Linux Device Drivers, Second Edition

The Header File blk.h

All block drivers should include the header file <linux/blk.h>. This file defines much of the common code that is used in block drivers, and it provides functions for dealing with the I/O request queue.

Actually, the blk.h header is quite unusual, because it defines several symbols based on the symbol MAJOR_NR, which must be declared by the driver before it includes the header. This convention was developed in the early days of Linux, when all block devices had preassigned major numbers and modular block drivers were not supported.

If you look at blk.h, you’ll see that several device-dependent symbols are declared according to the value of MAJOR_NR, which is expected to be known in advance. However, if the major number is dynamically assigned, the driver has no way to know its assigned number at compile time and cannot correctly define MAJOR_NR. If MAJOR_NR is undefined, blk.h can’t set up some of the macros used with the request queue. Fortunately, MAJOR_NR can be defined as an integer variable and all will work fine for add-on block drivers.

blk.h makes use of some other predefined, driver-specific symbols as well. The following list describes the symbols in <linux/blk.h> that must be defined in advance; at the end of the list, the code used in sbull is shown.

MAJOR_NR

This symbol is used to access a few arrays, in particular blk_dev and blksize_size. A custom driver like sbull, which is unable to assign a constant value to the symbol, should ...