Christopher Hallinan - Embedded Linux Primer - A Practical, Real-World Approach

One of the fundamental purposes of a device driver is to isolate the user's programs from ready access to critical kernel data structures and hardware devices. Furthermore, a well-written device driver hides the complexity and variability of the hardware device from the user. For example, a program that wants to write data to the hard disk need not care if the disk drive uses 512-byte or 1024-byte sectors. The user simply opens a file and issues a write command. The device driver handles the details and isolates the user from the complexities and perils of hardware device programming. The device driver provides a consistent user interface to a large variety of hardware devices. It provides the basis for the familiar UNIX/Linux convention that everything must be represented as a file.

Unlike some other operating systems, Linux has the capability to add and remove kernel components at runtime. Linux is structured as a monolithic kernel with a well-defined interface for adding and removing device driver modules dynamically after boot time. This feature not only adds flexibility to the user, but it has proven invaluable to the device driver development effort. Assuming that your device driver is reasonably well behaved, you can insert and remove the device driver from a running kernel at will during the development cycle instead of rebooting the kernel every time a change occurs.

Loadable modules have particular importance to embedded systems. Loadable modules enhance field upgrade capabilities; the module itself can be updated in a live system without the need for a reboot. Modules can be stored on media other than the root (boot) device, which can be space constrained.

Of course, device drivers can also be statically compiled into the kernel, and, for many drivers, this is completely appropriate. Consider, for example, a kernel configured to mount a root file system from a network-attached NFS server. In this scenario, you configure the network-related drivers (TCP/IP and the network interface card driver) to be compiled into the main kernel image so they are available during boot for mounting the remote root file system. You can use the initial ramdisk functionality as described in Chapter 6, "System Initialization," as an alternative to having these drivers compiled statically as part of the kernel proper. In this case, the necessary modules and a script to load them would be included in the initial ramdisk image.

Loadable modules are installed after the kernel has booted. Startup scripts can load device driver modules, and modules can also be "demand loaded" when needed. The kernel has the capability to request a module when a service is requested that requires a particular module.

Terminology has never been standardized when discussing kernel modules. Many terms have been and continue to be used interchangeably when discussing loadable kernel modules. Throughout this and later chapters, the terms device driver, loadable kernel module (LKM), loadable module, and module are all used to describe a loadable kernel device driver module.

The basic Linux device driver model is familiar to UNIX/Linux system developers. Although the device driver model continues to evolve, some fundamental constructs have remained nearly constant over the course of UNIX/Linux evolution. Device drivers are broadly classified into two basic categories: character devices and block devices . Character devices can be thought of as serial streams of sequential data. Examples of character devices include serial ports and keyboards. Block devices are characterized by the capability to read and write blocks of data to and from random locations on an addressable medium. Examples of block devices include hard drives and floppy disk drives.

Because Linux supports loadable device drivers, it is relatively easy to demonstrate a simple device driver skeleton. Listing 8-1 illustrates a loadable device driver module that contains the bare minimum structure to be loaded and unloaded by a running kernel.

Listing 8-1. Minimal Device Driver

/* Example Minimal Character Device Driver */

#include

static int __init hello_init(void) {

printk("Hello Example Init\n");

return 0;

}

static void __exit hello_exit(void) {

printk("Hello Example Exit\n");

}

module_init(hello_init);

module_exit(hello_exit);

MODULE_AUTHOR("Chris Hallinan");

MODULE_DESCRIPTION("Hello World Example");

MODULE_LICENSE("GPL");

The skeletal driver in Listing 8-1 contains enough structure for the kernel to load and unload the driver, and to invoke the initialization and exit routines. Let's look at how this is done because it illustrates some important high-level concepts that are useful for device driver development.

A device driver is a special kind of binary module. Unlike a stand-alone binary executable application, a device driver cannot be simply executed from a command prompt. The 2.6 kernel series requires that the binary be in a special "kernel object" format. When properly built, the device driver binary module contains a .ko suffix. The build steps and compiler options required to create the .ko module object can be quite complex. Here we outline a set of steps to harness the power of the Linux kernel build system without requiring you to become an expert in it, which is beyond the scope of this book.

A device driver must be compiled against the kernel on which it will execute. Although it is possible to load and execute kernel modules built against a different kernel version, it is risky to do so unless you are certain that the module does not rely on any features of your new kernel. The easiest way to do this is to build the module within the kernel's own source tree. This ensures that as the developer changes the kernel configuration, his custom driver is automatically rebuilt with the correct kernel configuration. It is certainly possible to build your drivers outside of the kernel source tree. However, in this case, you are responsible for making sure that your device driver build configuration stays in sync with the kernel you want to run your driver on. This typically includes compiler switches, location of kernel header files, and kernel configuration options.

For the example driver introduced in Listing 8-1, the following changes were made to the stock Linux kernel source tree to enable building this example driver. We explain each step in detail.

1.Starting from the top-level Linux source directory, create a directory under .../drivers/char called examples.

2.Add a menu item to the kernel configuration to enable building examples and to specify built-in or loadable kernel module.

3.Add the new examples subdirectory to the .../drivers/char/Makefile conditional on the menu item created in step 2.

4.Create a makefile for the new examples directory, and add the hello1.o module object to be compiled conditional on the menu item created in step 2.

5.Finally, create the driver hello1.c source file from Listing 8.1.

Adding the examples directory under the .../drivers/char subdirectory is self-explanatory. After this directory is created, two files are created in this directory: the module source file itself from Listing 8-1 and the makefile for the examples directory. The makefile for examples is quite trivial. It will contain this single line: