Yup - as I mentioned, that commit is causing an error to be printed, but I am fairly certain that specific error is spurious/noise and not actual root cause of the problem, since the fallback behavior does appear to be working.
I'm almost done rebuilding a version reverted to 5.11, will post log excerpts once I finish that and run it.
October 20 build:
[ 8.275807] usbcore: registered new interface driver cdc_acm
[ 8.281476] cdc_acm: USB Abstract Control Model driver for USB modems and ISDN adapters
[ 8.289964] Loading modules backported from Linux version v5.10.68-0-g4d8524048a35
[ 8.297575] Backport generated by backports.git v5.10.68-1-0-ga4f9ba32
[ 8.304737] usbcore: registered new interface driver ems_usb
[ 8.310881] usbcore: registered new interface driver esd_usb2
[ 8.317301] usbcore: registered new interface driver gs_usb
[ 8.325894] usbcore: registered new interface driver kvaser_usb
[ 9.344435] mcp251x spi0.0: MCP251x didn't enter in conf mode after reset
[ 9.351377] mcp251x spi0.0: Probe failed, err=110
[ 9.359357] mcp251x: probe of spi0.0 failed with error -110
[ 9.397682] usbcore: registered new interface driver peak_usb
[ 9.403884] slcan: serial line CAN interface driver
[ 9.408790] slcan: 10 dynamic interface channels.
[ 9.415306] usbcore: registered new interface driver ums-alauda
[ 9.421595] usbcore: registered new interface driver ums-cypress
[ 9.427942] usbcore: registered new interface driver ums-datafab
[ 9.434207] usbcore: registered new interface driver ums-freecom
[ 9.440617] usbcore: registered new interface driver ums-isd200
[ 9.446881] usbcore: registered new interface driver ums-jumpshot
[ 9.453250] usbcore: registered new interface driver ums-karma
[ 9.459437] usbcore: registered new interface driver ums-sddr09
[ 9.465695] usbcore: registered new interface driver ums-sddr55
[ 9.471911] usbcore: registered new interface driver ums-usbat
[ 9.478293] usbcore: registered new interface driver usb_8dev
[ 9.485077] usbcore: registered new interface driver usbhid
[ 9.490652] usbhid: USB HID core driver
[ 9.496413] usbcore: registered new device driver usbip-host
[ 9.503400] usbcore: registered new interface driver usbserial_generic
[ 9.509970] usbserial: USB Serial support registered for generic
[ 9.516324] vcan: Virtual CAN interface driver
[ 9.521587] vhci_hcd vhci_hcd.0: USB/IP Virtual Host Controller
[ 9.527524] vhci_hcd vhci_hcd.0: new USB bus registered, assigned bus number 3
[ 9.534773] vhci_hcd: created sysfs vhci_hcd.0
[ 9.539298] usb usb3: New USB device found, idVendor=1d6b, idProduct=0002, bcdDevice= 5.10
[ 9.547569] usb usb3: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 9.554792] usb usb3: Product: USB/IP Virtual Host Controller
[ 9.560531] usb usb3: Manufacturer: Linux 5.10.72 vhci_hcd
[ 9.566014] usb usb3: SerialNumber: vhci_hcd.0
[ 9.570629] hub 3-0:1.0: USB hub found
[ 9.574395] hub 3-0:1.0: 8 ports detected
[ 9.578727] vhci_hcd vhci_hcd.0: USB/IP Virtual Host Controller
[ 9.584658] vhci_hcd vhci_hcd.0: new USB bus registered, assigned bus number 4
[ 9.591974] usb usb4: We don't know the algorithms for LPM for this host, disabling LPM.
