Linux Device Drivers

The Header File blk.h

Since (for the most part) block drivers are device-independent, the kernel developers tried to simplify driver code by concentrating most of the common code in one header, <linux/blk.h>. Therefore, every block driver should include that header. The most important function defined in <linux/blk.h> is end_request, which is declared as static. Having it be static allows different drivers to have a correctly defined end_request, without each needing to write its own implementation.

In Linux 1.2, the header should be included as <linux/../../ drivers/block/blk.h>. The reason for this is that custom block drivers were not supported until later, and the header was originally local to the drivers/block source directory.

Actually, the blk.h header is quite unusual, as it defines several symbols based on the symbol MAJOR_NR, which must be declared by the driver before it includes the header. Once again, this shows that blk.h was not designed with custom drivers in mind.

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 in advance and cannot correctly define MAJOR_NR. If MAJOR_NR is undefined, blk.h can’t set up some of the macros used in end_request. Therefore, in order for a custom driver to benefit from the general-purpose end_request function and to ...