$ (Make)-C $ (kerneldir) M = $ (PWD) Modules

In the compilation makefile of the LED driver programming of the fl2440 data, the-C indicates that the makefile is executed in the $ (kerneldir) directory, while M is not the makefile option, is the variable used in makefile under the kernel root directory.

# Use make m = dir to specify directory of external module to build
# Old syntax make... subdirs = $ PWD is still supported
# Setting the environment variable kbuild_extmod take precedence
Ifdef subdirs
Kbuild_extmod? = $ (Subdirs)
Endif
Ifdef M // If M is not defined or assigned a value, M is not defined here (undefined)
Ifeq ("$ (origin m)", "command line") // if defined, this sentence is used to determine whether M is from the command line
Kbuild_extmod: = $ (m)
Endif
Endif