PM / genirq: Document rules related to system suspend and interrupts

System Suspend and Device Interrupts

Copyright (C) 2014 Intel Corp.

Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Suspending and Resuming Device IRQs

-----------------------------------

Device interrupt request lines (IRQs) are generally disabled during system

suspend after the "late" phase of suspending devices (that is, after all of the

->prepare, ->suspend and ->suspend_late callbacks have been executed for all

devices).  That is done by suspend_device_irqs().

The rationale for doing so is that after the "late" phase of device suspend

there is no legitimate reason why any interrupts from suspended devices should

trigger and if any devices have not been suspended properly yet, it is better to

block interrupts from them anyway.  Also, in the past we had problems with

interrupt handlers for shared IRQs that device drivers implementing them were

not prepared for interrupts triggering after their devices had been suspended.

In some cases they would attempt to access, for example, memory address spaces

of suspended devices and cause unpredictable behavior to ensue as a result.

Unfortunately, such problems are very difficult to debug and the introduction

of suspend_device_irqs(), along with the "noirq" phase of device suspend and

resume, was the only practical way to mitigate them.

Device IRQs are re-enabled during system resume, right before the "early" phase

of resuming devices (that is, before starting to execute ->resume_early

callbacks for devices).  The function doing that is resume_device_irqs().

The IRQF_NO_SUSPEND Flag

------------------------

There are interrupts that can legitimately trigger during the entire system

suspend-resume cycle, including the "noirq" phases of suspending and resuming

devices as well as during the time when nonboot CPUs are taken offline and

brought back online.  That applies to timer interrupts in the first place,

but also to IPIs and to some other special-purpose interrupts.

The IRQF_NO_SUSPEND flag is used to indicate that to the IRQ subsystem when

requesting a special-purpose interrupt.  It causes suspend_device_irqs() to

leave the corresponding IRQ enabled so as to allow the interrupt to work all

the time as expected.

Note that the IRQF_NO_SUSPEND flag affects the entire IRQ and not just one

user of it.  Thus, if the IRQ is shared, all of the interrupt handlers installed

for it will be executed as usual after suspend_device_irqs(), even if the

IRQF_NO_SUSPEND flag was not passed to request_irq() (or equivalent) by some of

the IRQ's users.  For this reason, using IRQF_NO_SUSPEND and IRQF_SHARED at the

same time should be avoided.

System Wakeup Interrupts, enable_irq_wake() and disable_irq_wake()

------------------------------------------------------------------

System wakeup interrupts generally need to be configured to wake up the system

from sleep states, especially if they are used for different purposes (e.g. as

I/O interrupts) in the working state.

That may involve turning on a special signal handling logic within the platform

(such as an SoC) so that signals from a given line are routed in a different way

during system sleep so as to trigger a system wakeup when needed.  For example,

the platform may include a dedicated interrupt controller used specifically for

handling system wakeup events.  Then, if a given interrupt line is supposed to

wake up the system from sleep sates, the corresponding input of that interrupt

controller needs to be enabled to receive signals from the line in question.

After wakeup, it generally is better to disable that input to prevent the

dedicated controller from triggering interrupts unnecessarily.

The IRQ subsystem provides two helper functions to be used by device drivers for

those purposes.  Namely, enable_irq_wake() turns on the platform's logic for

handling the given IRQ as a system wakeup interrupt line and disable_irq_wake()

turns that logic off.

Calling enable_irq_wake() causes suspend_device_irqs() to treat the given IRQ

in a special way.  Namely, the IRQ remains enabled, by on the first interrupt

it will be disabled, marked as pending and "suspended" so that it will be

re-enabled by resume_device_irqs() during the subsequent system resume.  Also

the PM core is notified about the event which casues the system suspend in

progress to be aborted (that doesn't have to happen immediately, but at one

of the points where the suspend thread looks for pending wakeup events).

This way every interrupt from a wakeup interrupt source will either cause the

system suspend currently in progress to be aborted or wake up the system if

already suspended.  However, after suspend_device_irqs() interrupt handlers are

not executed for system wakeup IRQs.  They are only executed for IRQF_NO_SUSPEND

IRQs at that time, but those IRQs should not be configured for system wakeup

using enable_irq_wake().

Interrupts and Suspend-to-Idle

------------------------------

Suspend-to-idle (also known as the "freeze" sleep state) is a relatively new

system sleep state that works by idling all of the processors and waiting for

interrupts right after the "noirq" phase of suspending devices.

Of course, this means that all of the interrupts with the IRQF_NO_SUSPEND flag

set will bring CPUs out of idle while in that state, but they will not cause the

IRQ subsystem to trigger a system wakeup.

System wakeup interrupts, in turn, will trigger wakeup from suspend-to-idle in

analogy with what they do in the full system suspend case.  The only difference

is that the wakeup from suspend-to-idle is signaled using the usual working

state interrupt delivery mechanisms and doesn't require the platform to use

any special interrupt handling logic for it to work.

IRQF_NO_SUSPEND and enable_irq_wake()

-------------------------------------

There are no valid reasons to use both enable_irq_wake() and the IRQF_NO_SUSPEND

flag on the same IRQ.

First of all, if the IRQ is not shared, the rules for handling IRQF_NO_SUSPEND

interrupts (interrupt handlers are invoked after suspend_device_irqs()) are

directly at odds with the rules for handling system wakeup interrupts (interrupt

handlers are not invoked after suspend_device_irqs()).

Second, both enable_irq_wake() and IRQF_NO_SUSPEND apply to entire IRQs and not

to individual interrupt handlers, so sharing an IRQ between a system wakeup

interrupt source and an IRQF_NO_SUSPEND interrupt source does not make sense.