Chapter 2.Writing Linux Drivers

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In this page we are going to learn how to write a very basic Linux driver.

Required

• Linux box with Kernel Source
• gcc Compiler
• gnu make

Assumptions

• You are familiar with Linux
• You are comfortable in C (gcc), makefiles
• You have some system knowledge

Lets start

If you have not installed Kernel Source already, get the kernel-source-<kernel-version>.rpm from the installation CD.

Or you can download kernel source from www.kernel.org

If you want to know about kernel compilation click here.

Preparing the Makefile

You need to prepare a makefile to compile the modules. Here is the standard Makefile.

#Makefile

KERNEL_DIR=linux-2.4.20
CFLAGS += -I/usr/src/${KERNEL_DIR}/include
CFLAGS += -DMODULE -D__KERNEL__
CFLAGS += -Wall
CFLAGS +=-I/usr/src/${KERNEL_DIR}/drivers/scsi


CFLAGS += -DMODVERSIONS -include /usr/src/${KERNEL_DIR}/include/linux/modversions.h -I.

#CFLAGS += -save-temps

all: module.o

module.o: mod1.o mod2.o
   $(LD) -r $^ -o $@

mod1.o: mod1.c mod1.h

mod2.o: mod2.c mod2.h

clean:
   rm -f module.o mod1.o mod2.o

This makefile builds the module module.o from the sub modules mod1.o and mod2.o.

You can change KERNEL_DIR variable according to your kernel version. For example if your kernel version is 2.4.20-2 then set KERNEL_DIR variable as

KERNEL_DIR=2.4.20-22

Our first module

As everybody starts, we will also start with "Hello World". Our module will say Hello World from Kernel Space.

Makefile hello.mak

#Makefile

KERNEL_DIR=linux-2.4.20
CFLAGS += -I/usr/src/${KERNEL_DIR}/include
CFLAGS += -DMODULE -D__KERNEL__
CFLAGS += -Wall

all: hello.o

clean:
   rm -f hello.o

Explanation

• KERNEL_DIR is a variable name set to the kernel source directory for which we are writing the module.
• CFLAGS is a Makefile variable, we can use it to pass the arguements to GCC.
• MODULE symbol is defined to indicate the hello.o is a module ( dynamically loadable module )
• __KERNEL__ symbol is defined so that kernel specific content will be included.
• -Wall - Enables all kind of warnings.

hello.c

#include <linux/module.h>
#include <linux/kernel.h>

MODULE_LICENSE("GPL");

int init_module(void)
{
   printk("<1>Hello World\n");
   return 0;
}

void cleanup_module()
{
   printk("<1>Bye\n");
}

Explanation

• module.h is included so that our module will work as a loadable module.
• kernel.h is included here for printk prototype.
• MODULE_LICENSE macro defined for avoiding the "module will taint the kernel" warning message.

init_module is the entry point for the module, ie it will be called when the module is linked with kernel by default. init_module takes no arguement and returns int. If the return value is non-zero,it is taken as error and the module is not inserted/linked. Typically this function will do the detection/initialization of hardware, memory allocation etc.

cleanup_module is the exit point for the module, it will be called when the module is to be unloaded. It returns nothing and takes no arguement. Typically this function can do the tasks such as  freeing the allocated memory, completing the I/O requests etc.

printk is similar to printf in user space. The only difference between printk and printf is printk does not support floating point format. <1> inside the arguement is the priority of the arguement. <0> is the highest priority and <7> is lowest priority. Depending upon the syslog configuration, the messages will be displayed on the console. You can get these messages from the file /var/log/messages also.

Download these two files ( hello.c and hello.mak ) and then run

   $make -f hello.mak

to build hello.o

If compilation is not successful, please click here.

You must be a superuser to run the following command

    $insmod ./hello.o

which installs (ie links the module into kernel) the hello.o module.

If the insmod is success, you should be able to see the message Hello World in the console. If you are not able to see the message, run 'dmesg' to see the driver messages. Even if you are not able to see this message in dmesg output, the module either is not inserted properly or the entry points are not right one. Check init_module method.

If insmod is failed, please click here.

hello.c can be written in more perfect way like this so this module can be statically linked with the kernel. We will see later about static linking of modules into kernel.

hello1.c

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>

MODULE_LICENSE("GPL");

int hello_init(void)
{
   printk("<1>Hello World\n");
   return 0;
}

void hello_exit()
{
   printk("<1>Bye\n");
}

module_init(hello_init);
module_exit(hello_exit);

Explanation

• init.h is included so that module_init and module_exit macros will work.
• These macros can be used when the module will be used as loadable module and static linking with kernel.
• Also module_init & module_exit macros provide the flexibility to have our own entry and exit points to our module.

Removing the module

Use  rmmod module-name (without .o) to remove the loaded module. In this case

    $rmmod hello

will unload the hello module and Bye will be displayed on the console.

If you face problems in unloading the module, please click here.

Listing the modules

Use  lsmod to display the currently loaded modules.  "Used by" column gives the list of processes using this driver.

Compilation Problems

• Check the Kernel source directory as per KERNEL_DIR variable in the Makefile.
• Check for the definition of __KERNEL__ symbol. Without this symbol, module will not compile.

Prolems while loading

• If you get unresolved symbol error for kernel API's, make sure you have included all needed header files.
• Make sure your function is defined and linked in the final object file.
• If you get the "Operation not permitted" error, login as super user (root)

Prolems while Unloading

• Dont give the module name with .o extension, it should be rmmod module-name
• If you get the "Operation not permitted" error, login as super user (root)
• If you "Resource or device busy", make sure no other programs are using that module and make sure that "Used by" column in lsmod output is zero.

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