module compiling : asm/linkage.h file not found

Question

I am trying to compile an example of "hello world" Kernel Module, problems found on ubuntu 11.04, kernel 3.2.6, gcc 4.5.2 and fedora 16, kernel 3.2.7, gcc 4.6.7.

code:

#include <linux/module.h>
#include <linux/init.h>
MODULE_LICENSE("GPL");

static int __init hello_init (void)
{
printk("Hello module init\n");
return 0;
}
static void __exit hello_exit (void)
{
printk("Hello module exit\n");
}
module_init(hello_init);
module_exit(hello_exit);

compiled with:

gcc -D__KERNEL__ -I /usr/src/linux/include/ -DMODULE -Wall -O2 -c hello.c -o hello.o

error:

In file included from /usr/src/linux/include/linux/kernel.h:13:0, from /usr/src/linux/include/linux/cache.h:4, from /usr/src/linux/include/linux/time.h:7, from /usr/src/linux/include/linux/stat.h:60, from /usr/src/linux/include/linux/module.h:10, from hello.c:1: /usr/src/linux/include/linux/linkage.h:5:25: fatal error: asm/linkage.h: file not found

then I found in /usr/src/linux/include/ there is no folder named 'asm' but 'asm-generic'; so I made a soft link 'asm' to 'asm-generic', and compiled agail:

this time the error was:

In file included from /usr/src/linux/include/linux/preempt.h:9:0, from /usr/src/linux/include/linux/spinlock.h:50, from /usr/src/linux/include/linux/seqlock.h:29, from /usr/src/linux/include/linux/time.h:8, from /usr/src/linux/include/linux/stat.h:60, from /usr/src/linux/include/linux/module.h:10, from hello.c:1: /usr/src/linux/include/linux/thread_info.h:53:29: fatal error: asm/thread_info.h: file not found

So I realized I was wrong, but why ? T_T

Answer 1

obj-m += hello.o

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

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

is a proper way to build modules see kbuild documentation

And to see difference beetween your compiler invocation you could

cat /lib/modules/$(shell uname -r)/build/Makefile

And analyze an output

Answer 2

obj-m += hello.o

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

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

Here hello.c is your kernel source file. just use make to build your hello.ko module.

Answer 3

asm should be a link to the actual architecture you're compiling for, not to asm-generic.
You can't compile a generic kernel module, that would work on a generic architecture. You have to compile it for the particular architecture you're going to use.

I don't know why the asm didn't exist. It should be created as part of the configuration process.
You might get other errors later, if configuration is incomplete in other ways.