Bit with Dithering"), so HDR should also work, if there is no more bandwidth available than already used for SDR.

If you care for more detail https://tomverbeure.github.io/video_timings_calculator has always been a great resource for calculating the peak bandwidth, also I would not rely on it to be 100% exact, but close to the actual numbers. (Most monitors and the U2723QE use CVT-RB btw.)

 

Regarding the U2723QE: I own a U3223QE and have previously observed that the display remains dark (claiming "no input connected") on newer Nvidia GPUs (supporting DSC) for driver version 510.x-511.x, when MST/Daisychaining was enabled on the display. This has been fixed for me with driver version 512. So this might be the reason the two U2723QE remain blank for you in direct mode.

I have in the past observed slight differences between Intel and Nvidia, how much compressed bandwidth, and therefore which resolutions I could drive with a single HBR3 displayport, Nvidia allowed slightly more for me.

 

Also on Intel GPUs I have observed problems, with a second monitor daisychained behind my U3223QE not switching to 10 bit for HDR, but correctly doing this when attached directly. This happened even when enough bandwidth was available. On Nvidia this could be manually selected, whereas this option was not exposed on older Intel drivers.

*correction: "Dell monitor info changes color depth from 24bit to 32bit 30bit"

2 Bronze
2 Bronze

jphughan, first, thank you for the detailed guide on WD19TB since the way it works is quite a mistery even for IT people. Your "DSC" question is answered below by the way.

Second, I still cannot understand well by how much HDR changes the bandwidth requirements:

So I have Dell XPS 9710 (Intel 11800H+RTX 3060) connected to WD19TBS connected to 2x Dell U2723QE (4k 60hz) with 2 DP cables. This seemed to work in both HDR and non-HDR mode (HDR changes 8 bit depth per color to 10 bit according to GPU, Dell monitor info changes color depth from 24bit to 32bit).

However, I noticed that Intel GPU limits performance of RTX 3060 because of new Windows 11 smart GPU scheduling that sometimes uses Intel GPU instead of RTX even if I set the priority to RTX. For that reason I changed "Graphics Controller Mode" to Direct in BIOS to bypass Intel. Now the performance is fine and RTX used whenever needed. However, 2 Display Ports are no longer capable of delivering HDR content to 2 monitors - when I switch on HDR, 2 monitors go blank. After your explanation I realized that "Graphics Controller Direct Mode" switched WD19TB from HBR3 to HBR2 because the Thunderbolt controller is built in the Intel CPU and now WD19TB connected to the laptop via USB-C via RTX GPU. This is also proved by the fact that when I connect one of the monitors to the Thunderbolt port on WD19TB, they both work in HDR mode (DP monitor HBR2 5.4Gbps, Thunderbolt USB-C monitor HBR2 5.4Gbps DSC).

Thank you for helping with unraveling of this mess.

Do you know what would be the benefits of upgrading WD19TB with WD22TB module? WD19TB has USB 3.1 vs WD22TB USB 3.2 so this part will stay. Do you think the lane allocation and DP bandwidth will change if I attach WD22TB module to WD19TB?

I wanted to provide an update about my issue regarding the reliability of dual 5120x1440 monitors.  I had the following suggestion from Dell Tech support:

Use one of the regular ports on the back, then use the Thunderbolt port (highlighted in the image below). That port has a separate allocation of bandwidth. 

alexcapone_2-1647578916809.png

So I have an HDMI cable for Monitor 1 and DP to USB-C port going into the Thunderbolt port for Monitor 2.  This is working reliably for me.  Honestly, I never tested the Thunderbolt port and I don't know how I missed it.

I will add one more caveat.  After trying this troubleshooting step the monitors worked about 85% of the time (it was still an improvement).  Then I came across an app called Awake which is a module in Windows PowerToys.  In case you're curious here is some info about it:

 
After installing I am getting 100% reliability.  The interesting thing is I have the module running but it is not set to stay awake or configured to run anything outside of the default power plan set in Windows settings.  I don't know if there is any correlation to Awake but nevertheless, everything is working great for me now.


@jphughan wrote:

@Ray519  and @XPSnerd I don’t see the point of using two WD19TBS docks to achieve quad 4K 60 Hz


you're absolutely right and I feel a bit stupid now even mentioning such wasteful setup, I merely wanted to emphasize that a 4x 4K setup over just two Thunderbolt ports actually works.

The proposed Lenovo TB4 dock got me seriously interested (teardown here https://dancharblog.wordpress.com/2021/09/22/lenovo-thunderbolt-4-dock-unboxing-and-teardown/)

  • 4x 4K over just one TB cable
  • 2x 4K over USB-C
  • based on newest TB4 JHL8440 chipset, no need for a noisy fan
  • 100W power delivery should be plenty enough for office use with the XPS17

However many question marks whether DSC actually works as advertised, based on prior experiences with several cable matters DP1.4/DSC /HBR3 gen USB-C docks DSC is still an unreliable mess even with new Tiger Lake laptops

I may either order that Lenovo dock or wait for a hopefully this year to be released TB4 based Dell "WD22TB" with 130W power delivery (if Dell keeps his release cycle)

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