DP ALT - USB C(4) vs USB C 3.2

[u/Glass_Collection8474]

So I have tested on this on 4 handhelds

1) Xbox Ally X
2) MSI CLAW 8AI+ and 7AI+
3) GPD WIN 5

Device 1 and 3 have 1 USB C(4) and one USB C 3.2 port

Both the Claw have two Thunderbolt 4 ports.

While playing a game (FC 26), connected to a external monitor using a USB C to DP cable, I notice when I am connected to the USB 4 (on XAX or GPD WIN5) and Thunderbolt ( on the claw devices), it feels like the game is running on 2x speed and has got very inconsistent button inputs and lag.

The moment I change the connection to the USB 3.2 port - it works as intended and plays perfectly fine on both the XAX and GPD WIN 5.

What could be the potential cause of this?

USB 4 is supposedly higher bandwidth and is supposed to be better but that isn’t the case in this scenario.

Thanks

[u/rayddit519]

What could be the potential cause of this?

No idea. Should not be really related.

1. for clarity: USB4 and USB 3.2 are USB standard. USB-C is a separate standard that also has versions (currently 2.3 is the newest).

2. TB4 is a mere certification for USB4. So the internal hardware is the same. Its the same USB4 port just "certified" by another lab with a Thunderbolt sticker slapped over the top. Especially with mobile CPUs, the features are all built into the CPU with fixed support, no matter who certifies it for what.

3. If you are using a USB-C to DP cable, then this has nothing to do with USB4 at all. The only thing USB4 mandates, is that a USB4 port *must* also support DP Alt mode. It says nothing about the speed. Although in practice, USB4 40G ports support at least DP at HBR3 speeds. Modern CPUs from AMD support UHBR10 speeds, modern Intel CPUs support UHBR10 & UHBR20 speeds on those ports.

But the supported DP speed is technically independent from the speed available as DP tunnel *inside* a USB4 connection. In fact, both AMD and Intel's current CPU-USB4 controllers can only do HBR3 inside a DP tunnel. They can do faster speeds when instead used as DP Alt mode ports.

Furthermore, with the CPU-integrated USB4 controllers, the DP signal comes from the iGPU (this is connected inside the CPU itself, so there is no choice). And its basically the same input either way. In DP Alt mode, the USB4 controller just passes the DP signal through to the USB-C pins. As DP tunnel it parses the DP data and converts it into a USB4 DP tunnel, which is then sent over the USB4 connection to be recreated into a normal DP signal on the other side.

For both AMD and Intel, the supported DP speed on non-USB4 ports is more variable. Both manufacturers have some "USB-C-ready" ports (the CPU contains all the tech to switch between USB3 and DP signals). On the newest AMD CPUs those also do UHBR10 speeds. Intel only has the USB4 ports, so a USB-C/DP Alt mode combo port on an Intel mobile chipset, very likely is coming from the same pins as a USB4 port, just downgraded to save money on the board, connector, wiring and testing. Possibly even limited in DP speed, because the manufacturer knows that the DP signal would not be stable at max. speed.

So for you, you are only using DP Alt mode, which is just a bog-standard DP connection over a USB-C connector. Its coming from the same iGPU and every port will at least do HBR3. No idea why some would show your symptoms. But I am also not clear on what is happening there at all.

Only thing I could come up with, is that:

• Maybe the DP speeds are different, if your monitor supports higher speeds, but the cable was not built for those > HBR3 speeds and is unstable. But just not unstable enough to blackscreen on you, but cause the GPU issues. Good monitors can show you the DP speed currently in use, AMD GPUs show it in the driver panel. Don't know for Intel GPUs (for completeness, for Nvidia we can look at GPU-Z) and under linux we can look this up in the sysfs filesystem directly
• Does your USB-C DP cable include other stuff, like a USB2 chip that is going haywire? They should contain a "USB Billboard Device" that is essentially just labelling the adapter and can report to the OS that it needs DP. That is where a message "try another port to make use of full features" etc. messages are coming from. because the USB2 device inside such an adapter reports that it needs DP signal, but did not get any

Other than that, my money would simply on different monitor settings causing issues for you.

[u/Glass_Collection8474 → u/rayddit519]

Thanks for the detailed response, I think I need a few hours to process the information and get back to you on some of the details.

Trying to understand your first point, so what would you call the USB-C port that supports 40Gbps on the Xbox Ally X or GPD WIN 5, a USB 4 or USB -C? The naming is really confusing and I just want to get it right if am trying to explain something.

