device_probe_and_attach() fails in kernel built w/ clang

Hi everyone,

I have a driver for PCI devices that have onboard I2C buses, so my
driver needs to create an iicbus child for the PCI device.  So in my
driver I use DRIVER_MODULE(iicbus, <my_pci_driver>, iicbus_driver,
iicbus_devclass, NULL, NULL).  This works because iicbus_driver and
iicbus_devclass are declared extern in iicbus.h.

Then during device attachment I create the iicbus child using:

device_add_child(parent, "iicbus", -1);
....
device_probe_and_attach(iicbus_dev);

This all works fine for a kernel built w/ gcc.  But if I build the
same kernel w/ clang, the device_probe_and_attach() call returns
ENXIO.  I was able to debug this far enough to determine that
device_probe_child() in subr_bus.c wasn't finding any matching
drivers, so first_matching_driver() returned NULL and the matching
loops fell through.

Interestingly enough, I have another driver for a similar class of
devices that also uses iicbus, and if I load that driver after loading
the first one, the *first* driver will be re-probed, and this time it
will successfully create its iicbus devices and attach to the PCI
devices.  The *second* driver, though, will still fail to create its
iicbuses.

I'm not sure if this is some weird problem w/ the iicbus driver, with
the kernel's probing routines, or with the module data structures and
init calls generated by DRIVER_MODULE().  Whatever it is seems to be
clang-related; removing optimization and march= from CFLAGS had no
effect.  I'm using r224899/amd64.

Any help would be really appreciated.

Cheers,
Jason
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