Years ago, when working on a temperature sensor device driver, I had an amusing experience (though it wasn't quite so amusing at the time). Attempting to express a temperature value in millidegrees Celsius as a "regular" temperature value in degrees Celsius, I did something like the following:
double temp;
[... processing ...]
temp = temp / 1000.0;
printk(KERN_INFO "temperature is %.3f degrees C\n", temp);
It all went bad from there!
The venerable LDD (Linux Device Drivers, by Corbet, Rubini, and G-K-Hartman) book pointed out my error – floating-point (FP) arithmetic is not allowed in kernel space! It's a conscious design decision – saving processor (FP) state, turning on the FP unit, working on and then turning off and restoring the FP state is just not considered a worthwhile thing to do while in the kernel. The kernel (or driver) developer is well advised to just not attempt performing FP work...
