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WD19DC compatibility question
I'm considering to purchase Precision 7750 (with Quadro RTX4000) and WD19DC. I'd like to use WD19DC to connect one U4320Q (4K 10bit Dell monitor) and two U2412M (8 bit 1920x1200 Dell monitor) monitors (and a USB hub). I want them all to refresh with 60Hz frequency.
Is it possible to achieve that?
I found that Dell documentation is very unclear on that.
The only WD19C manual I found is this.
It mentions the following promising points:
- "HBR3 (HBR3 x4 lanes - 25.9 Gbps) With discrete graphics card" - this sounds like something got HBR3, so 7750 is likely to have it too.
- "dual-monitor: DP1.4 + MFDP Type-C: 4K (3840 x 2160) @60 Hz" - this sound like a very explicit proof, that there's enough throughput to drive 4K+2K+2K@60Hz too
- "triple-monitor: DP1.4 + DP1.4 + MFDP TypeC: QHD (2560 x 1440) @60hz" - this sounds like a proof that some 3-monitor configs are possible. So.. Is bandwidth the only limit for such configs?
, but later it mentions some random incompatibilities for some specific models:
- Precision 7540 and Precision 7740 with AMD Graphics can support up to six displays dependent on GPU capability. With Nvidia Graphics, it can support up to four displays
- Precision 7530 and Precision 7730 with AMD Graphics can only support dual 4K@60 Hz non-HDR monitors when, both monitors are plugged to the two DP ports (not HDMI).
and this looks a bit scary, cause 7750 is not mentioned at all. Where's the list of the 7750 incompatibilities?
Finally, there's the most weird statement.
- Configuration that is supported with AMD and Nvidia in special-graphics mode. This mode is listed in the BIOS for the Dell Precision mobile workstations 7530/7730 will require enabling special-graphics mode when Switchable is enabled.
it looks like it should refer to some specific configuration, but someone forgot to put a link... It looks like I both don't understand the statement and what it refers to. Does this statement mean that I'll have to daily change that setting in the bios? What specific port is that? What's the alternative mode?
So, personally, I still got got the following:
- is this document related to Precision 7750 (with RTX4000)? AFAIU, this is not mentioned anywhere.
- is Precision 7750 (with RTX4000) equipped with HBR3 on those GPU-USB-C port at all? AFAIU, this is not mentioned anywhere.
- is it possible to connect WD19C and U4320Q via just a single type-C to type-C connection (for both data and video)? E.g. via "MFDP Type-C". Or, do I need separate "usb-c to DisplayPort" and "usb-A - usb-C" cables?
- is "triple-monitor: DP1.4(1920x1200@8bit@60Hz) + DP1.4(1920x1200@8bit@60Hz) + MFDP TypeC: QHD (3840x2160@10bit@60Hz)" config possible?
jphughan
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November 2nd, 2020 23:00
@i3vi3vi3v I can help with at least some of this.
The 7750 is likely not mentioned because that system is newer than the WD19DC's documentation, or at least the current revision of it as of this writing.
Yes, it is possible to run 4K + QHD + QHD, dual 4K is possible, and 2x QHD is less total pixel area than 1x 4K. (But fyi, referring to QHD as "2K" is a bad practice, because 2K technically refers to a film resolution of 2048x1080, and when used in a consumer context it rightfully refers to the nearest and slightly LOWER resolution of 1920x1080, not 2560x1440 -- just as 4K is a film resolution of 4096x2160 but in the consumer space refers to a lower resolution of 3840x2160. But the fact that 2K is so often incorrectly used to refer to QHD/1440p means that it's basically useless since nobody knows what somebody means when they say it, and ambiguity is bad when asking technical questions. QHD is only one more character to type than 2K, but it adds a lot of disambiguation value. )
Yes, bandwidth is the main factor to consider here. But GPUs can also have limits on the total number of displays they can drive, as mentioned in some of the notes you quoted from the documentation. Still, 3 displays isn't a problem, even if you wanted to keep the built-in display running as a fourth display.
The "special graphics mode" reference is a BIOS option that when enabled causes the discrete GPU to take direct control of the display outputs, rather than the Intel GPU. You'll need that enabled because the Intel GPU in that system only supports DP 1.2/HBR2, which means less bandwidth and no formal HDR support, and the Intel GPU is also limited to 3 simultaneous independent displays total, including the built-in display if you keep it active. All of the discrete GPU capabilities mentioned in the other notes you cited assume that the discrete GPU is in fact controlling the display outputs, which is achieved by enabling that BIOS option. You don't have to change it on a daily basis though; you just have to set it once. Technically you should expect reduced battery life if you ever plan to use an external display while running on battery power, since the discrete GPU will have to remain active whenever an external display is active, even if nothing graphics-intensive is going on, but if you have a 7750 I'm guessing that battery life isn't a major concern for you.
