Makefile: installing external Linux kernel module

Question

I am writing a simple Makefile for an external kernel module.

Building it with:

obj-m += usbtherm.o

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

compiles only the external module, which is nice.

But installing it with:

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

installs all modules in the kernel source tree, and I don't know how I can have the module installed to drivers/usb/misc.

So I am installing the module like this:

install:
    cp $(shell pwd)/usbtherm.ko /lib/modules/$(shell uname -r)/kernel/drivers/usb/misc
    depmod -a

which will install only the external module.

But it doesn't look too elegant to me - what am I missing?

Answer 1

When I installed with sudo

sudo -E make install

Installed just my external module and reran depmod.

Running with out the environment preservation flags resulted in reinstalling all of the modules built in the kernel source tree I used to update my kernel. Seems like using sudo may not have PWD set correctly by the time make changes into the kernel tree possibly, but maybe that's will help someone else that runs into this.

Answer 2

I had another, closer look at modules_install. In the Linux Makefile:

# Target to install modules
PHONY += modules_install
modules_install: _modinst_ _modinst_post

PHONY += _modinst_
_modinst_:
        @rm -rf $(MODLIB)/kernel
        @rm -f $(MODLIB)/source
        @mkdir -p $(MODLIB)/kernel
        @ln -s `cd $(srctree) && /bin/pwd` $(MODLIB)/source
        @if [ ! $(objtree) -ef  $(MODLIB)/build ]; then \
                rm -f $(MODLIB)/build ; \
                ln -s $(CURDIR) $(MODLIB)/build ; \
        fi
        @cp -f $(objtree)/modules.order $(MODLIB)/
        @cp -f $(objtree)/modules.builtin $(MODLIB)/
        $(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modinst

What I understand from it is that it expects a source tree, uninstalls existing modules, does some cleanup and then runs scripts/Makefile.modinst to install the modules - which installs external modules to extra:

# Modules built outside the kernel source tree go into extra by default
INSTALL_MOD_DIR ?= extra

And actually, when building an external module on a system with no kernel sources but the headers installed, the module is built in /usr/src/linux-headers-$(uname -r) which is symlinked from /lib/modules/$(uname -r)/build, and while the all and clean targets succeed, modules_install fails because i.e. modules.order doesn't exist if only the headers are installed.

But installing the external module by copying the *.ko and running depmod -a works fine.