138 lines
7.1 KiB
ReStructuredText
138 lines
7.1 KiB
ReStructuredText
====================================
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System Suspend and Device Interrupts
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====================================
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Copyright (C) 2014 Intel Corp.
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Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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Suspending and Resuming Device IRQs
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-----------------------------------
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Device interrupt request lines (IRQs) are generally disabled during system
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suspend after the "late" phase of suspending devices (that is, after all of the
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->prepare, ->suspend and ->suspend_late callbacks have been executed for all
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devices). That is done by suspend_device_irqs().
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The rationale for doing so is that after the "late" phase of device suspend
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there is no legitimate reason why any interrupts from suspended devices should
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trigger and if any devices have not been suspended properly yet, it is better to
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block interrupts from them anyway. Also, in the past we had problems with
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interrupt handlers for shared IRQs that device drivers implementing them were
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not prepared for interrupts triggering after their devices had been suspended.
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In some cases they would attempt to access, for example, memory address spaces
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of suspended devices and cause unpredictable behavior to ensue as a result.
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Unfortunately, such problems are very difficult to debug and the introduction
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of suspend_device_irqs(), along with the "noirq" phase of device suspend and
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resume, was the only practical way to mitigate them.
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Device IRQs are re-enabled during system resume, right before the "early" phase
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of resuming devices (that is, before starting to execute ->resume_early
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callbacks for devices). The function doing that is resume_device_irqs().
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The IRQF_NO_SUSPEND Flag
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------------------------
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There are interrupts that can legitimately trigger during the entire system
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suspend-resume cycle, including the "noirq" phases of suspending and resuming
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devices as well as during the time when nonboot CPUs are taken offline and
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brought back online. That applies to timer interrupts in the first place,
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but also to IPIs and to some other special-purpose interrupts.
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The IRQF_NO_SUSPEND flag is used to indicate that to the IRQ subsystem when
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requesting a special-purpose interrupt. It causes suspend_device_irqs() to
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leave the corresponding IRQ enabled so as to allow the interrupt to work as
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expected during the suspend-resume cycle, but does not guarantee that the
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interrupt will wake the system from a suspended state -- for such cases it is
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necessary to use enable_irq_wake().
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Note that the IRQF_NO_SUSPEND flag affects the entire IRQ and not just one
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user of it. Thus, if the IRQ is shared, all of the interrupt handlers installed
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for it will be executed as usual after suspend_device_irqs(), even if the
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IRQF_NO_SUSPEND flag was not passed to request_irq() (or equivalent) by some of
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the IRQ's users. For this reason, using IRQF_NO_SUSPEND and IRQF_SHARED at the
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same time should be avoided.
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System Wakeup Interrupts, enable_irq_wake() and disable_irq_wake()
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------------------------------------------------------------------
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System wakeup interrupts generally need to be configured to wake up the system
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from sleep states, especially if they are used for different purposes (e.g. as
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I/O interrupts) in the working state.
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That may involve turning on a special signal handling logic within the platform
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(such as an SoC) so that signals from a given line are routed in a different way
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during system sleep so as to trigger a system wakeup when needed. For example,
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the platform may include a dedicated interrupt controller used specifically for
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handling system wakeup events. Then, if a given interrupt line is supposed to
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wake up the system from sleep states, the corresponding input of that interrupt
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controller needs to be enabled to receive signals from the line in question.
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After wakeup, it generally is better to disable that input to prevent the
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dedicated controller from triggering interrupts unnecessarily.
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The IRQ subsystem provides two helper functions to be used by device drivers for
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those purposes. Namely, enable_irq_wake() turns on the platform's logic for
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handling the given IRQ as a system wakeup interrupt line and disable_irq_wake()
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turns that logic off.
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Calling enable_irq_wake() causes suspend_device_irqs() to treat the given IRQ
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in a special way. Namely, the IRQ remains enabled, but on the first interrupt
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it will be disabled, marked as pending and "suspended" so that it will be
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re-enabled by resume_device_irqs() during the subsequent system resume. Also
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the PM core is notified about the event which causes the system suspend in
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progress to be aborted (that doesn't have to happen immediately, but at one
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of the points where the suspend thread looks for pending wakeup events).
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This way every interrupt from a wakeup interrupt source will either cause the
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system suspend currently in progress to be aborted or wake up the system if
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already suspended. However, after suspend_device_irqs() interrupt handlers are
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not executed for system wakeup IRQs. They are only executed for IRQF_NO_SUSPEND
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IRQs at that time, but those IRQs should not be configured for system wakeup
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using enable_irq_wake().
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Interrupts and Suspend-to-Idle
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------------------------------
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Suspend-to-idle (also known as the "freeze" sleep state) is a relatively new
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system sleep state that works by idling all of the processors and waiting for
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interrupts right after the "noirq" phase of suspending devices.
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Of course, this means that all of the interrupts with the IRQF_NO_SUSPEND flag
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set will bring CPUs out of idle while in that state, but they will not cause the
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IRQ subsystem to trigger a system wakeup.
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System wakeup interrupts, in turn, will trigger wakeup from suspend-to-idle in
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analogy with what they do in the full system suspend case. The only difference
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is that the wakeup from suspend-to-idle is signaled using the usual working
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state interrupt delivery mechanisms and doesn't require the platform to use
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any special interrupt handling logic for it to work.
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IRQF_NO_SUSPEND and enable_irq_wake()
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-------------------------------------
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There are very few valid reasons to use both enable_irq_wake() and the
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IRQF_NO_SUSPEND flag on the same IRQ, and it is never valid to use both for the
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same device.
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First of all, if the IRQ is not shared, the rules for handling IRQF_NO_SUSPEND
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interrupts (interrupt handlers are invoked after suspend_device_irqs()) are
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directly at odds with the rules for handling system wakeup interrupts (interrupt
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handlers are not invoked after suspend_device_irqs()).
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Second, both enable_irq_wake() and IRQF_NO_SUSPEND apply to entire IRQs and not
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to individual interrupt handlers, so sharing an IRQ between a system wakeup
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interrupt source and an IRQF_NO_SUSPEND interrupt source does not generally
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make sense.
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In rare cases an IRQ can be shared between a wakeup device driver and an
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IRQF_NO_SUSPEND user. In order for this to be safe, the wakeup device driver
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must be able to discern spurious IRQs from genuine wakeup events (signalling
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the latter to the core with pm_system_wakeup()), must use enable_irq_wake() to
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ensure that the IRQ will function as a wakeup source, and must request the IRQ
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with IRQF_COND_SUSPEND to tell the core that it meets these requirements. If
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these requirements are not met, it is not valid to use IRQF_COND_SUSPEND.
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