In terms of your questions at the end of your post:
Fair warning though: I have not actually tested the above setup. I'm just providing information based on my knowledge of the capabilities of the WD19 family dock models and the underlying technologies in general. But unfortunately not everything that SHOULD work in the tech world always DOES work. For example, there are multiple reports of the XPS 13 9300 not running dual 4K 60 Hz displays properly through a WD19TB dock, even though the technology and Dell's own documentation indicate that this should work. That case appears to be a firmware/driver issue, but of course for the affected customers, the bottom line is that it doesn't work.
jphughan
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November 3rd, 2020 00:00
@i3vi3vi3v I figured I'd add a little bit more about the special graphics mode and switchable graphics. The default mode is to have the Intel GPU control all display outputs, including the one running the built-in display. And then the discrete GPU when its additional performance is needed acts as a render-only device that does the work and passes completed video frames to the Intel GPU for output to the displays. NVIDIA calls this technology NVIDIA Optimus. The main benefit to this design is battery life, since it means that the discrete GPU can be completely shut down when its performance isn't needed, whereas if the discrete GPU has direct control of any display outputs, it has to remain enabled whenever a display connected to that output is active, even if nothing graphics-intensive is going on. But the downside to this design is that there are certain capabilities that Intel GPUs don't support passing through and certain capabilities that require a discrete GPU to have direct control of the display output rather than an Intel GPU. A few of these are VR, NVIDIA G-Sync, AMD FreeSync, 5K resolution (only supported by the very latest Intel GPUs), DisplayPort 1.4/HBR3 (same note), stereoscopic 3D, and higher quantities of simultaneous independent displays. And there are also some applications that simply don't work well when using the discrete GPU indirectly through Optimus, although these applications are fairly rare these days since Optimus has been around for a while.
Enabling the special graphics mode option in the BIOS gives the discrete GPU direct control of the display outputs, while keeping the Intel GPU running the built-in display. If you choose to disable switchable graphics entirely, then the discrete GPU controls everything, including the built-in display. But that will impose a likely noticeable battery life penalty, since in that configuration the discrete GPU can never be completely shut down, even when only the built-in display is active and nothing graphics-intensive is going on. And at least at one point with the Precision 7000 Series, I remember reading a note that disabling switchable graphics could leave you with a blank built-in display when undocking or something, but I haven't kept up on that. The only real reason to disable switchable graphics entirely would be if you have some application that doesn't work with Optimus and only works properly when the Intel GPU is completely out of the loop, AND you need to run that application on the built-in display.
jphughan
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November 3rd, 2020 16:00
@i3vi3vi3v I'll try to address what I can, again bearing in mind that I'm speaking purely from theory rather than actual experience with this setup, which can be hazardous when dealing with tech products.
As for reasons why special graphics mode isn't the default, it could be inertia given that some early Precision 7000 Series models didn't even offer this option; you could just turn switchable graphics completely on or off. In fact I'm pretty sure that this option was introduced as part of a BIOS update for an existing model -- maybe the 7x20 models? -- rather than being a brand new feature with a brand new product generation. In that case, defaulting it to on would have changed the operation of the system. And I suppose for people who use their battery while connected to external displays, such as when giving presentations, but don't need any of the capabilities unlocked by graphics special mode, then having the Intel GPU control everything makes sense.
In terms of video and USB data running over a single cable between the WD19TB and the U4320Q, I'm really not sure. But if you'll be connecting this display to a WD19TB, it's also not a major issue. If you can't run your desired video setup and USB 3.x, then use a DisplayPort cable for video and a USB-C to USB-A cable to connect the display's USB-C input to one of the USB-A ports on the dock, giving you a USB 3.x data path. Just make sure you get a C to A cable specifically rated for USB 3.x, since some are only rated for USB 2.0. In any case, if 4K 60 Hz 10-bit requires more than 2x HBR3 lanes, the most you could get with USB data over a USB-C link is USB 2.0, since there are dedicated pins within a USB-C connector to carry that. Dell has implemented this setup in the past. For example, the U4919DW is a 5120x1440 display that has a USB-C input, but since it only supports DP 1.2, if you connect to it over USB-C, it only runs USB 2.0 data. However, that display also includes a USB-B "upstream" port so that you can still get USB 3.x running. This display doesn't offer that. And then yes, there are other Dell displays that offer a USB-C Prioritization option, which determines whether the display sets up the USB-C link for 4 lanes of HBR video and only USB 2.0, or 2 lanes of HBR video and 2 lanes for USB 3.x. That's handy with 4K 60 Hz displays because it means that DP 1.2 systems can use the High Resolution option to run 4K 60 Hz over USB-C, with the tradeoff that data speeds are limited to USB 2.0. Meanwhile, DP 1.4 systems can use the High Data Speed option to run 4K 60 Hz and USB 3.x. (This option is also handy on QHD displays since the USB-C Prioritization option can be used to decide whether you want enough bandwidth for a dual QHD daisy chain or are willing to sacrifice that to keep USB 3.x.) But I'm admittedly not sure how this all works when 10-bit color is involved. However, as I said earlier, worst case you can just run two cables between the dock and that display to get video and USB 3.x instead of one.
