Thus, in the kernel, goto is often used as shown here to deal with errors. Under Unix, the kernel executes in the highest level also called supervisor mode , where everything is allowed, whereas applications execute in the lowest level the so-called user mode , where the processor regulates direct access to hardware and unauthorized access to memory. Come along; we can always use the help. It should tell the kernel, “I’m not there anymore; don’t ask me to do anything else. Those who run odd-numbered versions of the kernel are usually skilled enough to dig in the code without the need for a textbook, which is another reason why we don’t talk about development kernels here. Const 4 9.

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One of the good features of Linux is the ability to extend at runtime the linjx of features offered by the kernel. Figure shows how function calls and function pointers are used in a module to add new functionality to a running kernel.

So, as you dig into the kernel, the following issues should be kept in mind. Thus, one modprobe command can sometimes replace several invocations of insmod although you’ll still need insmod when loading your own modules from the current directory, because modprobe looks only in the standard installed module directories.

examples / Linux Device Drivers 3rd Edition ยท GitLab

Kernel code cannot do floating point arithmetic. Module stacking is implemented in edvice mainstream kernel sources as well: For each facility, there is a specific kernel function that accomplishes this registration. All current processors have at least two protection levels, and some, like the x86 family, have more levels; when several levels exist, the highest and lowest levels are used.

With an understanding of peripheral buses in place, we can take a detailed look at the Linux device model, which is the abstraction layer used by the kernel to describe the hardware and software resources it is managing. Those programs can range from simple utilities to complete graphical applications. It is not strictly necessary, but lds3 module really should specify which license applies to its code.


If your module needs to export symbols for other modules to use, the following macros should linkx used.

The answer, of course, is that the kernel build system handles the rest. Clearly, if a module is to work with a given kernel, it must be built with the same understanding of the target processor as that kernel was.

Linux Device Drivers, 3rd Edition

Once that task is taken care of, you’ll be ready to start playing with kernel modules. The success of Linux is clearly a testament to its technical quality and to the numerous benefits of free software in general. Programmers wanting dveice keep up with kernel programming changes are encouraged to join the mailing lists and to make use of the web sites listed in the bibliography.

But for this edition, I’m mostly grateful to Jon and Greg, who have been great mates in this work; it couldn’t have existed without each and both of them, as the code base is bigger and tougher, while my time is a scarcer resource, always contended for by clients, free software issues, and expired deadlines. The modprobe utility is worth a quick mention. We see how to trace kernel errors in Chapter 4.

Linux Device Drivers, Third Edition

Four devices, each consisting of a memory area that is both global and persistent. You really do want to take care to do the right thing when an initialization error occurs.

Moreover, of course, Linux can run on symmetric multiprocessor SMP systems, with the result that your driver could be executing concurrently on more than one CPU. That means that you are free to download and redistribute it. The module loader refuses to accept more values than will fit in the array. Unfortunately, not everyone is able to run the sample code for these chapters, because some hardware support is actually needed to test the software interface interrupts.


The chances of a static number having already been assigned for the use of your new driver are small, however, and new numbers are not being assigned. Many driver authors do not need lrd3 of this material, but we encourage you to go on reading anyway. If it turns out that your module simply cannot load after a particular type of failure, you must undo any registration activities performed before the failure. The GPL allows anybody to redistribute, and even sell, a product covered by the GPL, as long as the recipient has access to the source and is able to exercise the same rights.

Equally important for those who would hack on contemporary kernels is the management lddd3 concurrency and race conditions.

Getting scull to export information via sysfs is beyond the scope of this chapter, however; we’ll return to this topic in Chapter Note that module removal fails if the kernel believes that the module is still in use e. Often a client library is provided as well, which provides capabilities that ddevice not need to be implemented as part of ld3 driver itself.

This is, on the surface, a book about writing device drivers for the Linux system. The minor number is used by the kernel to determine exactly which device is being referred to.