Linux Device Drivers

Other Portability Issues

In addition to data typing, there are a few other software issues to keep in mind when writing a driver if you want it to be portable across Linux platforms:

Time intervals

When dealing with time intervals, don’t assume that there are 100 jiffies per second. Although this is currently true for Linux-x86, not every Linux platform runs at 100Hz. The assumption can be false even for the x86 if you play with the HZ value, and nobody knows what will happen in future kernels. Whenever you calculate time intervals using jiffies, scale your times using HZ. For example, to check against a timeout of half a second, compare the elapsed time against HZ/2. More generally, the number of jiffies corresponding to msec milliseconds is always msec*HZ/1000. This detail had to be fixed in many network drivers when porting them to the Alpha; some drivers originally designed for the PC included an explicit jiffy value for the timeout, but the Alpha has a different HZ value.

Page size

When playing games with memory, remember that a memory page is PAGE_SIZE bytes, not 4KB. Assuming that the page size is 4KB and hard-coding the value is a common error among PC programmers--the Alpha has pages twice as big. The relevant macros are PAGE_SIZE and PAGE_SHIFT. The latter contains the number of bits to shift an address to get its page number. The number currently is 12 or 13, for 4KB and 8KB pages. The macros are defined in <asm/page.h>.

Let’s look at a non-trivial situation. ...