:= or += when compiling Linux kernel modules?

Question

Tutorials for compiling the kernel module for Linux, use different syntax for the Makefile.

Example 1

obj-m += rpi-pwm.o 

Example 2

obj-m := nothing.o

What are the differences, and is there a preferred way?

Answer 1

:= would set the variable obj-m to nothing.o. This implies that if obj-m was previously set then it would be replaced by nothing.o.

+= would add rpi-pwm.o to the variable obj-m. If obj-m was previously set to nothing.o, then it'd become nothing.o rpi-pwm.o.

Saying

obj-m += rpi-pwm.o

is equivalent to saying

obj-m := $(obj-m) rpi-pwm.o