[ 9.600122] usb usb4: New USB device found, idVendor=1d6b, idProduct=0003, bcdDevice= 5.10
[ 9.608387] usb usb4: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 9.615611] usb usb4: Product: USB/IP Virtual Host Controller
[ 9.621350] usb usb4: Manufacturer: Linux 5.10.72 vhci_hcd
[ 9.626832] usb usb4: SerialNumber: vhci_hcd.0
[ 9.631418] hub 4-0:1.0: USB hub found
[ 9.635190] hub 4-0:1.0: 8 ports detected
[ 9.650604] xt_time: kernel timezone is -0000
[ 9.687717] usbcore: registered new interface driver cp210x
[ 9.693332] usbserial: USB Serial support registered for cp210x
[ 9.700497] usbcore: registered new interface driver ftdi_sio
[ 9.706313] usbserial: USB Serial support registered for FTDI USB Serial Device
[ 9.724849] usbcore: registered new interface driver pl2303
[ 9.730451] usbserial: USB Serial support registered for pl2303
[ 9.749160] brcmfmac: brcmf_fw_alloc_request: using brcm/brcmfmac43455-sdio for chip BCM4345/6
[ 9.967654] brcmfmac: brcmf_fw_alloc_request: using brcm/brcmfmac43455-sdio for chip BCM4345/6
[ 9.985887] brcmfmac: brcmf_c_preinit_dcmds: Firmware: BCM4345/6 wl0: Mar 23 2020 02:20:01 version 7.45.206 (r725000 CY) FWID 01-febaba43
[ 10.027828] usbcore: registered new interface driver brcmfmac
[ 10.034048] kmodloader: done loading kernel modules from /etc/modules.d/*
[ 11.433123] bcmgenet fd580000.ethernet: configuring instance for external RGMII (RX delay)
[ 11.441813] bcmgenet fd580000.ethernet eth0: Link is Down
[ 11.447446] br-lan: port 1(eth0) entered blocking state
[ 11.452680] br-lan: port 1(eth0) entered disabled state
[ 11.458181] device eth0 entered promiscuous mode
[ 12.609598] netlink: 'iw': attribute type 302 has an invalid length.
[ 16.942550] IPv6: ADDRCONF(NETDEV_CHANGE): wlan0: link becomes ready
Latest git - driver loads, but link never comes up, and attempting to run ifconfig hangs the system:
[ 8.367110] usbcore: registered new interface driver cdc_acm
[ 8.372770] cdc_acm: USB Abstract Control Model driver for USB modems and ISDN adapters
[ 8.381152] Loading modules backported from Linux version v5.15-rc6-0-g519d81956ee2
[ 8.388823] Backport generated by backports.git v5.15-rc6-1-0-gd44432d6
[ 8.395875] usbcore: registered new interface driver ems_usb
[ 8.401905] usbcore: registered new interface driver esd_usb2
[ 8.410525] usbcore: registered new interface driver kvaser_usb
[ 8.442865] usbcore: registered new interface driver peak_usb
[ 8.449127] slcan: serial line CAN interface driver
[ 8.454007] slcan: 10 dynamic interface channels.
[ 8.460228] usbcore: registered new interface driver ums-alauda
[ 8.466789] usbcore: registered new interface driver ums-cypress
[ 8.473129] usbcore: registered new interface driver ums-datafab
[ 8.479455] usbcore: registered new interface driver ums-freecom
[ 8.485771] usbcore: registered new interface driver ums-isd200
[ 8.492005] usbcore: registered new interface driver ums-jumpshot
[ 8.498392] usbcore: registered new interface driver ums-karma
[ 8.504795] usbcore: registered new interface driver ums-sddr09
[ 8.511009] usbcore: registered new interface driver ums-sddr55
[ 8.517262] usbcore: registered new interface driver ums-usbat
[ 8.523718] usbcore: registered new interface driver usb_8dev
[ 8.530472] usbcore: registered new interface driver usbhid
[ 8.536056] usbhid: USB HID core driver
[ 8.541661] usbcore: registered new device driver usbip-host
[ 8.548685] usbcore: registered new interface driver usbserial_generic
[ 8.555270] usbserial: USB Serial support registered for generic
[ 8.561773] vcan: Virtual CAN interface driver
[ 8.567208] vhci_hcd vhci_hcd.0: USB/IP Virtual Host Controller
[ 8.573137] vhci_hcd vhci_hcd.0: new USB bus registered, assigned bus number 3
[ 8.580399] vhci_hcd: created sysfs vhci_hcd.0
[ 8.584937] usb usb3: New USB device found, idVendor=1d6b, idProduct=0002, bcdDevice= 5.10
[ 8.593197] usb usb3: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 8.600422] usb usb3: Product: USB/IP Virtual Host Controller
[ 8.606169] usb usb3: Manufacturer: Linux 5.10.76 vhci_hcd
[ 8.611647] usb usb3: SerialNumber: vhci_hcd.0
[ 8.616292] hub 3-0:1.0: USB hub found
[ 8.620053] hub 3-0:1.0: 8 ports detected
[ 8.624400] vhci_hcd vhci_hcd.0: USB/IP Virtual Host Controller
[ 8.630316] vhci_hcd vhci_hcd.0: new USB bus registered, assigned bus number 4
[ 8.637582] usb usb4: We don't know the algorithms for LPM for this host, disabling LPM.
