@magnum117 I may be able to clarify at least a few things. Unfortunately I doubt you'll get any help from Dell or Apple on this, since neither company seems very interested in helping their mutual customers with issues that occur when pairing their own products with the other company's products. And Intel is even worse, probably because they're not even really in the direct retail business.
But there's quite a bit of technical stuff going on here, so settle in. I'll try to lay it out as clearly as I can.
First, it's important to note that "5K" has become a completely ambiguous term. The original 5K resolution was 5120x2880, but it has since been used to refer to 5120x1440 (also called dual QHD, since it's dual 2560x1440 displays side-by-side). For a while, the former was sometimes distinguished as "5K2K", but now both 5K and 5K2K are also being used to refer to 5120x2160 (which is also sometimes called UHD ultra-wide, since it's essentially a wider 4K display). All three of those are very different resolutions. 5120x2880 is twice as many pixels as 5120x1440, for example. 5120x2160 falls in the middle. This also means they have very different bandwidth requirements that can be met in different ways. And it certainly means that reading a support article that discusses support for 5120x1440 will not necessarily be relevant to your 5120x2160 display.
It's also important to understand that Thunderbolt is capable of carrying dual GPU interfaces across a single cable. That can come into play because even though there's only a single cable, the GPU still sees two interfaces active, and not every scenario allows aggregating two interfaces, even when both are on the same cable, to drive a single display. The reason this part is important is that at least on the Windows side, Intel GPUs have not supported that in the past. I was surprised to see your quote about running “dual port 5K” from Intel Graphics. Maybe the very latest Intel GPUs support that, but that's news to me (EDIT: It seems that even older Intel GPUs now support aggregation, I guess thanks to driver updates?) For Dell's own displays that require two connections to run them properly, namely their current 8K display and their old 5K (5120x2880) display that they created before it was possible to run that setup over a single cable, Dell specifically notes that those displays only work at their native resolutions and refresh rates with NVIDIA and AMD GPUs, since only they supported link aggregation at least back then.
All that said, I suppose it's possible that this link aggregation limitation on Windows has been due to Intel Graphics drivers on the Windows side, and that macOS works differently. The fact that your 2016 13" MacBook Pro, with only Intel Graphics, is capable of running a 5120x2880 display suggests that’s the case. The Intel GPU in that system would only have supported DisplayPort 1.2 due to limitations of Intel GPUs in that era, and a single DP 1.2 interface doesn't have enough bandwidth for that, at least not at the standard 60 Hz refresh rate. But TB3 can carry two full DP 1.2 interfaces, and two interfaces would run 5120x2880. So if that system can run 5120x2880 over a single cable, that seems like the only way it could be achieved.
Which brings me to your 5120x2160 display. At first thought, you'd expect that to work too, since after all that's less resolution than 5120x2880. I suspect, but cannot be certain, that the issue here is that even if your MacBook Pro supports link aggregation, the U4021QW doesn't support being driven that way. Instead, it may be designed to be driven only by a single DisplayPort 1.4 interface, which supports higher bandwidth than DP 1.2, enough to run 5120x2160 at 60 Hz. But your 2016 13” MBP doesn’t support DP 1.4, so if the display doesn’t support being driven by dual DP 1.2, you’re at an impasse.
As for the 15" MacBook Pro working, its display outputs are driven by the discrete GPU, not the Intel GPU. And the discrete GPUs of that era supported DisplayPort 1.4, even though the contemporary Intel GPUs didn't. So it would be capable of running the U4021QW as it's designed to be driven.
So why are you limited to 2560x1080 specifically rather than something higher, like 3440x1440, which is a 21:9 resolution that can be carried even over DP 1.2? The only idea I have there is that the display itself might not have a scaler chip. The reason I say that is that 2560x1080 is exactly one-quarter of 5120x2160, so even if the display didn't have a scaler chip to allow it to scale arbitrary non-native resolutions to map them onto its physical pixels, it could still easily handle a 2560x1080 source signal, since in that case each pixel in the signal would occupy a 2x2 physical pixel grid, which is a simple process. This isn't unprecedented. The first 2560x1600 displays didn't have scaler chips, and consequently they could only be run at their native resolution or....1280x800, which is one-quarter of their native resolution. Intermediate resolutions were impossible.
