-10 panic during dbcool attach

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

A (custom) -10/amd64 kernel panics on certain hardware during dbcool attach:

[  1.0438364] piixpm0 at pci0 dev 2 function 0: ServerWorks HT1000SB South Bridge (rev. 0x00)
[  1.0438364] piixpn0: polling
[  1.0438364] iic0 at piixpm0 port 0: I2C bus
[  1.0438364] spdmen0 at iic0 addr 0x50
[  1.0438364] spdmem0: DDR2 SDRAM, data ECC, 2GB, 800MHz (PC2-6400)
[  1.0438364] spdnen1 at iicß addr 0x51
[  1.0438364] spdmen1: DDR2 SDRAM, data ECC, 2GB, 800MHz (PC2-6400)
[  1.0438364] dbcool0 at iic0 addr 0x2euvm_fault (0xffffffff810f0340, 0x0, 2) -> e
[  1.0438364] fatal page fault in supervisor mode
[  1.0438364] trap type 6 code 0x2 rip 0xffffffff80adb8b5 cs 0x8 rflags 0x10202 cr2 0x8 ilevel 0x8 rsp 0xffffffff813b93e8
[  1.04383641 curlwp 0xffffffff81034440 pid 0.0 lowest kstack 0xffffffff813b42c0
kernel: page fault trap, code=0
Stopped in pid 0.0 (system) at	netbsd:_atomic_inc_32_nv+0x5:	lock xaddl	%eax,0(%rdi)

db{0}> bt
atomic_inc32_nv() at netbsd:_atomic inc 32 nv+0x5
config_attach_internal() at netbsd:config_attach_internal+0x214
config _attach() at netbsd:config_attach+0x83
iic search() at netbsd:iic_search+0x1d3
mapply() at netbsd:mapply+0x2f
config_search_internal() at netbsd:config_search_internal+0xd7
config_search() at netbsd:config_search+0x7d
iic_attach() at netbsd:iic_attach+0x41a
config_attach_internal() at netbsd:config_attach_internal+0x214
config_found() at netbsd:config_found+0x112
piixpm_rescan() at netbsd:piixpm_rescan+0xbe
piixpm_attach() at netbsd:piixpm_attach+0x1da
config_attach_internal() at netbsd:config_attach_internal+0x214
config_found () at netbsd:config_found+0x112
pci_probe_device1() at netbsd:pci_probe_devicel+0x66f
pci_enumerate_bus1() at netbsd:pci_enumerate_bus1+0x1c8
pciattach() at netbsd:pciattach+0x18e
config_attach_internal() at netbsd:config_attach_internal+0x214
config_found() at netbsd:config_found+0x112
mp_pci_scan() at netbsd:mp_pci_scan+0xd6
amd64_mainbus_attach() at netbsd:and64 mainbus_attach+0x173
config_attach_internal() at netbsd:config_attach_internal+0x214
config_rootfound() at netbsd:config_rootfound+0x6e
cpu_configure() at netbsd:cpu_configure+0x2b
main() at netbsd:main+0x2f1

A (custom) -8 kernel attaches dbcool on that hardware:

piixpm0 at pci0 dev 2 function 0: vendor 1166 product 0205 (rev. 0x00)
piixpm0: polling
iic0 at piixpm0 port 0: I2C bus
spdmem0 at iic0 addr 0x50
spdmem0: DDR2 SDRAM, data ECC, 2GB, 800MHz (PC2-6400)
spdmem0: 14 rows, 10 cols, 2 ranks, 8 banks/chip, 2.50ns cycle time
spdmem0: tAA-tRCD-tRP-tRAS: 6-6-6-18
spdmem0: voltage SSTL 1.8V, refresh time 7.8us (self-refreshing)
spdmem1 at iic0 addr 0x51
spdmem1: DDR2 SDRAM, data ECC, 2GB, 800MHz (PC2-6400)
spdmem1: 14 rows, 10 cols, 2 ranks, 8 banks/chip, 2.50ns cycle time
spdmem1: tAA-tRCD-tRP-tRAS: 6-6-6-18
spdmem1: voltage SSTL 1.8V, refresh time 7.8us (self-refreshing)
dbcool0 at iic0 addr 0x2e
dbcool0: ADT7476 dBCool(tm) Controller (rev 0x0069)

The -10 kernel has been built from a netbsd-10 checkout (with local patches) 
last updated Nov 13.