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Comment by stephenr | Hacker News Reader

Technically macOS does support MST. But it only supports it to stitch together for a single display. It does not support daisy chaining two displays.

Thankfully, every Mac for the last 7 years has Thunderbolt3 at least, so getting dual-4K-display from a single port/cable is still very doable, you just need a TB3 to dual DisplayPort or HDMI adapter.

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Well except for some of the Apple Silicon machines. The M1 (and maybe M2?) only have two video output blocks, of which one is already used for the internal display. It’s honestly the biggest complaint I have about my M1 MBP. Yes DisplayLink or whatever it’s called exists but the performance is bad.

Ugh, right. Probably a freudian slip that I just mentally pretend those configs didn't come after half a decade of ubiquitous multi-display support on Macs.

I assume you got one of the original M1 MBPs. The more recent models have more display blocks. M1 Pro can drive two external displays, and I think the M1 Max can do three or four. I’m still slightly pissed Apple ever shipped the original M1 MBPs, they were horrible machines.

Although I wished i would have gotten the 15” and a bigger hard drive, I’m still very happy with my first gen 13” mbp. Also the fact that I can just plug in usb c/Thunderbolt monitors and the screen just instantly displays and windows reconfigure without flickering is amazing.

It is really my only complaint about it, and since I needed a new laptop at the time it still made sense. I would definitely rather one of the 14” ones now - but not enough to buy a new device.

But - and I can't believe I forgot this in my other reply - this is one thing that really grinds my gears about Apple's releases since 2018, on everything except the Mac Pro (both 2019 and 2023).

They hard-code one DisplayPort stream to *something* other than Thunderbolt.

On laptops, they hard-code one via eDP to the display, which is useless if it's in clamshell mode.

On Mac Mini, Mac Studio, and the MacBook Pro with HDMI port, one stream from the GPU is hard-coded to the HDMI port. If you want maximum displays, always has to be from HDMI.

But neither the 2019 or 2023 Mac Pro have this limitation. Even on the 2019 model where the HDMI ports were physically on the card - they could route all video streams via system TB3 ports.

I just checked and the base M2 Mini, and the M2 Pro MBP seem to finally allow using two video streams over the TB4 ports, - but the M2 Ultra Studio, with the exact same SoC as the M2 Ultra Mac Pro, still has this stupid artificial limit.

Unfortunately, although it can work with certain Thunderbolt devices, many TB4 docks with the video path based on MST don’t work, causing havoc for mixed environments

Doesn't MST merely chop up the bandwidth of the lanes you have? So why would you update different parts of a single display using MST (each stream only getting part of the bandwidth of the link), rather than using the whole link to update the display from top to bottom at the same pixel depth and clock?

From memory they use it on things like the Pro Display XDR (possibly some 5K’s too?) where the display supports DP1.2;

My understanding is, the spec doesn’t allow for 5K/6K at 60hz but does allow for MST to send two streams at up to 4K/60, so it’s a creative use of what the spec supports to allow a higher resolution than envisaged when the spec was published.

You can daisy chain the Studio Display and Pro Display, and you could daisy chain the old Thunderbolt Display. Is that using some custom thing over Thunderbolt rather than MST?

Yes that’s Thunderbolt daisy chaining.

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