Introduction to 64-Bit Driver Design

For drivers that only need support for the 32-bit kernel, existing 32-bit device drivers will continue to work without recompilation. However, most device drivers require some changes to run correctly in the 64-bit kernel, and all device drivers require recompilation to create a 64-bit driver module. The information in this appendix will help you to enable drivers for 32-bit and 64-bit environments to be generated from common source code, thus increasing code portability and reducing the maintenance effort.

Before starting to modify a device driver for the 64-bit environment, you should understand how the 32-bit environment differs from the 64-bit environment. In particular, you must be familiar with the C language data type models ILP32 and LP64. See the following table.

Table C–1 Comparison of ILP32 and LP64 Data Types

C Type	ILP32	LP64
char	8	8
short	16	16
int	32	32
long	32	64
long long	64	64
float	32	32
double	64	64
long double	96	128
pointer	32	64

The driver-specific issues due to the differences between ILP32 and LP64 are the subject of this appendix.

In addition to general code cleanup to support the data model changes for LP64, driver writers have to provide support for both 32-bit and 64-bit applications.

The ioctl(9E), devmap(9E), and mmap(9E) entry points enable data structures to be shared directly between applications and device drivers. If those data structures change size between the 32-bit and 64-bit environments, then the entry points must be modified so that the driver can determine whether the data model of the application is the same as that of the kernel. When the data models differ, data structures can be adjusted. See I/O Control Support for 64-Bit Capable Device Drivers, 32-bit and 64-bit Data Structure Macros, and Associating Kernel Memory With User Mappings.

In many drivers, only a few ioctls need this kind of handling. The other ioctls should work without change as long as these ioctls pass data structures that do not change in size.