I wouldn't count on DisplayPort DSC being in play. I'm not even sure the WD19TB supports it. There's no mention of it in its documentation, at least last time I checked. I also wouldn't put too much stock into the experience of Apple users. The sad truth is that lots of users are having multiple issues with multiple Mac laptops and desktops when using multiple Dell displays. It's not clear who's at fault here because each product works fine with equivalent products, i.e. Macs work with other similar displays and Dell displays work fine with non-Apple systems. It's an interoperability issue. And of course, both sides are recommending that affected customers work with the other vendor toward a resolution. Dell's basic position is that they don't test or officially support using their products with Apple systems, and Apple seems to take the same stance. Meanwhile, their mutual customers are caught in the middle. I personally think it would be in the interest of BOTH companies to work directly together to resolve these interoperability issues, regardless of whose fault it is, in the interest of serving their mutual customers, but what do I know? But for now, Dell display issues with Apple systems have gotten so bad that well respected display reviews site RTings.com has removed all Dell displays from their "Recommended for Mac users" list, specifically due to the number of complaints they received about such setups.
As for documentation, to my knowledge both DP outputs, the USB-C port, and the downstream TB3 port (on the WD19TB) all support DisplayPort 1.4/HBR3. But I'm not certain about that. The documentation about the effect of "graphics special mode" would rightfully be mentioned in the documentation of the laptop, not the dock. But I don't know if it's there, or how helpful the description of that option in the BIOS is. The real gap at the moment is that Dell is pretty good about indicating what's possible with their docks and displays based on what DisplayPort revision the attached system supports -- but the documentation of the systems doesn't indicate what DisplayPort revision they support, at least not over USB-C. THAT has been the major gap, and confusion around that has been responsible for a countless number of posts I've written to help people here. I reported this issue to an internal Dell contact, who escalated it to the Documentation team, and it does seem that things may be changing. The new XPS 15 9500 and XPS 17 9700 both had specs pages that specifically listed a DisplayPort revision level for their USB-C ports. Unfortunately, the XPS 15's specs incorrectly claimed that the non-Thunderbolt USB-C port on the right supported DP 1.4 (it's only DP 1.2), and the XPS 17 9700's specs only said all four TB3 ports supported DP 1.2, without mentioning that if you got a system with an RTX GPU and enabled a BIOS option very much like "graphics special mode", they suddenly support DP 1.4. (On the XPS 17, it's either all-Intel or all-NVIDIA, i.e. including the built-in display.) So that was sort of a one step forward, one step back situation. They finally started including details, but the details were wrong or incomplete. Sigh....
Yes, I too had thought of an interactive calculator that allows you to specify the resolutions, refresh rates, chroma subsampling levels, and color depths of your displays to find out their total bandwidth requirements. But that couldn't be printed into a PDF, and that has complications of its own. There are different timing standards that have varying amounts of overhead. And there can also be cases where the total bandwidth might be sufficient, but the display setup won't work because of how the HBR lanes are divided among the outputs. For example, the WD19TB (not DC) when connected to a Thunderbolt-capable DP 1.4 system allocates 4 HBR3 lanes to the main outputs and 1 HBR3 lane to a "downstream USB-C/TB3" port that doesn't exist on the other WD19 dock family models. Let's say you had a pair of displays, one of which requires 3 HBR3 lanes and the other that required 1.5 HBR3 lanes. That's less than the 5 HBR3 lanes of bandwidth coming into the dock, but you won't have a way to run that display setup through that dock, because if you put both displays on the outputs that are sharing 4 HBR3 lanes, there won't be enough bandwidth to run them. But if you move one of those displays to the downstream USB-C/TB3 port, there won't be enough bandwidth there either.
So while the question of, "Will my display setup work with my dock and system" is very easy to ask, it's not always so easy to answer.
i3vi3vi3v
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November 3rd, 2020 16:00
@jphughan , many thanks for the detailed answer!
> Technically you should expect reduced battery life if you ever plan to use an external display while running on battery power, since the discrete GPU will have to remain active whenever an external display is active
I was afraid that the "special graphics mode" is more-or-less the same as "fully disabling Intel GPU". Your additional information seems like this mode does not lead to permanent discrete GPU activity.. OK, thus that's probably not a problem.
