Linux Device Drivers, Second Edition

License Terms

Linux is licensed with the GNU General Public License (GPL), a document devised for the GNU project by the Free Software Foundation. The GPL allows anybody to redistribute, and even sell, a product covered by the GPL, as long as the recipient is allowed to rebuild an exact copy of the binary files from source. Additionally, any software product derived from a product covered by the GPL must, if it is redistributed at all, be released under the GPL.

The main goal of such a license is to allow the growth of knowledge by permitting everybody to modify programs at will; at the same time, people selling software to the public can still do their job. Despite this simple objective, there’s a never-ending discussion about the GPL and its use. If you want to read the license, you can find it in several places in your system, including the directory /usr/src/linux, as a file called COPYING.

Third-party and custom modules are not part of the Linux kernel, and thus you’re not forced to license them under the GPL. A module uses the kernel through a well-defined interface, but is not part of it, similar to the way user programs use the kernel through system calls. Note that the exemption to GPL licensing applies only to modules that use only the published module interface. Modules that dig deeper into the kernel must adhere to the “derived work” terms of the GPL.

In brief, if your code goes in the kernel, you must use the GPL as soon as you release the code. Although personal use of your changes doesn’t force the GPL on you, if you distribute your code you must include the source code in the distribution—people acquiring your package must be allowed to rebuild the binary at will. If you write a module, on the other hand, you are allowed to distribute it in binary form. However, this is not always practical, as modules should in general be recompiled for each kernel version that they will be linked with (as explained in Chapter 2, in Section 2.2.1, and Chapter 11, in Section 11.3). New kernel releases—even minor stable releases—often break compiled modules, requiring a recompile. Linus Torvalds has stated publicly that he has no problem with this behavior, and that binary modules should be expected to work only with the kernel under which they were compiled. As a module writer, you will generally serve your users better by making source available.

As far as this book is concerned, most of the code is freely redistributable, either in source or binary form, and neither we nor O’Reilly & Associates retain any right on any derived works. All the programs are available through FTP from ftp://ftp.ora.com/pub/examples/linux/drivers/, and the exact license terms are stated in the file LICENSE in the same directory.

When sample programs include parts of the kernel code, the GPL applies: the comments accompanying source code are very clear about that. This only happens for a pair of source files that are very minor to the topic of this book.