I started kernel configuration bisection in December but I do not remmeber the details (gcc 8 I think) so I need to redo it from start to see what exactly is the working configuration (is the one from hopkirk reproducibly working for me, first).
I restarted the kernel config bisection using 5.6.0-12706-gb032227c6293 and current gcc-9 (9.3.0 - Debian 9.3.0-10). So far I have not found a working configuration yet. From previous bisection saved configs and notes I came to a preliminary conclusion that when I turn off SMP from working configuration, I get a nonworking configuration that differs a lot (different RCU implementation chosen etc). I installed Debians latest UP (5.5.0-2-sparc64) and SMP (5.5.0-2-sparc64-smp) kernels on my single-CPU Ultra 45 and they both work for simple "apt update && apt install some-single-package" test. Earlier 5.2.0-3-sparc64 kernel also works well for compiling other kernels. Will continue searching for a working config and then config bisection to find something more precise. -- Meelis Roos <mroos@linux.ee>