puzzling boot issues with RasPi4b/8GB after kernel update: USB-C-OTG networking seems broken

[rcfa]

Setup: RasPi 4b w/ 8GB RAM booting of USB3 SSD

What worked: booting with the old 5.4.83:

• both with screen attached, or headless
• powered by USB powersupply, or by computer

What works: booting with the new 5.10.103:

• both with screen attached, or headless
• only powered by computer

It was difficult to track this down, because it was just due to the setup I have, that hooking the RasPi up to the screen meant I powered it for convenience from the computer. So at first it seemed the booting was dependent on a screen being attached, so I chased the wrong leads.

In reality, it's the power source, but that likely has a different reason:

I have USB-C networking enabled, so I can travel with the RasPi and hook up an iPad Pro with MagicKeyboard and access the RasPi with a fast, secure connection by xrdp via USB-C networking, and power the RasPi over the same cable (iPad Pro can easily be powered in turn by the USB-C port on the MagicKeyboard, but I digress...)

So what seems to have happened: with the change in kernel from 5.4.83 to 5.10.103, and the corresponding other updates, the USB-C-OTG networking was messed up. When enabled, the system only seems to boot when connected to a computer or some sort of USB host.
But as if that weren't enough: when that's the case, ifconfig reports the interface being up:

usb0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500        inet 10.123.45.6  netmask 255.255.255.248  broadcast 10.123.45.7
        ether 8a:d5:e4:fe:32:ef  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

But the host (in my case a Mac) sees the device as down.

So there have been some significant changes/bugs introduced with the latest updates, in that USB-C-OTG networking seems to be broken in significant ways...

There's one other option: that the way to enable/setup USB-C-OTG networking as significantly changed, but I'm not aware of any relevant changes in that regards.

[rcfa]

Well, things got more interesting: this seems to be a CABLE issue.
But it's not a bad cable: the cable worked fine with the old kernel, the cable works fine with drives on my Mac, the cable works fine for the initial boot.

So, my new working theory: RasPi 4 boots, and at some point switches to a high-power mode, notices the cable isn't designed for the kind of power requested, and halts the system.

It's not a shutdown, power LED still on, initial booting works; what's what made it so difficult to attribute it to the cable, especially given that the very same cable worked right up until the kernel update.

So: does anyone know what changed? Is there something that would support my theory?

PS: the issue with USB-C networking not working properly remains, but that seems to be independent from the boot issue, which is cable dependent, and thus (as my new working hypothesis) a matter of power delivery.