Jul 21, 2014

Linux Device Drivers

A driver drives, manages, controls, directs and monitors the entity under its command. What a bus driver does with a bus, a device driver does with a computer device (any piece of hardware connected to a computer) like a mouse, keyboard, monitor, hard disk, Web-camera, clock, and more.

Further, a “pilot” could be a person or even an automatic system monitored by a person (an auto-pilot system in airliners, for example). Similarly, a specific piece of hardware could be controlled by a piece of software (a device driver), or could be controlled by another hardware device, which in turn could be managed by a software device driver. In the latter case, such a controlling device is commonly called a device controller. This, being a device itself, often also needs a driver, which is commonly referred to as a bus driver.

General examples of device controllers include hard disk controllers, display controllers, and audio controllers that in turn manage devices connected to them. More technical examples would be an IDE controller, PCI controller, USB controller, SPI controller, I2C controller, etc. Pictorially, this whole concept can be depicted as in Figure 1.
Figure 1: Device and driver interaction

                       Device controllers are typically connected to the CPU through their respectively named buses (collection of physical lines) — for example, the PCI bus, the IDE bus, etc. In today’s embedded world, we encounter more micro-controllers than CPUs; these are the CPU plus various device controllers built onto a single chip. This effective embedding of device controllers primarily reduces cost and space, making it suitable for embedded systems. In such cases, the buses are integrated into the chip itself. Does this change anything for the drivers, or more generically, on the software front?

The answer is, not much — except that the bus drivers corresponding to the embedded device controllers are now developed under the architecture-specific umbrella. 

Drivers have two parts

           Bus drivers provide hardware-specific interfaces for the corresponding hardware protocols, and are the bottom-most horizontal software layers of an operating system (OS). Over these sit the actual device drivers. These operate on the underlying devices using the horizontal layer interfaces, and hence are device-specific. However, the whole idea of writing these drivers is to provide an abstraction to the user, and so, at the other “end”, these do provide an interface (which varies from OS to OS). In short, a device driver has two parts, which are: a) device-specific, and b) OS-specific. Refer to Figure 2.
Figure 2: Linux device driver partition
The device-specific portion of a device driver remains the same across all operating systems, and is more about understanding and decoding the device data sheets than software programming. A data sheet for a device is a document with technical details of the device, including its operation, performance, programming, etc. — in short a device user manual.
Later, I shall show some examples of decoding data sheets as well. However, the OS-specific portion is the one that is tightly coupled with the OS mechanisms of user interfaces, and thus differentiates a Linux device driver from a Windows device driver and from a MacOS device driver.


In Linux, a device driver provides a “system call” interface to the user; this is the boundary line between the so-called kernel space and user-space of Linux, as shown in Figure 2. Figure 3 provides further classification.
Figure 3: Linux kernel overview
Based on the OS-specific interface of a driver, in Linux, a driver is broadly classified into three verticals:
  • Packet-oriented or the network vertical
  • Block-oriented or the storage vertical
  • Byte-oriented or the character vertical
The CPU vertical and memory vertical, taken together with the other three verticals, give the complete overview of the Linux kernel, like any textbook definition of an OS: “An OS performs 5 management functions: CPU/process, memory, network, storage, device I/O.” Though these two verticals could be classified as device drivers, where CPU and memory are the respective devices, they are treated differently, for many reasons.

These are the core functionalities of any OS, be it a micro-kernel or a monolithic kernel. More often than not, adding code in these areas is mainly a Linux porting effort, which is typically done for a new CPU or architecture. Moreover, the code in these two verticals cannot be loaded or unloaded on the fly, unlike the other three verticals. Henceforth, when we talk about Linux device drivers, we mean to talk only about the latter three verticals in Figure 3.

Let’s get a little deeper into these three verticals. The network vertical consists of two parts: a) the network protocol stack, and b)the network interface card (NIC) device drivers, or simply network device drivers, which could be for Ethernet, Wi-Fi, or any other network horizontals. Storage, again, consists of two parts: a) File-system drivers, to decode the various formats on different partitions, and b) Block device drivers for various storage (hardware) protocols, i.e., horizontals like IDE, SCSI, MTD, etc.

With this, you may wonder if that is the only set of devices for which you need drivers (or for which Linux has drivers). Hold on a moment; you certainly need drivers for the whole lot of devices that interface with the system, and Linux does have drivers for them. However, their byte-oriented cessibility puts all of them under the character vertical — this is, in reality, the majority bucket. In fact,  because of the vast number of drivers in this vertical, character drivers have been further sub-classified — so you have tty drivers, input drivers, console drivers, frame-buffer drivers, sound drivers, etc. The typical horizontals here would be RS232, PS/2, VGA, I2C, I2S, SPI, etc.

Multiple-vertical drivers

One final note on the complete picture (placement of all the drivers in the Linux driver ecosystem): the horizontals like USB, PCI, etc, span below multiple verticals. Why is that?
Simple — you already know that you can have a USB Wi-Fi dongle, a USB pen drive, and a USB-to-serial converter — all are USB, but come under three different verticals!
In Linux, bus drivers or the horizontals, are often split into two parts, or even two drivers: a) device controller-specific, and b) an abstraction layer over that for the verticals to interface, commonly called cores. A classic example would be the USB controller drivers ohci, ehci, etc., and the USB abstraction, usbcore.

Summing up

So, to conclude, a device driver is a piece of software that drives a device, though there are so many classifications. In case it drives only another piece of software, we call it just a driver. Examples are file-system drivers, usbcore, etc. Hence, all device drivers are drivers, but all drivers are not device drivers.

Writing Your First Linux Driver in the Classroom

Dynamically loading drivers

These dynamically loadable drivers are more commonly called modules and built into individual files with a .ko (kernel object) extension. Every Linux system has a standard place under the root of the file system (/) for all the pre-built modules. They are organised similar to the kernel source tree structure, under /lib/modules/<kernel_version>/kernel, where <kernel_version> would be the output of the command uname -ron the system, as shown in Figure 1.

Figure 1: Linux pre-built modules
To dynamically load or unload a driver, use these commands, which reside in the /sbin directory, and must be executed with root privileges:
  • lsmod — lists currently loaded modules
  • insmod <module_file> — inserts/loads the specified module file
  • modprobe <module> — inserts/loads the module, along with any dependencies
  • rmmod <module> — removes/unloads the module
Let’s look at the FAT filesystem-related drivers as an example. Figure 2 demonstrates this complete process of experimentation. The module files would be fat.kovfat.ko, etc., in the fat (vfat for older kernels) directory under /lib/modules/`uname -r`/kernel/fs. If they are in compressed .gz format, you need to uncompress them with gunzip, before you can insmodthem.

Figure 2: Linux module operations
The vfat module depends on the fat module, so fat.ko needs to be loaded first. To automatically perform decompression and dependency loading, use modprobe instead. Note that you shouldn’t specify the .ko extension to the module’s name, when using the modprobe command. rmmod is used to unload the modules.

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.

/* 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: RTST registered");
    return 0;
static void __exit ofd_exit(void/* Destructor */
    printk(KERN_INFO "Sorry: RTST unregistered");
MODULE_AUTHOR("Vinod Kumar  <email_at__dot_com>");
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.

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

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