U2520D, HDMI and USB-C: resolution issue with M1 MacBook

@mdas  I'm not sure, but based on your findings, it seems macOS might treat HDMI displays differently, possibly to account for the possibility that they're TVs.  Or maybe Apple's own multi-port adapter is treated as a special case because sometimes Apple does that sort of thing.  I'd be curious what a third party USB-C to HDMI adapter yielded.  But the difference of those options suggests that in the first case, the systems sees the display as a Retina display, while in the latter case it doesn't.  Maybe one of those connection methods allows the OS to learn the physical size of the display, and therefore to calculate its pixel density, whereas the latter doesn't.  The actual pixel density of a 25" QHD display is 117 ppi, whereas Apple's reference densities are 100-110 ppi for standard displays and 200-220 ppi for Retina (on desktop and laptop displays, not tablets and smartphones).  Ironically when you have the Retina-style options, it's possible to force the other menu to become available by holding the Option key when you click the "Scaled" button -- but you seem to want to force options in the opposite direction, and I don't know of a way to do that.

I remember a Mac application called SwitchResX, which provided a lot more options over display resolutions and scaling.  I don't know if that's still in development and therefore compatible with newer macOS releases, but it may be worth looking into.  Even if it's not, you might find forum posts or something that suggest an equivalent alternative that's still current.

I'm curious too. DELL surely has 4k usb-c cables for their higher end models. Sadly, the support wasn't able to answer if a Windows 10 computer connected via USB-C will display the 3840x2160 resolution. With a HDMI cable, no adapters needed, it will. I tested it. They were also unwilling to send me any other USB-C cable to check if mine is 'defective'. Since I read amazon reviews mentioning resolution limitation with the USB-C supplied with this monitor, I will give it a shot and buy a 3rd party cable soon as I can. Testing the monitor on Windows 10 kinda convinced me is has nothing to do with the Apple adapter. SwitchResX might maybe enable a rescaled 720p resolution, but that would probably look massive. I didn't try it yet. What I want is a text scaled resolution of 1080p with usb-c, instead of the 'numerical' scaled resolution. This will be only possible if 3840x2160 is recognized. I know this will not scale as well as a true 4K monitor, but it is the sharpest resolution with a reasonable text size I could get. The HDMI solution is good, but I'd like to charge multiple devices using the monitor and usb-c capabilities.

@mdas  There's no such thing as a "4K USB-C cable".  Not all USB-C cables have the wiring to support USB 3.x and video, but the ones that do will be rated for at least DisplayPort 1.2/HBR2 since that is the oldest standard that was supported for video output over USB-C.  That is enough bandwidth to run 4K 60 Hz, although when paired with a system that only supports DP 1.2/HBR2 over USB-C, that will only be possible if USB speeds are limited to USB 2.0.  Systems that support DP 1.4/HBR3 over USB-C can run 4K 60 Hz and USB 3.x simultaneously, or dual 4K 60 Hz (or 8K 30 Hz) when limited to USB 2.0 -- and even higher if they also support DisplayPort DSC.  But the cable needs to be rated for DP 1.4 in that case.

That's interesting. 

I found in the user guide of my system that it 'offers the fastest transfer rate and the alternate mode with DP 1.4 supports a maximum resolution of 2560 x 1440...'

Also, a review at the German monitors Page: https://www.dell.com/de-de/shop/ultrasharp-25-usb-c-monitor-u2520d/apd/210-avbf/monitore-und-monitorzubehör

reports about 2K signal over 2 monitors not being possible with the supplied USB-C cable (they needed a thunderbolt 3 cable). 
If I understood you correctly, there is a trade off if you want high data speed or high resolution if the cable is rated for DP 1.2. My monitor has the 'USB-C Prioritization' option greyed out, so I can't test what would happen if I choose 'High resolution'.  Is it possible that the supplied cable is rated for DP 1.2?

