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mactinkerlover

macrumors regular
Original poster
Sep 20, 2020
169
111
Title says it. My question is why can it connect to one 6k pro display xdr but not 2 1080p displays? Is there a hardware limitation or is it just arbitrary software limitation so that apple can make people upgrade later on?
 

djlythium

macrumors 65816
Jun 11, 2014
1,158
1,606
Given that there are known simple strategies to get the M1s to use multiple monitors, this suggests that it’s neither hardware nor software limitations, but rather I would say that it‘s more Apple being cautious against too many failure points with such new hardware design.
 

mactinkerlover

macrumors regular
Original poster
Sep 20, 2020
169
111
Given that there are known simple strategies to get the M1s to use multiple monitors, this suggests that it’s neither hardware nor software limitations, but rather I would say that it‘s more Apple being cautious against too many failure points with such new hardware design.
Okay, so is it possible that apple could enable multiple display support in the future with an update if they wanted to (we obviously know they won't but I'm asking do they technically have the capability to?)
 

TGM85

macrumors 6502
Aug 29, 2005
268
754
I'm presuming it's a hardware limitation: something to do with the GPU part of the M1 SoC.

The M1 is basically an upscaled iPad SoC. An iPad GPU wasn't really designed to drive multiple displays and seems to be limited to driving two displays max.

That's my best guess. I'm very curious to find out more as well, because triple-display support is a must-have for me. As soon as they release an Apple Silicon SoC that supports triple displays, I'm buying.
 

turbineseaplane

macrumors P6
Mar 19, 2008
15,857
35,003
Whatever the issue is, whenever an Apple Silicon mac can drive more than one external display I sure hope that comes across the line.

Just because somebody has a MacBook Air does not mean they might not want to use multiple external displays
 

djlythium

macrumors 65816
Jun 11, 2014
1,158
1,606
Okay, so is it possible that apple could enable multiple display support in the future with an update if they wanted to (we obviously know they won't but I'm asking do they technically have the capability to?)
I would think they could flip that 'switch' with an update, yes, but I'm hard-pressed to think a time in the past when Apple has every allowed further or enhanced hardware capabilities with a software update. Stated another way, the probably of Apple doing this would be incredibly low, IMO. There are too many benefits for not doing so, not the least of which is the monetary potential of officially adding 'multiple display support' to future models.

Whatever Apple's motives, since we know the M1s actually can run multiple displays, is this something you're considering? Have you done it already?
 

mactinkerlover

macrumors regular
Original poster
Sep 20, 2020
169
111
I would think they could flip that 'switch' with an update, yes, but I'm hard-pressed to think a time in the past when Apple has every allowed further or enhanced hardware capabilities with a software update. Stated another way, the probably of Apple doing this would be incredibly low, IMO. There are too many benefits for not doing so, not the least of which is the monetary potential of officially adding 'multiple display support' to future models.

Whatever Apple's motives, since we know the M1s actually can run multiple displays, is this something you're considering? Have you done it already?
No, I was just curious as to why it only supported one display. I don't use external displays often, I was just wondering if it was another arbitrary thing that apple was doing
 

JTravers

macrumors 6502a
Jun 28, 2010
733
228
And why can the mini support a 2nd display via HDMI but not via Thunderbolt/USB-C?
 

AppleFeller

macrumors 6502
Oct 19, 2020
371
520
And why can the mini support a 2nd display via HDMI but not via Thunderbolt/USB-C?
the M1 seemingly has a single output through each controller. Mini gets two by design of having the extra HDMI output. M1 is in seemingly entry level devices, your not getting more then that unless you step up to the inevitable (4 thunderbolt) 13" refresh and or 16"
 

joevt

Contributor
Jun 21, 2012
6,837
4,133
Thunderbolt has two DisplayPort connections. The HDMI (for Mac mini) or laptop display (for MacBook Air or MacBook Pro) is a third DisplayPort connection (connected to an internal DisplayPort 1.4 to HDMI 2.0b converter.

Although Thunderbolt has two DisplayPort connections, they can only be used for a single display on an M1 Mac. The LG UltraFine 5K uses both DisplayPort connections to achieve 5K 60Hz 10bpc (with one connection it would be limited to 4K 60Hz 10bpc).

6K requires only one DisplayPort connection because it uses DSC on the M1 Mac.
 

Yebubbleman

macrumors 603
May 20, 2010
5,951
2,544
Los Angeles, CA
Title says it. My question is why can it connect to one 6k pro display xdr but not 2 1080p displays? Is there a hardware limitation or is it just arbitrary software limitation so that apple can make people upgrade later on?

