linux-rpi-5.4.27; kernel/firmware defect causes RPi4 to become nonfunctional
This one is proving nearly impossible to troubleshoot because of what's going on.
5.4.27 exhibits multiple issues indicating a severe kernel or firmware problem. This reproduced on 2 different RPi4 4GBs and failed to reproduce with 5.4.12 on the same hardware. Symptoms include complete loss of USB port functionality, and severe impacts to both Ethernet and WiFi in the form of 30%+ packet loss which jumps to 100% when attempting to pass any sort of significant traffic. As a direct result, it is impossible to diagnose or troubleshoot the system because there is literally no way to get into the system. (Because, again, the USB ports are also dead.)
This was able to be identified as specific to 5.4.27 as the exact same hardware fails to show these symptoms when running any other kernel or any other distribution. i.e. Alpine 5.4.12, Raspbian Lite, Ubuntu, etc. All other scenarios, the hardware behaves as expected with USB ports functioning and no packet loss.