EV3 FIQ

ARM processors have a feature called Fast Interrupts (FIQs). These work mostly like regular interrupts, except that they can actually interrupt regular hardware interrupts (IRQs). The linux kernel is not designed to handle this, so we have to be very careful when using them.

This article does a good job explaining why we need to use FIQs and has some nice pictures so you can see the difference of using a FIQ vs. a regular IRQ.

This article is by the author of the patches that our implementation (and in lms2012) is based on. The links to the patches in the article are broken, but it can be found here. A patch that is more like what was included in lms2012 is found here.