PM: Document rules on using pm_runtime_resume() in system suspend callbacks
It quite often is necessary to resume devices from runtime suspend during system suspend for various reasons (for example, if their wakeup settings need to be changed), but that requires middle-layer or subsystem code to follow additional rules which currently are not clearly documented. Namely, if a driver calls pm_runtime_resume() for the device from its ->suspend (or equivalent) system sleep callback, that may not work if the middle layer above it has updated the state of the device from its ->prepare or ->suspend callbacks already in an incompatible way. For this reason, all middle layers must follow the rule that, until the ->suspend callback provided by the device's driver is invoked, the only way in which the device's state can be updated is by calling pm_runtime_resume() for it, if necessary. Fortunately enough, all middle layers in the code base today follow this rule, but it is not explicitly stated anywhere, so do that. Note that calling pm_runtime_resume() from the ->suspend callback of a driver will cause the ->runtime_resume callback provided by the middle layer to be invoked, but the rule above guarantees that this callback will nest properly with the middle layer's ->suspend callback and it will play well with the ->prepare one invoked before. Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
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@ -328,7 +328,10 @@ the phases are: ``prepare``, ``suspend``, ``suspend_late``, ``suspend_noirq``.
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After the ``->prepare`` callback method returns, no new children may be
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registered below the device. The method may also prepare the device or
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driver in some way for the upcoming system power transition, but it
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should not put the device into a low-power state.
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should not put the device into a low-power state. Moreover, if the
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device supports runtime power management, the ``->prepare`` callback
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method must not update its state in case it is necessary to resume it
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from runtime suspend later on.
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For devices supporting runtime power management, the return value of the
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prepare callback can be used to indicate to the PM core that it may
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@ -356,6 +359,16 @@ the phases are: ``prepare``, ``suspend``, ``suspend_late``, ``suspend_noirq``.
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the appropriate low-power state, depending on the bus type the device is
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on, and they may enable wakeup events.
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However, for devices supporting runtime power management, the
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``->suspend`` methods provided by subsystems (bus types and PM domains
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in particular) must follow an additional rule regarding what can be done
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to the devices before their drivers' ``->suspend`` methods are called.
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Namely, they can only resume the devices from runtime suspend by
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calling :c:func:`pm_runtime_resume` for them, if that is necessary, and
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they must not update the state of the devices in any other way at that
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time (in case the drivers need to resume the devices from runtime
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suspend in their ``->suspend`` methods).
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3. For a number of devices it is convenient to split suspend into the
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"quiesce device" and "save device state" phases, in which cases
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``suspend_late`` is meant to do the latter. It is always executed after
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@ -729,6 +742,16 @@ state temporarily, for example so that its system wakeup capability can be
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disabled. This all depends on the hardware and the design of the subsystem and
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device driver in question.
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If it is necessary to resume a device from runtime suspend during a system-wide
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transition into a sleep state, that can be done by calling
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:c:func:`pm_runtime_resume` for it from the ``->suspend`` callback (or its
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couterpart for transitions related to hibernation) of either the device's driver
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or a subsystem responsible for it (for example, a bus type or a PM domain).
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That is guaranteed to work by the requirement that subsystems must not change
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the state of devices (possibly except for resuming them from runtime suspend)
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from their ``->prepare`` and ``->suspend`` callbacks (or equivalent) *before*
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invoking device drivers' ``->suspend`` callbacks (or equivalent).
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During system-wide resume from a sleep state it's easiest to put devices into
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the full-power state, as explained in :file:`Documentation/power/runtime_pm.txt`.
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Refer to that document for more information regarding this particular issue as
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