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Subject[PATCH] PM: Document rules on using pm_runtime_resume() in system suspend callbacks
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From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

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>
---

This is a follow-up for the system suspend callbacks discussion during
the Power Management and Energy-Awareness session at the LPC last week.

In particular, I have been thinking about using pm_runtime_resume() from
driver ->suspend callbacks and it actually appears to be quite defendable to
me as long as it is guaranteed that middle layers will not mess up with the
device state before the driver's ->suspend callback is invoked. If that
is the case, and I *think* that it currently is the case for all of the
middle layers in the tree unless I overlook something (USB anyone?), the
middle layer callbacks involved (->prepare, ->suspend and ->runtime_resume)
should actually nest properly and there should not be problems with that.
So, my proposal here would be to simply go ahead and document the rules
regarding this case without modifying the code.

At the same time, I see at least two general problems with calling
pm_runtime_force_suspend() from the ->suspend callbacks of device drivers
(unless the middle layers involved are trivial).

First, note that the middle layer callbacks involved in that case are
->prepare, ->suspend, ->runtime_suspend (called indirectly from within the
former) and then *also* ->suspend_late and ->suspend_noirq, because the PM
core will call the last two from the middle layer as it has no information
that pm_runtime_force_suspend() was called for the device in the "suspend"
phase. Of course, in general, what the middle layer ->suspend_late and
->suspend_noirq do is not guaranteed to play well with what its
->runtime_suspend does even if ->suspend itself is OK (but for ->runtime_resume
all of that actually works, because the state it leaves the device in should
be compatible with the system suspend callbacks invoked in the later phases).

Second, leaving devices in runtime suspend in the "suspend" phase of system
suspend is fishy even when their runtime PM is disabled, because that doesn't
guarantee anything regarding their children or possible consumers. Runtime
PM may still be enabled for those devices at that time and runtime resume may
be triggered for them later, in which case it all quickly falls apart.

IOW, there are reasons why the PM core bumps up the runtime PM usage counters
of all devices during system suspend and they also apply to runtime suspend
callbacks being invoked directly with runtime PM disabled for the given device.
Frankly, it generally is only safe to leave a device in runtime suspend during
system suspend if it can be guarateed that the system suspend callbacks in the
subsequent suspend phases will not be invoked for it at all.

Thanks,
Rafael

---
Documentation/driver-api/pm/devices.rst | 25 ++++++++++++++++++++++++-
1 file changed, 24 insertions(+), 1 deletion(-)

Index: linux-pm/Documentation/driver-api/pm/devices.rst
===================================================================
--- linux-pm.orig/Documentation/driver-api/pm/devices.rst
+++ linux-pm/Documentation/driver-api/pm/devices.rst
@@ -328,7 +328,10 @@ the phases are: ``prepare``, ``suspend``
After the ``->prepare`` callback method returns, no new children may be
registered below the device. The method may also prepare the device or
driver in some way for the upcoming system power transition, but it
- should not put the device into a low-power state.
+ should not put the device into a low-power state. Moreover, if the
+ device supports runtime power management, the ``->prepare`` callback
+ method must not update its state in case it is necessary to resume it
+ from runtime suspend later on.

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
@@ -356,6 +359,16 @@ the phases are: ``prepare``, ``suspend``
the appropriate low-power state, depending on the bus type the device is
on, and they may enable wakeup events.

+ However, for devices supporting runtime power management, the
+ ``->suspend`` methods provided by subsystems (bus types and PM domains
+ in particular) must follow an additional rule regarding what can be done
+ to the devices before their drivers' ``->suspend`` methods are called.
+ Namely, they can only resume the devices from runtime suspend by
+ calling :c:func:`pm_runtime_resume` for them, if that is necessary, and
+ they must not update the state of the devices in any other way at that
+ time (in case the drivers need to resume the devices from runtime
+ suspend in their ``->suspend`` methods).
+
3. For a number of devices it is convenient to split suspend into the
"quiesce device" and "save device state" phases, in which cases
``suspend_late`` is meant to do the latter. It is always executed after
@@ -729,6 +742,16 @@ state temporarily, for example so that i
disabled. This all depends on the hardware and the design of the subsystem and
device driver in question.

+If it is necessary to resume a device from runtime suspend during a system-wide
+transition into a sleep state, that can be done by calling
+:c:func:`pm_runtime_resume` for it from the ``->suspend`` callback (or its
+couterpart for transitions related to hibernation) of either the device's driver
+or a subsystem responsible for it (for example, a bus type or a PM domain).
+That is guaranteed to work by the requirement that subsystems must not change
+the state of devices (possibly except for resuming them from runtime suspend)
+from their ``->prepare`` and ``->suspend`` callbacks (or equivalent) *before*
+invoking device drivers' ``->suspend`` callbacks (or equivalent).
+
During system-wide resume from a sleep state it's easiest to put devices into
the full-power state, as explained in :file:`Documentation/power/runtime_pm.txt`.
Refer to that document for more information regarding this particular issue as
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