[ 8.645735] usb usb4: New USB device found, idVendor=1d6b, idProduct=0003, bcdDevice= 5.10
[ 8.653994] usb usb4: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 8.661214] usb usb4: Product: USB/IP Virtual Host Controller
[ 8.666961] usb usb4: Manufacturer: Linux 5.10.76 vhci_hcd
[ 8.672439] usb usb4: SerialNumber: vhci_hcd.0
[ 8.677042] hub 4-0:1.0: USB hub found
[ 8.680800] hub 4-0:1.0: 8 ports detected
[ 8.696146] xt_time: kernel timezone is -0000
[ 8.735522] usbcore: registered new interface driver cp210x
[ 8.741137] usbserial: USB Serial support registered for cp210x
[ 8.748035] usbcore: registered new interface driver ftdi_sio
[ 8.753812] usbserial: USB Serial support registered for FTDI USB Serial Device
[ 8.772297] usbcore: registered new interface driver pl2303
[ 8.777955] usbserial: USB Serial support registered for pl2303
[ 8.796737] brcmfmac: brcmf_fw_alloc_request: using brcm/brcmfmac43455-sdio for chip BCM4345/6
[ 8.805964] brcmfmac mmc1:0001:1: Direct firmware load for brcm/brcmfmac43455-sdio.raspberrypi,4-model-b.bin failed with error -2
[ 8.817647] brcmfmac mmc1:0001:1: Falling back to sysfs fallback for: brcm/brcmfmac43455-sdio.raspberrypi,4-model-b.bin
[ 9.042507] brcmfmac: brcmf_fw_alloc_request: using brcm/brcmfmac43455-sdio for chip BCM4345/6
[ 9.060938] brcmfmac: brcmf_c_preinit_dcmds: Firmware: BCM4345/6 wl0: Mar 23 2020 02:20:01 version 7.45.206 (r725000 CY) FWID 01-febaba43
[ 9.102143] usbcore: registered new interface driver brcmfmac
[ 9.108417] kmodloader: done loading kernel modules from /etc/modules.d/*
[ 12.143347] bcmgenet fd580000.ethernet: configuring instance for external RGMII (RX delay)
[ 12.151987] bcmgenet fd580000.ethernet eth0: Link is Down
[ 12.152375] br-lan: port 1(eth0) entered blocking state
[ 12.162672] br-lan: port 1(eth0) entered disabled state
[ 12.168103] device eth0 entered promiscuous mode
Revert to 5.11:
[ 8.274771] usbcore: registered new interface driver cdc_acm
[ 8.280432] cdc_acm: USB Abstract Control Model driver for USB modems and ISDN adapters
[ 8.288784] Loading modules backported from Linux version v5.11.22-0-gef8aed2fffe0
[ 8.296365] Backport generated by backports.git v5.11.22-1-0-g49432781
[ 8.303323] usbcore: registered new interface driver ems_usb
[ 8.309610] usbcore: registered new interface driver esd_usb2
[ 8.318262] usbcore: registered new interface driver kvaser_usb
[ 8.351311] usbcore: registered new interface driver peak_usb
[ 8.357684] slcan: serial line CAN interface driver
[ 8.362568] slcan: 10 dynamic interface channels.
[ 8.368603] usbcore: registered new interface driver ums-alauda
[ 8.374971] usbcore: registered new interface driver ums-cypress
[ 8.381271] usbcore: registered new interface driver ums-datafab
[ 8.387584] usbcore: registered new interface driver ums-freecom
[ 8.393874] usbcore: registered new interface driver ums-isd200
[ 8.400118] usbcore: registered new interface driver ums-jumpshot
[ 8.406486] usbcore: registered new interface driver ums-karma
[ 8.412652] usbcore: registered new interface driver ums-sddr09
[ 8.418870] usbcore: registered new interface driver ums-sddr55
[ 8.425108] usbcore: registered new interface driver ums-usbat
[ 8.431414] usbcore: registered new interface driver usb_8dev
[ 8.438154] usbcore: registered new interface driver usbhid
[ 8.443728] usbhid: USB HID core driver
[ 8.449157] usbcore: registered new device driver usbip-host
[ 8.456172] usbcore: registered new interface driver usbserial_generic
[ 8.462719] usbserial: USB Serial support registered for generic
[ 8.469085] vcan: Virtual CAN interface driver
[ 8.474355] vhci_hcd vhci_hcd.0: USB/IP Virtual Host Controller
[ 8.480282] vhci_hcd vhci_hcd.0: new USB bus registered, assigned bus number 3
[ 8.487544] vhci_hcd: created sysfs vhci_hcd.0
[ 8.492302] usb usb3: New USB device found, idVendor=1d6b, idProduct=0002, bcdDevice= 5.10
[ 8.500571] usb usb3: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 8.507794] usb usb3: Product: USB/IP Virtual Host Controller
[ 8.513532] usb usb3: Manufacturer: Linux 5.10.76 vhci_hcd
[ 8.519016] usb usb3: SerialNumber: vhci_hcd.0
[ 8.523634] hub 3-0:1.0: USB hub found
[ 8.527404] hub 3-0:1.0: 8 ports detected
[ 8.531747] vhci_hcd vhci_hcd.0: USB/IP Virtual Host Controller
[ 8.537676] vhci_hcd vhci_hcd.0: new USB bus registered, assigned bus number 4
[ 8.545028] usb usb4: We don't know the algorithms for LPM for this host, disabling LPM.