So, it looks like the only reasons behind that "special graphics mode" is not the default are:
OK then...
> if you have a 7750 I'm guessing that battery life isn't a major concern for you.
Well, 99% of time, I don't care about that. But sometimes I do. At least when planning a purchase . And, BTW, it's quite weird that although 95WHr LongLife 1000-cycles battery is mentioned in the docs, only "ExpressCharge Capable" version is actually available, and the "LongLife" version is not even mentioned on the "customization" page - yet another misleading point.
> Yes, you should be able to run a 4K 60 Hz HDR display via the Type-C output.
That's reassuring, thanks! Posts like this (looking like there's some static bandwidth allocation) look scary.
Data+video?
You've omitted the "a single type-C to type-C connection (for both data and video)?" part though... And I can't blame you for that - that's probably the most self-contradictory part
The docs say:
Thus, it looks like 4K@10bit would require slightly more than 2 HBR3 lanes "effective data rate". Well, there also might be some DSC compression, which makes this even more complicated. But still, it looks like there's a high chance that it won't fit into two lanes.
Meanwhile, DP can only use 1,2, or 4 lanes. Thus, it would use 4 lanes, AFAIU.
Thus, even if everything is HBR3, I won't get USB3 data via the same cable?
Thus, I'll probably need USB-C -> DP and USB-A -> USB-C cable combo?
High data speed: High data speed is for DP and USB 3.0. For max supported resolution, please check the Display info Link Rate (Current). For HBR3, maximum supported resolution is 3840 x 2160@60 Hz, For HBR2, maximum supported resolution is 3840 x 2160@30 Hz with USB 3.0 data." - this looks like U4320Q is HBR3 - capable, and USB 3.0 data is possible.
Thus, probably, I should purchase a USB-C to DP cable, if I want USB3.
Dell docs
> If Dell tried to list every possible combination of heterogeneous display setups anyone might want to run, taking into account different resolutions, refresh rates, color depths, etc., the documentation would obviously get unwieldy.
I'm not saying Dell should list all possible combinations. But explicitly documenting which ports are HBR3-capable and listing "special graphics mode" effects would be much appreciated. Even if HDR3 sounds trivial and "default". A handy table with resolution*refresh_rate*bit_depth+overhead=bandwidth would be nice too. Or a calculator , with a compatibility info.
jphughan
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November 3rd, 2020 17:00
@i3vi3vi3v While writing a reply to somebody else, I remembered an experiment I ran that is germane to your questions. I have this Anker "mini-dock" adapter. It supports DP 1.4/HBR3 source devices, but it also runs USB 3.x, so it only gets two HBR3 lanes. Despite that, and despite this capability NOT being mentioned anywhere in the product's marketing material or specs, I was able to run 4K 60 Hz HDR to my TV through this adapter from a DP 1.4 source device. So it would seem that 2 HBR3 lanes is enough for that, and thus 4 HBR3 lanes should be enough for dual 4K 60 Hz 10-bit.
i3vi3vi3v
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November 5th, 2020 11:00
@jphughan , thanks again for lots of useful info!
> In terms of video and USB data running over a single cable between the WD19TB and the U4320Q
It was a bit surprising to find WD19TB here, cause I was only considering WD19DC everywhere, except mentioning that WD19TB got limitations, not directly related to "total bandwidth"...
Nevertheless, AFAIU, this your paragraph (and the next one) applies to both WD19DC and WD19TB.
> I reported this issue to an internal Dell contact, who escalated it to the Documentation team, and it does seem that things may be changing.
Thanks for your concern! Hopefully Dell would ultimately sort it out and produce some more thoughtful documentation standards for that.
jphughan
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November 5th, 2020 12:00
@i3vi3vi3v Happy to help. Yes, the question of how to connect the 4320Q would apply equally to the WD19TB and WD19DC. I would keep your focus on the DC though, since the WD19TB can only supply up to 130W of power to the attached system, which isn't enough to properly power a Precision 7000 Series system. The whole reason the WD19DC exists is to offer a way to push up to 210W to the attached system, by plugging into two USB-C ports, specifically to provide a docking solution for that model line that doesn't involve connecting the system's power adapter separately. The WD19TB does get access to more video bandwidth though. Whereas the DC can tap into 4 lanes of either HBR3 or HBR2, depending on what the system supports, the TB can tap into 5 lanes of HBR3 or 8 lanes of HBR2 -- because the TB is a Thunderbolt 3 dock, whereas the DC is essentially a "dual USB-C" dock. But it doesn't even make full use of the available connectivity. In theory it could run 6 HBR lanes (with 2 high speed lanes in one USB-C connector being needed for USB 3.x), but for whatever reason it doesn't seem to do that.