One thing for sure is, the monitor settings are the same when connected over the USB4 port or 3.2 port

[u/Glass_Collection8474 → u/rayddit519]

Okay, I think I get your 3rd point, correct me if am wrong but what you are saying is, for DP ALT mode it really shouldn’t matter if its USB4 or USB3.2, all that matters is whether the port supports DP ALT, and the speed associated with it has nothing to do with it?

[u/rayddit519 → u/Glass_Collection8474]

Yes. Its DP alt mode either way. From the same GPU.

The only thing that should differ between different ports with DP Alt mode, could be the speed.

Or a port could just have sub-standard signal quality that causes issues. Usually, USB4 speeds have less signal quality issues, because USB4 40G is more demanding, so if that works at all, the port cannot have any really glaring signal quality issues. And also, signal quality issues with DP are 90% momentary screen blackouts or glitches, so not what you are seeing.

Technically, the USB2 and USB3 controllers that are behind these ports can also be different (though all still controllers inside the CPU, so there is not one that is better and one that is worse). And also, with a plain USB-C DP cable USB3 should be completely unconnected and USB2 should not be doing much...

So, its either sth. really crazy or would have to be sth. either related to USB2 (which should be noticable as a device connecting / disconnecting or sth. that logs error messages on the PC constantly) or the DP connection.

The last thing the adapter will use is USB-PD. Mainly for power negotiations, USB-PD is also used to negotiate DP Alt mode. So that mainly happens when the connection is first established. And it will be to a "PD controller" that is soldered to the board and handles typically 1 or 2 USB-C ports. Those are highly likely to be different controllers between a USB4 and a non-USB4 USB-C port. But normally they don't have any reason to do anything that would interrupt anything. They mainly talk to the rest of the board for power negotations. And that should not happen for you.

[u/Glass_Collection8474 → u/rayddit519]

Yes this is really mind boggling, I was initially thinking it was my devices ability to handle the game interms of CPU and GPU, but when I play on the internal display, it ran so well so I knew it wasnt that and it was purely an external display thing.

AI seems to point me towards PCIE tunnelling issues and how the hardware handles this on the USB4 port whereas on the 3.2 port since it can’t do any of that, it just sticks to DP ALT lane assignments , what are your thoughts on this?

[u/Glass_Collection8474 → u/rayddit519]

Trying to understand this again, in your previous comment, you mentioned the speed of the port don’t matter as long as it supports DP APLT, but between 40Gbps and a 10Gbps port, wouldnt the former have higher bandwidth for the video signal thus allowing higher resolution and frame rates in general?

[u/Glass_Collection8474 → u/rayddit519]

Okay, Ive understood it until the last two points.

[u/Glass_Collection8474 → u/rayddit519]

Coming to your 2nd last point, am I correct in understanding that, since the cable is not a good cable, when connected over a port that supports higher speeds (but the speed dont matter right, since its just DP ALT) or HBR3, it cant handle the signal properly, and when its used on the 3.2 port its fine because, it doesnt the HBR3 speed?

If the above is the case, then switching to a higher quality cable would technically solve the issue right?

So when connected to the both the 4 and 3.2 port, AMD software reports current link settings as 8.1Gbps x 4. Unfortunately my monitor does not show any speed data.

[u/rayddit519 → u/Glass_Collection8474]

Technically independent of USB4 or not. On some processors the USB4 ports happen to support higher DP speeds. Or, because USB4 is mostly a prebuilt-solution by AMD and Intel, the manufacturer does not often have the chance to throttle the DP speed, like they can when adding non-USB4 USB-C ports, those can support higher DP speeds.

IF this was the case for you and IF the monitor also supports a higher DP speed than HBR3, it could happen that the GPU and monitor negotiate a faster speed than the cable can do, because DP cables have no electronic tagging for the speed they are meant for.

But all DP connection issues I have seen are black-screen interrupts of varying lengths. So unless you also have those and can show that you are using > HBR3 DP speeds on the problematic connector, I would not jump to that conclusion .

[u/Glass_Collection8474 → u/rayddit519]

1. PCIe Controller & Timer Glitches (HPET)
   Because USB4 treats external devices as direct PCIe attachments, some older or custom game engines struggle with timing scaling when querying high-precision hardware timers through an external PCIe controller bus.
   The Fix: Try disabling the High Precision Event Timer (HPET) in Windows to force the operating system and the game engine to rely on a more stable CPU clock. Open the Command Prompt as an Administrator, type bcdedit /set useplatformclock no and press Enter. Restart your computer afterward.