[ 8.553171] usb usb4: New USB device found, idVendor=1d6b, idProduct=0003, bcdDevice= 5.10
[ 8.561436] usb usb4: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 8.568656] usb usb4: Product: USB/IP Virtual Host Controller
[ 8.574404] usb usb4: Manufacturer: Linux 5.10.76 vhci_hcd
[ 8.579882] usb usb4: SerialNumber: vhci_hcd.0
[ 8.584493] hub 4-0:1.0: USB hub found
[ 8.588250] hub 4-0:1.0: 8 ports detected
[ 8.603504] xt_time: kernel timezone is -0000
[ 8.642216] usbcore: registered new interface driver cp210x
[ 8.647866] usbserial: USB Serial support registered for cp210x
[ 8.654781] usbcore: registered new interface driver ftdi_sio
[ 8.660562] usbserial: USB Serial support registered for FTDI USB Serial Device
[ 8.679272] usbcore: registered new interface driver pl2303
[ 8.684887] usbserial: USB Serial support registered for pl2303
[ 8.700055] kmodloader: done loading kernel modules from /etc/modules.d/*
[ 8.710489] brcmfmac: brcmf_fw_alloc_request: using brcm/brcmfmac43455-sdio for chip BCM4345/6
[ 8.928184] brcmfmac: brcmf_fw_alloc_request: using brcm/brcmfmac43455-sdio for chip BCM4345/6
[ 8.949766] brcmfmac: brcmf_c_preinit_dcmds: Firmware: BCM4345/6 wl0: Mar 23 2020 02:20:01 version 7.45.206 (r725000 CY) FWID 01-febaba43
[ 8.992812] usbcore: registered new interface driver brcmfmac
[ 11.695257] bcmgenet fd580000.ethernet: configuring instance for external RGMII (RX delay)
[ 11.703911] bcmgenet fd580000.ethernet eth0: Link is Down
[ 11.706939] br-lan: port 1(eth0) entered blocking state
[ 11.714676] br-lan: port 1(eth0) entered disabled state
[ 11.720088] device eth0 entered promiscuous mode
BusyBox v1.34.1 (2021-11-01 15:34:36 UTC) built-in shell (ash)
_______ ________ __
| |.-----.-----.-----.| | | |.----.| |_
| - || _ | -__| || | | || _|| _|
|_______|| __|_____|__|__||________||__| |____|
|__| W I R E L E S S F R E E D O M
-----------------------------------------------------
OpenWrt SNAPSHOT, r17781+152-a889dcd3f2
-----------------------------------------------------
root@Automation-pibridge164:/# [ 37.031759] IPv6: ADDRCONF(NETDEV_CHANGE): wlan0: link becomes ready
Note that it took a LONG time for the link to come up, and WLAN performance is extremely poor (to the point where LuCi is unusable due to XHR timeouts). There is a possibility that is due to lacking a Raspberry Pi specific backport from the Pi kernel repos that I have been procrastinating on upstreaming here. ifconfig and iw don't hang the system
Edit: Performance issues seem to be either a need to reboot at least once with a "sane" configuration, or due to lacking AC mode on Raspberry Pi 4 WiFi - #37 by Entropy512
Edit 2: I mentioned on github that sysupgrade, even of a locally obtained file (such as from USB storage) was failing with the "bad" brcmfmac - somehow, the bad driver was causing the kernel to behave EXTREMELY strangely - in addition to ifconfig and iw hanging the kernel silently, a sysupgrade would cause the console to spam a bunch of USB port errors, an mmcblk0 error, then the Pi would completely power off. The USB port errors would not occur during a transfer - manually copying an image from USB to /tmp would work, but then sysupgrading would send the system out to lunch, and I was able to tell from uname that sysupgrade didn't flash anything (the system had been running a kernel prior to the 5.10.76 bump, and a sysupgrade success should have taken it to 5.10.76)