Why do we need to disable interrupt before WFI in ARM Linux cpu_idle

Question

The Linux kernel for ARM basically does CPU_idle in a loop:

while (1) {
    disalbe_irq
    wfi
    enable_irq
}

I can understand that this logic works because "wfi" wakes up ARM regardless of IRQ/FIQ status. However, why "wfi" has to be bracketed by disable_irq and eanble_irq in the first place?

The source code /arch/arm/process.c has the following commends:

* We need to disable interrupts here
* to ensure we don't miss a wakeup call.

But I can't make sense of it. Can someone enlighten me in which scenario we would miss a wakeup call ?

Answer 1

The whole 'going to sleep' sequence in the main loop is split into two steps:

1. Realize that you don't have work to do;
2. Try to sleep (i.e. WFI)

The WFI instruction will act as a NOP if there are some interrupt flags still set, which allows the main loop to go back running the required tasks. So far so good.

There's a problem though if an interrupt occurs right after step 1 and before step 2. If that happens then the interrupt flags will be cleared upon exiting the ISR, and when control goes back to the main loop it will hit the WFI instruction with all interrupt flags cleared, causing the CPU to go into sleep before the main loop has had a chance to execute whatever tasks were required by the ISR.