userspace code test: bit ops, calculate max continous 1
kernel code reading: sysfs create dir
kernel code reading: irq on power

Each kernel irq will have a irq_desc.

First the irq_desc->handle will be invoked, and in this irq_desc->handle the
irqaction->handle is called. By doing so, it creates a chain.

Two questions:
1. who and how locate the correct irq_desc?
2. will one irq_desc associate with several kernel irq number?
kernel code reading: do_irq on power
kernel code test: code clean
kernel code test: rbtree example
kernel code test: rb_tree make it more visible