Commit f71495f3 authored by Rafael J. Wysocki's avatar Rafael J. Wysocki

PM / sleep: Update device PM documentation to cover direct_complete

Update the device PM documentation in devices.txt and runtime_pm.txt
to reflect the changes in the system suspend and resume handling
related to the introduction of the new power.direct_complete flag.
Signed-off-by: default avatarRafael J. Wysocki <>
Acked-by: default avatarAlan Stern <>
parent aae4518b
......@@ -2,6 +2,7 @@ Device Power Management
Copyright (c) 2010-2011 Rafael J. Wysocki <>, Novell Inc.
Copyright (c) 2010 Alan Stern <>
Copyright (c) 2014 Intel Corp., Rafael J. Wysocki <>
Most of the code in Linux is device drivers, so most of the Linux power
......@@ -326,6 +327,20 @@ the phases are:
driver in some way for the upcoming system power transition, but it
should not put the device into a low-power state.
For devices supporting runtime power management, the return value of the
prepare callback can be used to indicate to the PM core that it may
safely leave the device in runtime suspend (if runtime-suspended
already), provided that all of the device's descendants are also left in
runtime suspend. Namely, if the prepare callback returns a positive
number and that happens for all of the descendants of the device too,
and all of them (including the device itself) are runtime-suspended, the
PM core will skip the suspend, suspend_late and suspend_noirq suspend
phases as well as the resume_noirq, resume_early and resume phases of
the following system resume for all of these devices. In that case,
the complete callback will be called directly after the prepare callback
and is entirely responsible for bringing the device back to the
functional state as appropriate.
2. The suspend methods should quiesce the device to stop it from performing
I/O. They also may save the device registers and put it into the
appropriate low-power state, depending on the bus type the device is on,
......@@ -400,12 +415,23 @@ When resuming from freeze, standby or memory sleep, the phases are:
the resume callbacks occur; it's not necessary to wait until the
complete phase.
Moreover, if the preceding prepare callback returned a positive number,
the device may have been left in runtime suspend throughout the whole
system suspend and resume (the suspend, suspend_late, suspend_noirq
phases of system suspend and the resume_noirq, resume_early, resume
phases of system resume may have been skipped for it). In that case,
the complete callback is entirely responsible for bringing the device
back to the functional state after system suspend if necessary. [For
example, it may need to queue up a runtime resume request for the device
for this purpose.] To check if that is the case, the complete callback
can consult the device's power.direct_complete flag. Namely, if that
flag is set when the complete callback is being run, it has been called
directly after the preceding prepare and special action may be required
to make the device work correctly afterward.
At the end of these phases, drivers should be as functional as they were before
suspending: I/O can be performed using DMA and IRQs, and the relevant clocks are
gated on. Even if the device was in a low-power state before the system sleep
because of runtime power management, afterwards it should be back in its
full-power state. There are multiple reasons why it's best to do this; they are
discussed in more detail in Documentation/power/runtime_pm.txt.
gated on.
However, the details here may again be platform-specific. For example,
some systems support multiple "run" states, and the mode in effect at
......@@ -2,6 +2,7 @@ Runtime Power Management Framework for I/O Devices
(C) 2009-2011 Rafael J. Wysocki <>, Novell Inc.
(C) 2010 Alan Stern <>
(C) 2014 Intel Corp., Rafael J. Wysocki <>
1. Introduction
......@@ -444,6 +445,10 @@ drivers/base/power/runtime.c and include/linux/pm_runtime.h:
bool pm_runtime_status_suspended(struct device *dev);
- return true if the device's runtime PM status is 'suspended'
bool pm_runtime_suspended_if_enabled(struct device *dev);
- return true if the device's runtime PM status is 'suspended' and its
'power.disable_depth' field is equal to 1
void pm_runtime_allow(struct device *dev);
- set the power.runtime_auto flag for the device and decrease its usage
counter (used by the /sys/devices/.../power/control interface to
......@@ -644,6 +649,18 @@ place (in particular, if the system is not waking up from hibernation), it may
be more efficient to leave the devices that had been suspended before the system
suspend began in the suspended state.
To this end, the PM core provides a mechanism allowing some coordination between
different levels of device hierarchy. Namely, if a system suspend .prepare()
callback returns a positive number for a device, that indicates to the PM core
that the device appears to be runtime-suspended and its state is fine, so it
may be left in runtime suspend provided that all of its descendants are also
left in runtime suspend. If that happens, the PM core will not execute any
system suspend and resume callbacks for all of those devices, except for the
complete callback, which is then entirely responsible for handling the device
as appropriate. This only applies to system suspend transitions that are not
related to hibernation (see Documentation/power/devices.txt for more
The PM core does its best to reduce the probability of race conditions between
the runtime PM and system suspend/resume (and hibernation) callbacks by carrying
out the following operations:
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