Latest revision as of 14:42, 26 June 2015

Linux Driver for ARMEBS3 development guide

What is a driver ? (or why do I need a new driver?)

In Linux (like most OSes) a simple application can't access hardware. If you want accessing the hardware, you must use the kernel to acess it. Userland applications generally access the hardware through a (pseudo-)file located in the /dev Drivers are not always hardware related, many drivers simply add new functionalities to the kernel.

Where to start

A good starting point for writing drivers is the Linux device drivers book download (available in printed form from O'Reilly).

Here are some informations specific to ARMEBS3 drivers.

Step-by-step hello world driver

• Create a new directory (change actual directory to it)

 mkdir /tmp/hello-module
 cd /tmp/hello-module
 /tmp/hello-module

• Create the file Makefile, containing this:

 obj-m := hello.o

• Create the file hello.c, containing this:

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

 static int hello_init(void)
 {
       printk(KERN_ALERT "Hello, world\n");
       return 0;
 }
 static void hello_exit(void)
 {
       printk(KERN_ALERT "Goodbye, cruel world\n");
 }
 module_init(hello_init);
 module_exit(hello_exit);

 MODULE_LICENSE("GPL");

• Compile it

 make -C /opt/armebs3/linux/linux-2.6.15.4-armebs3/ M={{{`pwd`}}} modules
 make: Entering directory `/opt/armebs3/linux/linux-2.6.15.4-armebs3'
 CC [M]  /tmp/hello-module/hello.o
 Building modules, stage 2.
 MODPOST
 CC      /tmp/hello-module/hello.mod.o
 LD [M]  /tmp/hello-module/hello.ko
 make: Leaving directory `/opt/armebs3/linux/linux-2.6.15.4-armebs3'

• install it (supposes NFS development environment)

 su -c "make -C /opt/armebs3/linux/linux-2.6.15.4-armebs3/ M={{{`pwd`}}} INSTALL_MOD_PATH=/opt/armebs3/nfs modules_install"
   Password:
   make: Entering directory `/opt/armebs3/linux/linux-2.6.15.4-armebs3'
     INSTALL /tmp/hello-module/hello.ko
   make: Leaving directory `/opt/armebs3/linux/linux-2.6.15.4-armebs3'

• Test it

 dmesg -c > /dev/null
 depmod -a
 modprobe hello
 Hello, world
 rmmod hello
 dmesg -c
 Goodbye, cruel world

You can download the source code here: hello.c. Here is the makefile to compile the driver: Makefile.

Including the driver into the kernel

Now we've got a new driver as a module, we're satisfied with it's functionnality, and we want to include it into the kernel. For doing this we must include it into the kernel sources.