Again, I can't be certain of any of this, but based on my general understanding of the underlying technology, it's the only explanation I can think of that would account for your findings.
Hopefully this has at least been interesting reading, even if it may not be entirely correct -- and obviously doesn't provide a solution!
@jphughan Thanks so much for your detailed reply. I would of paid you for that response if I could.
That's okay if it's not a solution, all I'm looking for really is a technical explanation as to why it happens, which you helped a lot with.
While looking at a potential software workaround, I think I found the core reason it doesn't work. Check this link out for similar problem: click here.
Pretty interesting, coming from a resolution expert that created that software, looks like nothing we can do.
@magnum117 Happy to do what I can to help. As a tech geek myself, I know it's already frustrating not to have things work, but to not even understand WHY it doesn't work when there isn't an obvious explanation makes it even more frustrating.
With respect to 5120x1440, that's a bit of a different scenario because that lower resolution is within the bandwidth capabilities of DisplayPort 1.2, whereas 5120x2160 is not. But a similar scenario to what's described on that site played out on the Windows side. In January 2020, Intel released a driver update that broke 5120x1440 support, and since in some cases it was pushed automatically by Windows Update, several users of such displays suddenly found them broken for no immediately obvious reason. Intel released a later update that restored that capability, but now there are registry tweaks that have to be applied to make it work on GPUs of that era, which is why Dell has this patch for their own 5120x1440 display. It seems Apple did something similar to restore that capability for macOS 11 after dropping it for a while. But that was a case of a pure software limitation. This issue around 5120x2160 appears to be an "impasse" as I called it above, where the devices on each end are capable of running 5120x2160, but they do not have a mutually supported hardware/firmware/software means to do so.
On a side note, I'm a big fan of SwitchResX. I used it long ago when Retina displays first arrived on Macs to tinker around. Glad to see it's still actively maintained.
jphughan
9 Legend
•
14K Posts
3
April 17th, 2021 20:00
@magnum117 I may be able to clarify at least a few things. Unfortunately I doubt you'll get any help from Dell or Apple on this, since neither company seems very interested in helping their mutual customers with issues that occur when pairing their own products with the other company's products. And Intel is even worse, probably because they're not even really in the direct retail business.
But there's quite a bit of technical stuff going on here, so settle in. I'll try to lay it out as clearly as I can.
First, it's important to note that "5K" has become a completely ambiguous term. The original 5K resolution was 5120x2880, but it has since been used to refer to 5120x1440 (also called dual QHD, since it's dual 2560x1440 displays side-by-side). For a while, the former was sometimes distinguished as "5K2K", but now both 5K and 5K2K are also being used to refer to 5120x2160 (which is also sometimes called UHD ultra-wide, since it's essentially a wider 4K display). All three of those are very different resolutions. 5120x2880 is twice as many pixels as 5120x1440, for example. 5120x2160 falls in the middle. This also means they have very different bandwidth requirements that can be met in different ways. And it certainly means that reading a support article that discusses support for 5120x1440 will not necessarily be relevant to your 5120x2160 display.
It's also important to understand that Thunderbolt is capable of carrying dual GPU interfaces across a single cable. That can come into play because even though there's only a single cable, the GPU still sees two interfaces active, and not every scenario allows aggregating two interfaces, even when both are on the same cable, to drive a single display. The reason this part is important is that at least on the Windows side, Intel GPUs have not supported that in the past. I was surprised to see your quote about running “dual port 5K” from Intel Graphics. Maybe the very latest Intel GPUs support that, but that's news to me (EDIT: It seems that even older Intel GPUs now support aggregation, I guess thanks to driver updates?) For Dell's own displays that require two connections to run them properly, namely their current 8K display and their old 5K (5120x2880) display that they created before it was possible to run that setup over a single cable, Dell specifically notes that those displays only work at their native resolutions and refresh rates with NVIDIA and AMD GPUs, since only they supported link aggregation at least back then.
