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1 | System Suspend and Device Interrupts |
2 | ||
3 | Copyright (C) 2014 Intel Corp. | |
4 | Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com> | |
5 | ||
6 | ||
7 | Suspending and Resuming Device IRQs | |
8 | ----------------------------------- | |
9 | ||
10 | Device interrupt request lines (IRQs) are generally disabled during system | |
11 | suspend after the "late" phase of suspending devices (that is, after all of the | |
12 | ->prepare, ->suspend and ->suspend_late callbacks have been executed for all | |
13 | devices). That is done by suspend_device_irqs(). | |
14 | ||
15 | The rationale for doing so is that after the "late" phase of device suspend | |
16 | there is no legitimate reason why any interrupts from suspended devices should | |
17 | trigger and if any devices have not been suspended properly yet, it is better to | |
18 | block interrupts from them anyway. Also, in the past we had problems with | |
19 | interrupt handlers for shared IRQs that device drivers implementing them were | |
20 | not prepared for interrupts triggering after their devices had been suspended. | |
21 | In some cases they would attempt to access, for example, memory address spaces | |
22 | of suspended devices and cause unpredictable behavior to ensue as a result. | |
23 | Unfortunately, such problems are very difficult to debug and the introduction | |
24 | of suspend_device_irqs(), along with the "noirq" phase of device suspend and | |
25 | resume, was the only practical way to mitigate them. | |
26 | ||
27 | Device IRQs are re-enabled during system resume, right before the "early" phase | |
28 | of resuming devices (that is, before starting to execute ->resume_early | |
29 | callbacks for devices). The function doing that is resume_device_irqs(). | |
30 | ||
31 | ||
32 | The IRQF_NO_SUSPEND Flag | |
33 | ------------------------ | |
34 | ||
35 | There are interrupts that can legitimately trigger during the entire system | |
36 | suspend-resume cycle, including the "noirq" phases of suspending and resuming | |
37 | devices as well as during the time when nonboot CPUs are taken offline and | |
38 | brought back online. That applies to timer interrupts in the first place, | |
39 | but also to IPIs and to some other special-purpose interrupts. | |
40 | ||
41 | The IRQF_NO_SUSPEND flag is used to indicate that to the IRQ subsystem when | |
42 | requesting a special-purpose interrupt. It causes suspend_device_irqs() to | |
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43 | leave the corresponding IRQ enabled so as to allow the interrupt to work as |
44 | expected during the suspend-resume cycle, but does not guarantee that the | |
45 | interrupt will wake the system from a suspended state -- for such cases it is | |
46 | necessary to use enable_irq_wake(). | |
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47 | |
48 | Note that the IRQF_NO_SUSPEND flag affects the entire IRQ and not just one | |
49 | user of it. Thus, if the IRQ is shared, all of the interrupt handlers installed | |
50 | for it will be executed as usual after suspend_device_irqs(), even if the | |
51 | IRQF_NO_SUSPEND flag was not passed to request_irq() (or equivalent) by some of | |
52 | the IRQ's users. For this reason, using IRQF_NO_SUSPEND and IRQF_SHARED at the | |
53 | same time should be avoided. | |
54 | ||
55 | ||
56 | System Wakeup Interrupts, enable_irq_wake() and disable_irq_wake() | |
57 | ------------------------------------------------------------------ | |
58 | ||
59 | System wakeup interrupts generally need to be configured to wake up the system | |
60 | from sleep states, especially if they are used for different purposes (e.g. as | |
61 | I/O interrupts) in the working state. | |
62 | ||
63 | That may involve turning on a special signal handling logic within the platform | |
64 | (such as an SoC) so that signals from a given line are routed in a different way | |
65 | during system sleep so as to trigger a system wakeup when needed. For example, | |
66 | the platform may include a dedicated interrupt controller used specifically for | |
67 | handling system wakeup events. Then, if a given interrupt line is supposed to | |
68 | wake up the system from sleep sates, the corresponding input of that interrupt | |