@mdas  It seems like that was in the user guide of the display, not your system.  But in any case, DP 1.4 support on makes a difference on a QHD display if you plan to run a daisy chain, because even if your system and display only supported DP 1.2, you'd be able to run QHD 60 Hz and USB 3.x simultaneously.  But the U2520D does in fact have a daisy chain output, and that's when the USB-C Prioritization option comes into play.  Note: This is all going to be irrelevant to you since macOS still doesn't support DisplayPort MST in order to run a DisplayPort daisy chain, which has caused complaints from Mac users for years on this forum, Apple's forum, and elsewhere.  But for Windows systems, if you had a system that only supported DP 1.2, and wanted to run a dual QHD daisy chain, you would set USB-C Prioritization to High Resolution in order to have enough bandwidth to run both displays at 30 Hz, with the tradeoff being that you'd be limited to USB 2.0.  Or technically you could keep USB 3.x if you were willing to run both displays at USB 3.x.  But if you had a system that supported DP 1.4, you'd have enough bandwidth to run dual QHD 60 Hz and USB 3.x simultaneously.  The reason the option is grayed out for you is that you're not running a daisy chain -- but since you're running macOS, you wouldn't be able to anyway.  In fact the initial batch of M1 MacBook systems only support a single external display total, regardless of how they're connected, so you have that additional limitation.  The M1 Mac Mini supports two displays because it doesn't have a built-in display.

I'm not sure what that reviewer was talking about in their setup.  A Thunderbolt 3 cable isn't necessary here since the display doesn't actually support Thunderbolt.  But if they wanted dual QHD and USB 3.x, then they would've needed a USB-C cable rated for DP 1.4.  I would expect the included cable to support that given that the display does, but I haven't tested it myself.  Maybe that person received a problematic cable.

@mdas  Short of MAYBE a cable that had some sort of physical fault like partial damage, there is no scenario where a USB-C port and cable combination would limit you to a single display at 1080p.  If that user was running a daisy chain, then they might be limited to dual 1080p, but not single 1080p.  I've seen enough reviews from users who made claims based on not noticing a key bit of information or not understanding how the underlying technology actually works to put much faith into any single review.  As just one example, I remember a review on Amazon about a 128 GB flash drive claiming that it didn't have as much capacity as advertised.  The user made this claim based on having an existing 128 GB flash drive and not being able to copy all of its current contents over to this new 128 GB flash drive they were reviewing.  They saw an error that the flash drive had run out of space.  In reality, the reason for the difference was that their original flash drive was formatted NTFS, which uses 4KB allocation units by default, while the new one came from the factory formatted as exFAT, which uses 128KB allocation units by default.  If you have a lot of small files, this can result in wasted space.  This could have been solved by simply reformatting the drive as NTFS, but the user didn't even notice this file system difference in their own screenshots, and probably wouldn't have known about the different allocation unit size and the implications of that even if they had.  That's just one of many examples of cases where I've seen people claim something that was clearly wrong because there was something wrong with their test conditions.

Hi, it's also not possible for me to set up scaled resolution on my Dell U2520D - I have a MacBook Pro 2019 (so not M1). I'm using USB-C cable that came with monitor.

I've had some success using SwitchResX application - I've changed "Scaled resolution base" to 3840x2160 (higher than display native resolution). I also added custom scaled resolution 3840x2160 (though I'm not sure if both steps were necessary). After reboot, SwitchResX allowed me to pick 1920x1080 (59,88Hz) HiDPI option, which makes image crystal clear - and scaled to my liking.

While this is a some kind of solution, I would like to use default system settings for dpi scaling, without need for 3rd party apps - who knows if things won't break after future system update? Seeing that this thread is rather fresh, I wanted to ask - did you have any success with thunderbolt usb-c cable?

Apples thunderbolt cable didn't work, kinda as I expected it not to.

You could still give the apple dongle a try. As I wrote earlier, it worked in my case, but I have a different processor.