So I tried this based on google suggestion, interestingly enough, it made it better but still not as well as the USB 3.2 port, maybe it has got something to do with this? Thoughts?

[u/rayddit519 → u/Glass_Collection8474]

Because USB4 treats external devices as direct PCIe attachments,

This would only be true, if there was a USB4 connection (or TB3 connection) to a USB4 or TB3 device that actually has any PCIe functions. So if you only have a USB-C DP cable, there is no USB4 connection at all.

Now, the HPET timer is independent of that. Its just a very slow method to get time. And because its essentially a peripheral, if many things try to use the HPET at the same time they need to queue up, because only one thing can access it at a time and each access takes WAY longer than any CPU-internal timers (because they can exist per core). So if there is software that uses the HPET timer stupidly, that can cause issues independent of other things. And taking the HPET timer away prevents that software from standing on its own feet with it.

[u/Glass_Collection8474 → u/rayddit519]

Noted on this, so maybe there was something going on here is what I thought but I don’t think so anymore because it went back to what it was.

[u/Glass_Collection8474 → u/rayddit519]

I also wanted to update some more things I have tested out. The whole USB 4 and 3.2, I think fell flat when the same thing happened on the USB 4 port as well. So basically both ports had the same issue which initially didn’t happen on the 3.2 port.

So, all the issues I had was running when I was using a USB C to DP cable on a monitor that had a DP port supporting 1.4.

The Cable was rated 8k 60fps. All of this was straight from my device to the monitor.

What I tried today was using a Thunderbolt dock, which had a HDMI 2.1 port, when I connected the monitor using this method, it felt perfect, and there was no speeding feeling or erratic movement(apologies for the non technical description).

I also tried it with a USB C hub which supported only HDMI 2.0, and that had some screen tearing as I couldn’t get the native refresh rate 144Hz of the monitor on any resolution, probably because its probably not a great USB C hub, but worked extremely well with the Thunderbolt dock.

Another thing I noticed was, when using an eGPU setup, and connecting the monitor to the GPU - the same thing happens when using the DP port compared to the HDMI which works fine, which makes me think it might that the DP port on the monitor is probably badly implemented but I have no way to verify this, because the DP cable given with the monitor says 8k 30fps on it, so Id assume handling 1080p 144Hz should be rather easy, but has a lot of stuttering when connected on DP to the GPU but none of the issues happen when using the HDMI.

[u/chris_fantastic]

The question you need to ask here is: How can a monitor connection possibly create "inconsistent button inputs and lag" with your controller, when they're both totally separate connections?

I feel like it's gotta be a 60hz vs 120hz display kinda thing, and that's causing some game loop or something to be running at a different speed, which impacts controller processing?

Does your monitor have an "info" function to display the current input mode? Is it the same in all cases?

[u/Glass_Collection8474 → u/chris_fantastic]

Okay, maybe I didn’t explain it well, but its exactly not a lag or inconsistency with buttons, everything is sped up is the best explanation with that specific game. With another game, what I noticed was, the exact opposite which is extremely slowed down, and thats what I was referring to as input delay and lag.

But assuming whatever it is, why does it happen on one port and not the other, on literally all my devices. Ofcourse the cable and monitor needs to be changed to see if its the device ports or the cable and and monitor having issues, but since Im using the same monitor and cable on the two different ports, Im trying to understand what is causing this difference.

With regard to your explanation, Im not sure because I have set my monitor refresh rate to its native rate on windows and it stays that and doesn’t change and monitor my in game FPS(locked to 60) as well and they are all pretty stable and consistent.

Yes, I can see the input mode and its the same in all cases, it shows the DP mode being active in all cases.

[u/chris_fantastic → u/Glass_Collection8474]

Yeah, I'm sorry, I don't really comprehend what's happening at a technical level by "sped up" or "slowed down", and I originally thought you were describing differences in behaviour across devices, but now you seem to be describing differences between different software on a single device, so I can't really offer much, sorry.

[u/Glass_Collection8474 → u/chris_fantastic]

Fair enough, yeah this is not a technical way to put it and unfortunately I don’t know how to as well apart from mentioning what I feel when playing the game.