Our first Linux driver

Our first Linux driver

Before we write our first driver, let’s go over some concepts. A driver never runs by itself. It is similar to a library that is loaded for its functions to be invoked by a running application. It is written in C, but lacks a main() function. Moreover, it will be loaded/linked with the kernel, so it needs to be compiled in a similar way to the kernel, and the header files you can use are only those from the kernel sources, not from the standard /usr/include.
One interesting fact about the kernel is that it is an object-oriented implementation in C, as we will observe even with our first driver. Any Linux driver has a constructor and a destructor. The module’s constructor is called when the module is successfully loaded into the kernel, and the destructor when rmmod succeeds in unloading the module. These two are like normal functions in the driver, except that they are specified as the init and exit functions, respectively, by the macros module_init() and module_exit(), which are defined in the kernel header module.h.

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/* ofd.c – Our First Driver code */ #include <linux/module.h> #include <linux/version.h> #include <linux/kernel.h> static int __init ofd_init(void) /* Constructor */ {     printk(KERN_INFO "Namaskar: ofd registered");     return 0; } static void __exit ofd_exit(void) /* Destructor */ {     printk(KERN_INFO "Alvida: ofd unregistered"); } module_init(ofd_init); module_exit(ofd_exit); MODULE_LICENSE("GPL"); MODULE_AUTHOR("Anil Kumar Pugalia <email_at_sarika-pugs_dot_com>"); MODULE_DESCRIPTION("Our First Driver");

Given above is the complete code for our first driver; let’s call it ofd.c. Note that there is no stdio.h (a user-space header); instead, we use the analogous kernel.h (a kernel space header). printk() is the equivalent of printf(). Additionally, version.h is included for the module version to be compatible with the kernel into which it is going to be loaded. The MODULE_* macros populate module-related information, which acts like the module’s “signature”.

Building our first Linux driver

Once we have the C code, it is time to compile it and create the module file ofd.ko. We use the kernel build system to do this. The following Makefile invokes the kernel’s build system from the kernel source, and the kernel’s Makefile will, in turn, invoke our first driver’s Makefile to build our first driver.
To build a Linux driver, you need to have the kernel source (or, at least, the kernel headers) installed on your system. The kernel source is assumed to be installed at /usr/src/linux. If it’s at any other location on your system, specify the location in the KERNEL_SOURCE variable in this Makefile.

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# Makefile – makefile of our first driver # if KERNELRELEASE is defined, we've been invoked from the # kernel build system and can use its language. ifneq (${KERNELRELEASE},)     obj-m := ofd.o # Otherwise we were called directly from the command line. # Invoke the kernel build system. else     KERNEL_SOURCE := /usr/src/linux     PWD := $(shell pwd) default:     ${MAKE} -C ${KERNEL_SOURCE} SUBDIRS=${PWD} modules clean:     ${MAKE} -C ${KERNEL_SOURCE} SUBDIRS=${PWD} clean endif

With the C code (ofd.c) and Makefile ready, all we need to do is invoke make to build our first driver (ofd.ko).

$ make make -C /usr/src/linux SUBDIRS=... modules make[1]: Entering directory `/usr/src/linux'   CC [M]  .../ofd.o   Building modules, stage 2.   MODPOST 1 modules   CC      .../ofd.mod.o   LD [M]  .../ofd.ko make[1]: Leaving directory `/usr/src/linux'

Summing up

Once we have the ofd.ko file, perform the usual steps as the root user, or with sudo.

# su # insmod ofd.ko # lsmod | head -10

lsmod should show you the ofd driver loaded.
While the students were trying their first module, the bell rang, marking the end of the session. Professor Gopi concluded, “Currently, you may not be able to observe anything other than the lsmod listing showing the driver has loaded. Where’s the printk output gone? Find that out for yourselves, in the lab session, and update me with your findings. Also note that our first driver is a template for any driver you would write in Linux. Writing a specialised driver is just a matter of what gets filled into its constructor and destructor. So, our further learning will be to enhance this driver to achieve specific driver functionalities.”