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.Also, 5.5.0-1-sparc64 works.
Too early conclusion - compiled and installed a kernel but crashed on shutdown (for reboot):
[ 6871.421677] Unable to handle kernel NULL pointer dereference
[ 6871.489666] tsk->{mm,active_mm}->context = 0000000000000094
[ 6871.556530] tsk->{mm,active_mm}->pgd = fff00002756b8000
[ 6871.619231] \|/ ____ \|/
[ 6871.619231] "@'/ .. \`@"
[ 6871.619231] /_| \__/ |_\
[ 6871.619231] \__U_/
[ 6871.795790] kworker/0:1(1138): Oops [#1]
[ 6871.842860] CPU: 0 PID: 1138 Comm: kworker/0:1 Tainted: G E 5.5.0-1-sparc64 #1 Debian 5.5.13-2
[ 6871.962020] Workqueue: cgroup_destroy css_killed_work_fn
[ 6872.025824] TSTATE: 0000008080e01600 TPC: 00000000005d3028 TNPC: 00000000005d302c Y: 00000000 Tainted: G E
[ 6872.160601] TPC: <memcg_drain_all_list_lrus+0x108/0x160>
[ 6872.224377] g0: 000000000000009e g1: fff000027638b980 g2: fff000027638b980 g3: 0000000100864288
[ 6872.328872] g4: fff00002753c8000 g5: 000000000021a774 g6: fff00002745f4000 g7: 0000000000000000
[ 6872.433358] o0: 0000000000d5d598 o1: fff00002762e4010 o2: 0000000000000000 o3: 0000000000000000
[ 6872.537843] o4: fff000027638b980 o5: fff0000276a80000 sp: fff00002745f71d1 ret_pc: fff0000274a22280
[ 6872.646506] RPC: <0xfff0000274a22280>
[ 6872.690402] l0: 0000000000000001 l1: 0000000010000000 l2: 00000000005c96c0 l3: 00000000005c9720
[ 6872.794943] l4: 0000000000d58800 l5: 0000000000c29c48 l6: 0000000000dbac00 l7: 0000000000000001
[ 6872.899429] i0: 0000000000000038 i1: fff0000274472000 i2: 0000000000d5d5b0 i3: 0000000000d5d400
[ 6873.003915] i4: fff0000274a22280 i5: fff000027c154470 i6: fff00002745f7281 i7: 0000000000623c18
[ 6873.111709] I7: <memcg_offline_kmem.part.0+0x98/0xe0>
[ 6873.174017] Call Trace:
[ 6873.204979] [0000000000623c18] memcg_offline_kmem.part.0+0x98/0xe0
[ 6873.281892] [0000000000626094] mem_cgroup_css_offline+0xb4/0xe0
[ 6873.355768] [00000000004eaed8] css_killed_work_fn+0x38/0x120
[ 6873.426542] [0000000000482c54] process_one_work+0x194/0x460
[ 6873.496229] [0000000000483064] worker_thread+0x144/0x540
[ 6873.562782] [0000000000488cdc] kthread+0xdc/0x120
[ 6873.622005] [0000000000405fa4] ret_from_fork+0x1c/0x2c
[ 6873.686410] [0000000000000000] 0x0
[ 6873.729990] Disabling lock debugging due to kernel taint
[ 6873.795475] Caller[0000000000623c18]: memcg_offline_kmem.part.0+0x98/0xe0
[ 6873.878743] Caller[0000000000626094]: mem_cgroup_css_offline+0xb4/0xe0
[ 6873.958837] Caller[00000000004eaed8]: css_killed_work_fn+0x38/0x120
[ 6874.035813] Caller[0000000000482c54]: process_one_work+0x194/0x460
[ 6874.111726] Caller[0000000000483064]: worker_thread+0x144/0x540
[ 6874.184407] Caller[0000000000488cdc]: kthread+0xdc/0x120
[ 6874.249765] Caller[0000000000405fa4]: ret_from_fork+0x1c/0x2c
[ 6874.320251] Caller[0000000000000000]: 0x0
[ 6874.369854] Instruction DUMP:
[ 6874.369856] ce5f0000
[ 6874.406890] c4584000
[ 6874.436667] c65f2008
[ 6874.466425] <d871e008>
[ 6874.496071] ce704000
[ 6874.525725] c470c000
[ 6874.555388] c670a008
[ 6874.584920] de770000
[ 6874.614471] de772008
--
Meelis Roos <mroos@linux.ee>