Diving into advanced peripheral IRQ management
In Chapter 3, Dealing with Kernel Core Helpers, we introduced peripheral IRQs, using request_irq() and request_threaded_irq(). With the former, you register a handler (top half) that will be executed in an atomic context, from which you can schedule a bottom half using one of the mechanisms discussed in that same chapter. On the other hand, with the _threaded variant, you can provide top and bottom halves to the function, so that the former will be run as the hard IRQ handler, which may decide to raise the second and threaded handler or not, which will be run in a kernel thread.
The problem with those approaches is that sometimes, drivers requesting an IRQ do not know about the nature of the interrupt controller that provides this IRQ line, especially when the interrupt controller is a discrete chip (typically a GPIO expander connected over SPI or I2C buses). Now comes the request_any_context_irq()function with which drivers requesting...