Chapter 11: Data Types in the Kernel
Before we go on to more advanced topics, we need to stop for a quick note on portability issues. Modern versions of the Linux kernel are highly portable, running on numerous different architectures. Given the multiplatform nature of Linux, drivers intended for serious use should be portable as well.
But a core issue with kernel code is being able both to access data items of known length (for example, filesystem data structures or registers on device boards) and to exploit the capabilities of different processors (32-bit and 64-bit architectures, and possibly 16 bit as well).
Several of the problems encountered by kernel developers while porting x86 code to new architectures have been related to incorrect data typing. Adherence to strict data typing and compiling with the -Wall -Wstrict-prototypes flags can prevent most bugs.
Data types used by kernel data are divided into three main classes: standard C types such as int, explicitly sized types such as u32, and types used for specific kernel objects, such as pid_t. We are going to see when and how each of the three typing classes should be used. The final sections of the chapter talk about some other typical problems you might run into when porting driver code from the x86 to other platforms, and introduce the generalized support for linked lists exported by recent kernel headers.
If you follow the guidelines we provide, your driver should compile and run even on platforms on which you are unable to test it.