I have 2 kernels: and I have an acx111 wireless pci card, which works fine with the default kernel. After installing my real-time kernel, I tried to compile the acx module that my card needs with the real-time kernel, to no avail:

make -C /usr/src/linux- M=/lib/modules/
make: Entering directory `/usr/src/linux-'
make: *** No targets specified and no makefile found. Stop.
make: Leaving directory `/usr/src/linux-'

I believe it's because i'm missing compiling stuff under the real-time kernel:
ls /usr/src/linux-
Module.symvers makedumpfile.config

For comparison purposes, here's what I have under the default kernel:
ls /usr/src/linux-obj/x86_64/default/
.config Module.symvers include makedumpfile.config scripts
Makefile arch include2 modules.alias