So again, a few issues. Even non-GPU devices, like storage, need compatible option ROMs. At least for boot. That may make a lot of PCIe storage cards non-bootable. (I haven't seen any reports of people booting from Thunderbolt attached PCIe storage yet on M1, I'd assume it probably doesn't work.)
That really doesn't line up. Here is a link to the Sonnet Technogiy PCI-e card
compatibility matrix for their Thunderbolt PCI-e enclosure boxes. It is a PDF file but I'll loop in a summary count for each of the categories of cards . The M1 gets its own column so it ia pretty widespread issue. These are counts of "Yes" compatible driver among the options and then the total number of cards ( followed by the percentage).
Category Yes Total %
Audio Pro Cards 0 24 0
Ethernet Adapter 1GbE 0 5 0
Ethernet Adapter 10GbE 2 12 17 ( two more Q1 33% )
Ethernet Adapter 25GbE 0 6 0 ( but none of these work on Intel Macs either. No drivers )
Fiber Channel 0 9 0
FireWire 2 5 40
GPU Cards 0 4 0 ( these aren't the GPU focused boxes )
PCI-e Bus Extender 0 4 0
SAS/SATA (RAID) 2 17 12 ( only two are Sonnet Tech )
SSD 9 15 60 ( if drop the ones were no coverage on Intel mac percentage 75% )
Signal Acquisition 0 4 0 ( no Intel Mac qualify either; no new issue here)
Video Capture Process 13 134 9 ( first 65 cards ran on Intel Macs at some point)
USB 3 Adatper 5 5 100
SSD drives work reasonably well. Enabling more external USB devices works just fine (so the boot drives there ... no problem). Even FireWire if wanted to boot relatively slow would be mostly OK.
Booting off of "real RAID" cards is a problem, but is that really a firmware problem or a don't have a System Extension replacement for their Kernel Extension (kext) problem? If those RAID card pretended to be a NVMe SSD or just a plain SATA drive , then it probably would work as well as the SSD category. Won't be surprising if the non-disk features ( metadata interace to RAID management ) was the bigger problematic issue here.
1GbE works good enough on USB 3 adapters ( I suspect this more 'old' , funky controllers assumptions than something to do with boot firmware). If there are two 10GbE adapters on an updated Mac Pro who has a deep need a 1GbE card anyway? The 25GbE clearly illustrates that macOS has a driver issue in general ( nothing really new about the M1 ).
Booting off of Fibre Channel probably isn't a show stopper for most folks.
So, as far as keyboard, mouse , storage getting going on the boot process isn't a huge problem if mainly depending upon an attached drive. Probably if toss out the older cards that have controllers that have been dropped by Apple and/or have abandoned/obsolete drivers there is already decent coverage.
GPUs are harder to tell because Apple just simply doesn't cover them at all and the Apple GPU is ubiquitous. Can get a boot screen in plug into the Apple GPU port on all M-series.
Video processor aren't necessary at boot. Drivers initializing the card post boot should be possible if the vendor is up to modernizing the drivers. That is a large category for Apple to completely walk away from. That is a bigger set than several of the other categories combined.
Audio is suggestive that it is in part simply drivers. There is link the matrix for the "No" for Rednet Dante card.
The PCI-e card is a bright red NO there. The dedicated TB device is a "Yes". ( if digging into the article on M1 thunderbolt drivers find out they are stll kext ; not System extensions. )
Apple's push to revamp the "extensions" system from kext to system extensions is probably going to be a bit of a "garbage collection" on cards and devices that have about zero driver R&D budget for upkeep and maintenance. That isn't really an M1 only problem since Apple is on path to dumping kext on Intel side over an extended period of time also.
The M1 intensifies Apple's 'war' on old, vintage kext drivers, but it is an issue that is in motion regardless of platform. There probably are a sizable number of board controllers that have BIOS and odd ball x86 instructional mode quirky incantations that have been in zombie support mode for a long while that will drop out. However, that is mostly a tangent to strict UEFI going away. ( UEFI has been used as a crutch to keep old BIOS cruft danging around. Apple not committed to BIOS and multiple decade old stuff shouldn't be a shocker. )
PCI-e has some standards augments that could help. There is a push to get PCI=e devices to authenticate themselves and their firmware metadata.
pcisig.com
Intel also has a
draft on PCI-e security enhancements. (NOTE: link to a PDF file)
The metadata about the card and its firmware is to be verfied
before load the drivers/bootstramp code from the PCI-e device. That "Vendor ID , device ID , firmware version" data is all the System Extenstion dispatch should need to play match maker between a still unitilized ( or hotplugged) device in the PCI-e device tree. Even if don't use the card's bootstrap code the singature of that code can be used as a "matchmaker" for a driver . That driver though would have to do the initial incantation to get the PCI-e device running.
There are tons of old cards that are not going to meet those security standards. but slots in a M-series Mac Pro would be just as much about new cards in the future.
In short, lack of UEFI should not be a show stopper for the post boot environment. There is a giant stack of not written yet software though.