69 | controller needs to be enabled to receive signals from the line in question. | |
70 | After wakeup, it generally is better to disable that input to prevent the | |
71 | dedicated controller from triggering interrupts unnecessarily. | |
72 | ||
73 | The IRQ subsystem provides two helper functions to be used by device drivers for | |
74 | those purposes. Namely, enable_irq_wake() turns on the platform's logic for | |
75 | handling the given IRQ as a system wakeup interrupt line and disable_irq_wake() | |
76 | turns that logic off. | |
77 | ||
78 | Calling enable_irq_wake() causes suspend_device_irqs() to treat the given IRQ | |
79 | in a special way. Namely, the IRQ remains enabled, by on the first interrupt | |
80 | it will be disabled, marked as pending and "suspended" so that it will be | |
81 | re-enabled by resume_device_irqs() during the subsequent system resume. Also | |
1f999d14 | 82 | the PM core is notified about the event which causes the system suspend in |
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83 | progress to be aborted (that doesn't have to happen immediately, but at one |
84 | of the points where the suspend thread looks for pending wakeup events). | |
85 | ||
86 | This way every interrupt from a wakeup interrupt source will either cause the | |
87 | system suspend currently in progress to be aborted or wake up the system if | |
88 | already suspended. However, after suspend_device_irqs() interrupt handlers are | |
89 | not executed for system wakeup IRQs. They are only executed for IRQF_NO_SUSPEND | |
90 | IRQs at that time, but those IRQs should not be configured for system wakeup | |
91 | using enable_irq_wake(). | |
92 | ||
93 | ||
94 | Interrupts and Suspend-to-Idle | |
95 | ------------------------------ | |
96 | ||
97 | Suspend-to-idle (also known as the "freeze" sleep state) is a relatively new | |
98 | system sleep state that works by idling all of the processors and waiting for | |
99 | interrupts right after the "noirq" phase of suspending devices. | |
100 | ||
101 | Of course, this means that all of the interrupts with the IRQF_NO_SUSPEND flag | |
102 | set will bring CPUs out of idle while in that state, but they will not cause the | |
103 | IRQ subsystem to trigger a system wakeup. | |
104 | ||
105 | System wakeup interrupts, in turn, will trigger wakeup from suspend-to-idle in | |
106 | analogy with what they do in the full system suspend case. The only difference | |
107 | is that the wakeup from suspend-to-idle is signaled using the usual working | |
108 | state interrupt delivery mechanisms and doesn't require the platform to use | |
109 | any special interrupt handling logic for it to work. | |
110 | ||
111 | ||
112 | IRQF_NO_SUSPEND and enable_irq_wake() | |
113 | ------------------------------------- | |
114 | ||
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115 | There are very few valid reasons to use both enable_irq_wake() and the |
116 | IRQF_NO_SUSPEND flag on the same IRQ, and it is never valid to use both for the | |
117 | same device. | |
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118 | |
119 | First of all, if the IRQ is not shared, the rules for handling IRQF_NO_SUSPEND | |
120 | interrupts (interrupt handlers are invoked after suspend_device_irqs()) are | |
121 | directly at odds with the rules for handling system wakeup interrupts (interrupt | |
122 | handlers are not invoked after suspend_device_irqs()). | |
123 | ||
124 | Second, both enable_irq_wake() and IRQF_NO_SUSPEND apply to entire IRQs and not | |
125 | to individual interrupt handlers, so sharing an IRQ between a system wakeup | |
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126 | interrupt source and an IRQF_NO_SUSPEND interrupt source does not generally |
127 | make sense. | |
128 | ||
129 | In rare cases an IRQ can be shared between a wakeup device driver and an | |
130 | IRQF_NO_SUSPEND user. In order for this to be safe, the wakeup device driver | |
131 | must be able to discern spurious IRQs from genuine wakeup events (signalling | |
132 | the latter to the core with pm_system_wakeup()), must use enable_irq_wake() to | |
133 | ensure that the IRQ will function as a wakeup source, and must request the IRQ | |
134 | with IRQF_COND_SUSPEND to tell the core that it meets these requirements. If | |
135 | these requirements are not met, it is not valid to use IRQF_COND_SUSPEND. |