rpi-4.19.y-rt USB FIQ exception handler still locking up on RPi zero

[Nedardo]

When using the PREEMPT-RT kernel, the usb FIQ exception handler is still giving me issues on the Raspberry pi Zero. The previous patches (#2244) seem to work fine in testing on Raspberry pi 3 and 4, but on the zero plugging anything in to the usb still causes immediate lock-up. The workaround of adding 'dwc_otg.fiq_enable=0 dwc_otg.fiq_fsm_enable=0' to cmdline.txt fixes this as expected, but at the cost of significant cpu cycle consumption.

Is this a known issue or is there an alternative patch that needs to be applied for the zero specifically? Pinging @TiejunChina, as I understand the rt code isn't maintained by the Raspberry Pi Foundation. Apologies if this isn't the best forum for this issue.

To reproduce
Compiled rpi-4.19.y-rt for the Pi zero as per https://www.raspberrypi.org/documentation/linux/kernel/building.md and loaded onto fresh sd card flashed with Raspbian 10 image from 2020-05-27. Boot Raspberry Pi Zero 1.3, then insert any usb device (tested with usb ethernet and flash drive).

Actual behaviour
Kernel locks up immediately with no output to console.

Additional context
Same code compiled and tested for and on Pi 4 and Pi 3's works without issue.

[pelwell]

Apologies if this isn't the best forum for this issue.

We're happy to host this issue but not support it, on the same basis that we host but don't support the -rt branch.