calling a static function from another module in the kernel?

[YONETANI Tomokazu <qhwt+dfly@xxxxxxxxxx>]

Hello.
While working on making acpica5 compile into the kernel, I've hit a
strange problem.  isab_attach() is defined as a static function
in /sys/bus/pci/pci_isab.c, and somehow declared in /sys/bus/isa/isavar.h .
However acpi_isab_attach() calls this function, so IIUC isab_attach()
should not be static.  In fact the kernel won't get linked without making
isab_attach() non-static.  What I don't understand is, that acpi driver as
a module doesn't complain about this when it's loaded.  Is this supposed
to work?