All that said, I suppose it's possible that this link aggregation limitation on Windows has been due to Intel Graphics drivers on the Windows side, and that macOS works differently. The fact that your 2016 13" MacBook Pro, with only Intel Graphics, is capable of running a 5120x2880 display suggests that’s the case. The Intel GPU in that system would only have supported DisplayPort 1.2 due to limitations of Intel GPUs in that era, and a single DP 1.2 interface doesn't have enough bandwidth for that, at least not at the standard 60 Hz refresh rate. But TB3 can carry two full DP 1.2 interfaces, and two interfaces would run 5120x2880. So if that system can run 5120x2880 over a single cable, that seems like the only way it could be achieved.
Which brings me to your 5120x2160 display. At first thought, you'd expect that to work too, since after all that's less resolution than 5120x2880. I suspect, but cannot be certain, that the issue here is that even if your MacBook Pro supports link aggregation, the U4021QW doesn't support being driven that way. Instead, it may be designed to be driven only by a single DisplayPort 1.4 interface, which supports higher bandwidth than DP 1.2, enough to run 5120x2160 at 60 Hz. But your 2016 13” MBP doesn’t support DP 1.4, so if the display doesn’t support being driven by dual DP 1.2, you’re at an impasse.
As for the 15" MacBook Pro working, its display outputs are driven by the discrete GPU, not the Intel GPU. And the discrete GPUs of that era supported DisplayPort 1.4, even though the contemporary Intel GPUs didn't. So it would be capable of running the U4021QW as it's designed to be driven.
So why are you limited to 2560x1080 specifically rather than something higher, like 3440x1440, which is a 21:9 resolution that can be carried even over DP 1.2? The only idea I have there is that the display itself might not have a scaler chip. The reason I say that is that 2560x1080 is exactly one-quarter of 5120x2160, so even if the display didn't have a scaler chip to allow it to scale arbitrary non-native resolutions to map them onto its physical pixels, it could still easily handle a 2560x1080 source signal, since in that case each pixel in the signal would occupy a 2x2 physical pixel grid, which is a simple process. This isn't unprecedented. The first 2560x1600 displays didn't have scaler chips, and consequently they could only be run at their native resolution or....1280x800, which is one-quarter of their native resolution. Intermediate resolutions were impossible.
Again, I can't be certain of any of this, but based on my general understanding of the underlying technology, it's the only explanation I can think of that would account for your findings.
Hopefully this has at least been interesting reading, even if it may not be entirely correct -- and obviously doesn't provide a solution!
magnum117
6 Posts
1
April 18th, 2021 17:00
@jphughan Thanks so much for your detailed reply. I would of paid you for that response if I could.
That's okay if it's not a solution, all I'm looking for really is a technical explanation as to why it happens, which you helped a lot with.
While looking at a potential software workaround, I think I found the core reason it doesn't work.
Check this link out for similar problem: click here.
Pretty interesting, coming from a resolution expert that created that software, looks like nothing we can do.
jphughan
9 Legend
•
14K Posts
2
April 18th, 2021 18:00
@magnum117 Happy to do what I can to help. As a tech geek myself, I know it's already frustrating not to have things work, but to not even understand WHY it doesn't work when there isn't an obvious explanation makes it even more frustrating.
With respect to 5120x1440, that's a bit of a different scenario because that lower resolution is within the bandwidth capabilities of DisplayPort 1.2, whereas 5120x2160 is not. But a similar scenario to what's described on that site played out on the Windows side. In January 2020, Intel released a driver update that broke 5120x1440 support, and since in some cases it was pushed automatically by Windows Update, several users of such displays suddenly found them broken for no immediately obvious reason. Intel released a later update that restored that capability, but now there are registry tweaks that have to be applied to make it work on GPUs of that era, which is why Dell has this patch for their own 5120x1440 display. It seems Apple did something similar to restore that capability for macOS 11 after dropping it for a while. But that was a case of a pure software limitation. This issue around 5120x2160 appears to be an "impasse" as I called it above, where the devices on each end are capable of running 5120x2160, but they do not have a mutually supported hardware/firmware/software means to do so.
On a side note, I'm a big fan of SwitchResX. I used it long ago when Retina displays first arrived on Macs to tinker around. Glad to see it's still actively maintained.