Definitely a hardware limitation. Remember that Apple is much newer to the GPU game (having only really produced their own since A11 first graced iPhones in 2017) than the CPU game (having produced their own CPU cores in SoCs since A4 in 2010) and that, at most, they've only needed to drive a single external display in mirrored fashion.

This is something that will most certainly get better with time (i.e. on newer/beefier Mac SoCs). I think Apple releasing M1 and on the MacBook Air, the 2-port version of the 13" MacBook Pro, and a Mac mini intended to only really replace the lower-end quad core 8th Gen Core i3 model of 2018 Mac mini was extremely strategic in nature. Those are the machines that are least likely to inconvenience customers buying them with limitations such as two Thunderbolt 3 ports (which, for the Air and 2-port 13" Pro was exactly the same pre-M1) and a 16GB maximum capacity of RAM. You could've certainly tried to drive more than one external display on a 2020 Ice Lake Y-series based MacBook Air, but hell if the experience would've been anything other than really crappy. That said, I think if Apple had waited on the Mac mini, we'd be substantially less aware of M1's inherent limitations.

As for the disparity of it being able to do 1 6K display but not 2 1080p displays, I'd imagine that's not a limitation of the GPU's raw power as much as it is a design limitation, again, because the most intense iOS/iPadOS/tvOS display arrangement that has ever come about is a single external display in mirrored fashion. I'd imagine that extended monitor support will eventually come to USB-C iPads and that Apple Silicon Macs will also be able to natively support more than 2 displays (internal integrated display included). Again, all three of these Macs are the type of Mac that would rarely inconvenience their target market audiences with such limitations.

Given that there are known simple strategies to get the M1s to use multiple monitors, this suggests that it’s neither hardware nor software limitations, but rather I would say that it‘s more Apple being cautious against too many failure points with such new hardware design.
The "known simple strategies" you refer to involve DisplayLink adapters. DisplayLink adapters are USB-driven and not 3D accelerated as they are not driven by a GPU, but rather by software (which is, again, not accelerated). If the things you put on those monitors are basic, like Word/Pages or Excel/Numbers documents, then that's fine. But usually anything higher-end on those displays suffers from weak performance. It's not at all the same as an internal GPU driving multiple external displays (as is still possible on Intel Macs).
I'm presuming it's a hardware limitation: something to do with the GPU part of the M1 SoC.

The M1 is basically an upscaled iPad SoC. An iPad GPU wasn't really designed to drive multiple displays and seems to be limited to driving two displays max.

That's my best guess. I'm very curious to find out more as well, because triple-display support is a must-have for me. As soon as they release an Apple Silicon SoC that supports triple displays, I'm buying.

I think your guess is 10000% on the money. I'd guess that the higher-end models and/or successors to these current M1 Macs will resolve the limitations. Again, it's rare that anyone ever endeavored to hook more than a single monitor up to any MacBook Air, a 2-port 13" MacBook Pro of any generation, or a Mac mini that wasn't intended to be lower-end. It made perfect sense for these Macs to be the ones to ship with Apple's FIRST Mac-centric SoC.

And why can the mini support a 2nd display via HDMI but not via Thunderbolt/USB-C?
If I had to guess, the internal displays on the M1 Air and 2-port 13" Pro are either connected via an HDMI based internal connection or some other connection that isn't using Thunderbolt/USB-C explicitly (thereby making the M1 Mac mini's HDMI connection akin to the M1 Air's and M1 2-port 13" Pro's own internal display. I'd imagine it's a limitation on Apple's custom Thunderbolt controller and how it interfaces with the GPU.
 

joevt

Contributor
Jun 21, 2012
6,837
4,133
This is something that will most certainly get better with time (i.e. on newer/beefier Mac SoCs).
There's already hints in the ioreg of the M1 Macs that four displays and four USB-C ports were or are being considered.

There's a device driver called "AppleDisplayCrossbar(display-crossbar0)". It is a sub class of IODPSwitch. The crossbar is a mux or switch that routes displays or DisplayPort connections to USB/Thunderbolt controllers. It lists four display connections "dispext0","dispext1","dispext2","dispext3" (only "dispext0" exists as a device in M1 Macs). It lists four USB/Thunderbolt controllers "atc0","atc1","atc2","atc3" (only "atc0" and "atc1" exist as a device in M1 Macs).

Each USB/Thunderbolt controller has two DisplayPort inputs (e.g. "atc0-dpin0", "atc0-dpin1" for "atc0").

