138 lines
7.1 KiB
ReStructuredText
138 lines
7.1 KiB
ReStructuredText
====================================
|
|
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 as
|
|
expected during the suspend-resume cycle, but does not guarantee that the
|
|
interrupt will wake the system from a suspended state -- for such cases it is
|
|
necessary to use enable_irq_wake().
|
|
|
|
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 causes 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 very few valid reasons to use both enable_irq_wake() and the
|
|
IRQF_NO_SUSPEND flag on the same IRQ, and it is never valid to use both for the
|
|
same device.
|
|
|
|
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 generally
|
|
make sense.
|
|
|
|
In rare cases an IRQ can be shared between a wakeup device driver and an
|
|
IRQF_NO_SUSPEND user. In order for this to be safe, the wakeup device driver
|
|
must be able to discern spurious IRQs from genuine wakeup events (signalling
|
|
the latter to the core with pm_system_wakeup()), must use enable_irq_wake() to
|
|
ensure that the IRQ will function as a wakeup source, and must request the IRQ
|
|
with IRQF_COND_SUSPEND to tell the core that it meets these requirements. If
|
|
these requirements are not met, it is not valid to use IRQF_COND_SUSPEND.
|