At best, we're back to the PPC days where PCIe card options were restricted down to companies like Sonnet on things like storage. That already makes an AS Mac Pro quite a bit less attractive.
The days were a significantly large chunk of card vendors didn't write macOS drivers really didn't change with Intel.
The macOS driver model isn't the windows driver model. That means work which some folks will punt on. MacOS on both Intel and M-series means there is twice as much work for those that have drivers with significantly large chunks of handwritten assembly in them.
Did macOS Intel have a large broach ecosystem of Infiniband cards avaialble (that were on Linux) ? No.
100GbE cards ? No.
> 10GbE cards ? No ( outlined that above already. )
MacOS just being > 10% of the market is a factor here at least as much as pre-boot roms.
But, even going down that route... size becomes a problem. A half height AS Mac Pro would barely have enough room for an MPX Slot. It's definitely not going to have a whole bunch of slots just based on a case layout alone. And height isn't the only problem. Length becomes another problem. You'd end up with a Mac Pro that looks like a giant sausage if it's half height but full length.
I have been though this before, but if just look at the core 'box' ignore the feet/handle , then if drop MPX bay 2 and combine top CPU and "bottom" MPX thermal zones then have dumped 1/3 of that height. So only 1/6 short of getting to a full overall height drop by a 1/2. So nuke the feet(substitute small fraction of an inch non-stracth pads and place on desktop ). That is a couple of inches there. So now down to about a 1/12 (or less) of getting to "half as tall". That's if just stick dogmatically to the "space frame" model. If you get rid of the patruding handles, feet, twist off handle on top ( some kind of more normal latch closed case with integrated handles ). That is the rest of the half.
That is just leaving the CPU thermal zone alone. If willing to get with two different sized fans on front then could possibly take the 1/6 off of that (and still keep some feet height ) .
The notion that is super hard to get to half spatially is only if looking for an excuse to get rid of the slots; not that it is necessary.
The fact that it isn't some perfect cube or whatever OCD geometric shape Apple industrial design might want doesn't really matter. The length would be serving a purpose. The holes in the grill serve a purpose. If magical oompa loompas could move the heat of the Mac Pro without moving air then they get rid of those holes too.
And is Apple willing to ship a Mac Pro without PCIe slots? Yes! They did with the 2013. It's completely reasonable, given all the issues and everything described, that Apple could ship a Mac Pro without PCIe slots. Especially given the last time they optimized for size they cut all the PCIe slots.
Let's say they bring back a 7" x 7" x 7" cube and put 6-12 Thunderbolt sockets on the back and they would have really solved a whole lot of nothing if do not have the driver creation problem sorted out. The number of solution suppliers collapses down to a smaller number and the solutions available greatly decrease then really haven't solved the issue ( if non pre-boot was a core issue).
The the thunderbolt driver issue gets solved then it that would help internal PCI-e slot also ( to the cards Thunderbolt is really largely transparent once get past having to deal with hot plug/unplug. ) Hot plug/unplug means card initalization isn't necessarily at overall system boot time.
All they would have done is balloon squeeze the PCI-e cards out to a more expensive Thunderbolt PCI-e slot external enclosures. As a net "save desktop footprint consumption" solution that is a spatial diaster. Now have the total desktop area consumption consists of the "cube" plus the TB eclosure which is going to include all of the "evil length" avoided by punting to another box.
If the Apple "Mac Pro" SoC can't provision a decent number of slot then that would be a rational path to dump them. For example only had 12 TB and 16-20 x1 PCI-v4 lanes coming out as general PCI-e related I/O then sure skip the slots. The widest lane bundle in that context is just x4 PCI-e v3. If that is the best they got and are trying to compete in the 2022 workstation SoC marketplace then they really do need some serious misdirection. "Don't look behind the curtain we are the all powerful Oz" is really all they got to work with.
If they leave enough space to put an user optional 2.5" SSD or a couple of M.2 SSD then they wouldn't have very tightly painted themselves back into the same corner that the Mac Pro 2013 did. There would be enough room to declare themselves "king of that small kingdom" but much of the same problems would be the same.
Minimally they'd need more than one storage device even if there are no legacy standard PCI-e slots. That one, and only one, storage device restriction was explicitly mentioned by Apple as being a mistake for the Mac Pro. Since the RAM is likely soldered down they'd need something users could insert to label this as part of the "Mac Pro" line up.
Otherwise this would be more of a "Mini Plus" . With the current Mini enclosure there is pretty good chance they could get the Jade2C ( 20 cores and 64 GPUs) into that box with some small board changes. Get some "front to back" reasonable speed airflow through there by making it taller they could get 40 cores and 128 GPUs could be possible if minimized any other additions.
However, what would be the point of a "Mac Pro" that largely just covered the same space as the Mini. Apple could through the "Mac Pro" name at it because the price tag was quite high for a Mini , but it pragmatically would just be a Mini.
if Apple only wants to do 2-3 M-series product lines then yeah the role that the Mac Pro has filled probably dry up. That would be almost entirely Apple's choice though, not precipitated by shift from UEFI.