[PATCH v2 2/2] powerpc:85xx: fix timebase sync issue when CONFIG_HOTPLUG_CPU=n

[Martin Kennedy]

Hi there,

I have bisected OpenWrt master, and then the Linux kernel down to this
change, to confirm that this change causes a kernel panic on my
P1020RDB-based, dual-core Aerohive HiveAP 370, at initialization of
the second CPU:

:
[    0.000000] Linux version 5.10.80 (labby at lobon)
(powerpc-openwrt-linux-musl-gcc (OpenWrt GCC 11.2.0
r18111+1-ebb6f9287e) 11.2.0, GNU ld (GNU Binutils) 2.37) #0 SMP Thu
Nov 25 02:49:35 2021
[    0.000000] Using P1020 RDB machine description
:
[    0.627233] smp: Bringing up secondary CPUs ...
[    0.681659] kernel tried to execute user page (0) - exploit attempt? (uid: 0)
[    0.766618] BUG: Unable to handle kernel instruction fetch (NULL pointer?)
[    0.848899] Faulting instruction address: 0x00000000
[    0.908273] Oops: Kernel access of bad area, sig: 11 [#1]
[    0.972851] BE PAGE_SIZE=4K SMP NR_CPUS=2 P1020 RDB
[    1.031179] Modules linked in:
[    1.067640] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.10.80 #0
[    1.139507] NIP:  00000000 LR: c0021d2c CTR: 00000000
[    1.199921] REGS: c1051cf0 TRAP: 0400   Not tainted  (5.10.80)
[    1.269705] MSR:  00021000 <CE,ME>  CR: 84020202  XER: 00000000
[    1.340534]
[    1.340534] GPR00: c0021cb8 c1051da8 c1048000 00000001 00029000
00000000 00000001 00000000
[    1.340534] GPR08: 00000001 00000000 c08b0000 00000040 22000208
00000000 c00032c4 00000000
[    1.340534] GPR16: 00000000 00000000 00000000 00000000 00000000
00000000 00029000 00000001
[    1.340534] GPR24: 1ffff240 20000000 dffff240 c080a1f4 00000001
c08ae0a8 00000001 dffff240
[    1.758220] NIP [00000000] 0x0
[    1.794688] LR [c0021d2c] smp_85xx_kick_cpu+0xe8/0x568
[    1.856126] Call Trace:
[    1.885295] [c1051da8] [c0021cb8] smp_85xx_kick_cpu+0x74/0x568 (unreliable)
[    1.968633] [c1051de8] [c0011460] __cpu_up+0xc0/0x228
[    2.029038] [c1051e18] [c0031bbc] bringup_cpu+0x30/0x224
[    2.092572] [c1051e48] [c0031f3c] cpu_up.constprop.0+0x180/0x33c
[    2.164443] [c1051e88] [c00322e8] bringup_nonboot_cpus+0x88/0xc8
[    2.236326] [c1051eb8] [c07e67bc] smp_init+0x30/0x78
[    2.295698] [c1051ed8] [c07d9e28] kernel_init_freeable+0x118/0x2a8
[    2.369641] [c1051f18] [c00032d8] kernel_init+0x14/0x124
[    2.433176] [c1051f38] [c0010278] ret_from_kernel_thread+0x14/0x1c
[    2.507125] Instruction dump:
[    2.542541] XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
XXXXXXXX XXXXXXXX
[    2.635242] XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
XXXXXXXX XXXXXXXX
[    2.727952] ---[ end trace 9b796a4bafb6bc14 ]---
[    2.783149]
[    3.800879] Kernel panic - not syncing: Fatal exception
[    3.862353] Rebooting in 1 seconds..
[    5.905097] System Halted, OK to turn off power

Without this patch, the kernel no longer panics:

[    0.627232] smp: Bringing up secondary CPUs ...
[    0.681857] smp: Brought up 1 node, 2 CPUs

Here is the kernel configuration for this built kernel:
https://git.openwrt.org/?p=openwrt/openwrt.git;a=blob_plain;f=target/linux/mpc85xx/config-5.10;hb=HEAD

In case a force-push is needed for the source repository
(https://github.com/Hurricos/openwrt/commit/ad19bdfc77d60ee1c52b41bb4345fdd02284c4cf),
here is the device tree for this board:
https://paste.c-net.org/TrousersSliced

Martin