M option in make command, Makefile

M is not an option for make. Note it lacks the hyphen. M is a variable assigned to the execution of make. If make executes a Makefile script, this script can read the variable M and use its contents.

In the example you provide, make will read Makefile in ~/kernel-2.6 and assign your present working directory to variable M. Typically, this will allow for make to return to your current directory after processing Makefile.

I had similar quiz with

make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

Here the make is called within my project's directory. -C is make option:

-C dir, --directory=dir Change to directory dir before reading the makefiles or doing anything else. If multiple -C options are specified, each is interpreted relative to the previous one: -C / -C etc is equivalent to -C /etc. This is typically used with recursive invocations of make.

M is not make option but argument passed to it. Since -C changes make directory we know that make will read make file in that directory. By inspection of make file in that directory I have discovered what it is going then to do with M:

From make file (named Makefile) in the directory pointed to by -C (btw it is kernel build 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 takes precedenceifdef SUBDIRS  KBUILD_EXTMOD ?= $(SUBDIRS)endif

Explanation from Linux Device Drivers, 3rd Edition, Jonathan Corbet et al.:

This command starts by changing its directory to the one provided with the -C option (that is, your kernel source directory). There it finds the kernel's top-level makefile. The M= option causes that makefile to move back into your module source directory before trying to build the modules target.

Read the manual for make:

-C dir, --directory=dir            Change to directory dir before reading the makefiles or doing anything else.

Your invocation changes the directory to ~/kernel and effectively calls make there, i.e. reading the Makefile from that directory. With the M variable, the makefile knows where your actual project files are and can change back to that location.

The point is that you don't write your own makefile, but use a single, once-and-for-all version.