Linux Device Driver Architecture Components

I have discussed about the module structures before. Now lets have a look on overall linux device driver architecture components. So that we can sort out, what should be the next step of my research work.

Device Drivers as modules

Linux device drivers are managed by kernel itself. Modules works here as device drivers. Modules can interact with kernels directly. When we write device drivers, we use the module structure to specify custom routines. (I have posted the basic module structure before, you can take a look on that).

To load the driver we use module_init and to unload we use module_exit. When the module_init is executed, the first routine is executed as well. We have seen this example when we wrote the elementary module on previous post. The kernel registers the driver by using a registration routine register_chrdev while loading the module and when unloading the module it uses unregister_chrdev. By the way, these are for character devices. I still don't know what actually these does, I'll find it out later.

Naming Devices

In linux, devices are labeled by numbers (0-255). These 256 numbers are known as major numbers. A major number is assigned to a particular device. So, Linux can handle 256 devices all together. Only 256 devices? No, they are major devices. Each major device can handle additional 256 devices of that type. Lets recalculate the number 256x256 = 65535. Well, now nobody can call this a small number. kernels documentation/devices.txt file is supposed to have the list of all major devices. I'll upload that file once I boot into ubuntu (Now I'm on Vista). By the way, the 0-th device is known as null device.

Applications can not access the device by the major number directly. Rather, apps use something called file system entries. I guess I didn't mention it earlier. The beauty of linux is, it can treat all physical devices, virtual devices and real file systems as file systems. The benefit of this is we can use the generic structure for all of them, without knowing what type of device we are using, still it is not that easy as it sounds right now. All driver entry points are located on a directory "/dev". For example, /dev/tty1 is the serial port 1 [COM1] . Applications which want to use any driver uses a system call to get the handle of the device. Once it gets the handle, it can gain access of the devices and can use the device by other system calls.


Driver Installation

Driver Installation is varied by linux distribution. As I have mentioned earlier. Linux uses modules as device drivers. All drivers are installed as modules and are located in directory "/lib/modules/kernel_x.x.xx". A configuration file located on /etc/modules.conf is used by the kernel [and user if you want to override it] to load and unload the modules. Module installing and uninstalling can be done by kernel module utilities like insmod, rmmod [we have used it on previous examples] and modprobe. Installing/inserting a module isn't everything, the module has to be registered as a device entry in the directory "/dev". The utility mknod does this job.

To see what drivers have been installed, we can see the contents of "/proc/module". Which will show the list of all loaded modules/devices.