What might be educational is a comparison of ioreg from an M1 Mac with a solo-DisplayPort display (4K) connected, and another ioreg from the same M1 Mac with a dual-DisplayPort display (5K). It's not entirely clear from the ioreg that I have with a solo-DisplayPort display how a dual-DisplayPort display works. The ioreg for the dual-DisplayPort display may show a third DisplayPort connection somewhere.
If I had to guess, the internal displays on the M1 Air and 2-port 13" Pro are either connected via an HDMI based internal connection or some other connection that isn't using Thunderbolt/USB-C explicitly (thereby making the M1 Mac mini's HDMI connection akin to the M1 Air's and M1 2-port 13" Pro's own internal display. I'd imagine it's a limitation on Apple's custom Thunderbolt controller and how it interfaces with the GPU.
The teardown of the M1 Mac mini shows that it uses a DisplayPort 1.4 to HDMI 2.0b converter chip. That means all display sources in the M1 Mac are DisplayPort. There is at least three of them (in order to support the HDMI or laptop display, and to support one 5K display).

It may be that the DisplayPort sources for a 5K display is a single source which is processed into tow separate DisplayPort signals. A comparison of an ioreg with a solo-DisplayPort display and an ioreg with a dual-DisplayPort display may shed some light on that.
 

Yebubbleman

macrumors 603
May 20, 2010
5,951
2,544
Los Angeles, CA
There's already hints in the ioreg of the M1 Macs that four displays and four USB-C ports were or are being considered.

There's a device driver called "AppleDisplayCrossbar(display-crossbar0)". It is a sub class of IODPSwitch. The crossbar is a mux or switch that routes displays or DisplayPort connections to USB/Thunderbolt controllers. It lists four display connections "dispext0","dispext1","dispext2","dispext3" (only "dispext0" exists as a device in M1 Macs). It lists four USB/Thunderbolt controllers "atc0","atc1","atc2","atc3" (only "atc0" and "atc1" exist as a device in M1 Macs).

Each USB/Thunderbolt controller has two DisplayPort inputs (e.g. "atc0-dpin0", "atc0-dpin1" for "atc0").

What might be educational is a comparison of ioreg from an M1 Mac with a solo-DisplayPort display (4K) connected, and another ioreg from the same M1 Mac with a dual-DisplayPort display (5K). It's not entirely clear from the ioreg that I have with a solo-DisplayPort display how a dual-DisplayPort display works. The ioreg for the dual-DisplayPort display may show a third DisplayPort connection somewhere.

The teardown of the M1 Mac mini shows that it uses a DisplayPort 1.4 to HDMI 2.0b converter chip. That means all display sources in the M1 Mac are DisplayPort. There is at least three of them (in order to support the HDMI or laptop display, and to support one 5K display).

It may be that the DisplayPort sources for a 5K display is a single source which is processed into tow separate DisplayPort signals. A comparison of an ioreg with a solo-DisplayPort display and an ioreg with a dual-DisplayPort display may shed some light on that.
That is all super cool info! Which Mac mini teardown are you referencing, by the way? Am curious. iFixit seemed to just try to do a brief three-for-one special with the M1 Macs. ?
 

AppleFeller

macrumors 6502
Oct 19, 2020
371
520
Pleeease don't throw this "entry level device" excuse. The predecessor (the non-Touch Bar MacBook Pro) supported at least two displays externally. My 500$ work laptop does too.
Well if a 32” Pro Display XDR isn’t enough monitor for you then idk what to say ?
 

joevt

Contributor
Jun 21, 2012
6,837
4,133
That is all super cool info! Which Mac mini teardown are you referencing, by the way? Am curious. iFixit seemed to just try to do a brief three-for-one special with the M1 Macs. ?
There's pictures at https://egpu.io/forums/desktop-comp...i-apple-silicon-m1-thunderbolt-4-usb4-pcie-4/

The picture labeled "2020 M1 Mac Mini Unboxing Processor Thunderbolt 4 Controllers" has a picture of the MegaChips MCDP2920A4 in the top left corner.
You can google MCDP2920A4 site:ifixit.com to see that the same chip is used in the Apple TV 4K, and Mac mini Late 2018.
 
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mmkerc

macrumors 6502
Jun 21, 2014
290
157
I am currently running two Dell 3220DGF monitors on my M1 Mac mini connected via HDMI, and an OWC Thunderbolt 3 dock (also using the HDMI connection). Additionally this link shows how you can run up to 6 monitors off an M1 Mini at 4k resolution.
 

kecinzer

macrumors 6502a
Oct 19, 2015
624
344
Czech republic
I have DisplayLink adapter for few days and I'm going to return it. Performance is very poor for me and the main reason is that when I use ma MB Air on battery, put it on sleep and ther connect it again into DisplayLink I have wrong resolution on internal display that can't be easy changed.
 
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