Linux Device Drivers, Second Edition

The kiobuf Interface

As of version 2.3.12, the Linux kernel supports an I/O abstraction called the kernel I/O buffer, or kiobuf. The kiobuf interface is intended to hide much of the complexity of the virtual memory system from device drivers (and other parts of the system that do I/O). Many features are planned for kiobufs, but their primary use in the 2.4 kernel is to facilitate the mapping of user-space buffers into the kernel.

The kiobuf Structure

Any code that works with kiobufs must include <linux/iobuf.h>. This file defines struct kiobuf, which is the heart of the kiobuf interface. This structure describes an array of pages that make up an I/O operation; its fields include the following:

int nr_pages;

The number of pages in this kiobuf

int length;

The number of bytes of data in the buffer

int offset;

The offset to the first valid byte in the buffer

struct page **maplist;

An array of page structures, one for each page of data in the kiobuf

The key to the kiobuf interface is the maplist array. Functions that operate on pages stored in a kiobuf deal directly with the page structures—all of the virtual memory system overhead has been moved out of the way. This implementation allows drivers to function independent of the complexities of memory management, and in general simplifies life greatly.

Prior to use, a kiobuf must be initialized. It is rare to initialize a single kiobuf in isolation, but, if need be, this initialization can be performed with kiobuf_init ...