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authorRafael J. Wysocki <rjw@sisk.pl>2011-07-06 10:51:58 +0200
committerRafael J. Wysocki <rjw@sisk.pl>2011-07-06 10:51:58 +0200
commit1e2ef05bb8cf851a694d38e9170c89e7ff052741 (patch)
treeff1dfccc622374f9039d342cc48d16d6636740ae /Documentation/power
parenteea3fc0357eb89d0b2d1af37bdfb83eb4076a542 (diff)
PM: Limit race conditions between runtime PM and system sleep (v2)
One of the roles of the PM core is to prevent different PM callbacks executed for the same device object from racing with each other. Unfortunately, after commit e8665002477f0278f84f898145b1f141ba26ee26 (PM: Allow pm_runtime_suspend() to succeed during system suspend) runtime PM callbacks may be executed concurrently with system suspend/resume callbacks for the same device. The main reason for commit e8665002477f0278f84f898145b1f141ba26ee26 was that some subsystems and device drivers wanted to use runtime PM helpers, pm_runtime_suspend() and pm_runtime_put_sync() in particular, for carrying out the suspend of devices in their .suspend() callbacks. However, as it's been determined recently, there are multiple reasons not to do so, inlcuding: * The caller really doesn't control the runtime PM usage counters, because user space can access them through sysfs and effectively block runtime PM. That means using pm_runtime_suspend() or pm_runtime_get_sync() to suspend devices during system suspend may or may not work. * If a driver calls pm_runtime_suspend() from its .suspend() callback, it causes the subsystem's .runtime_suspend() callback to be executed, which leads to the call sequence: subsys->suspend(dev) driver->suspend(dev) pm_runtime_suspend(dev) subsys->runtime_suspend(dev) recursive from the subsystem's point of view. For some subsystems that may actually work (e.g. the platform bus type), but for some it will fail in a rather spectacular fashion (e.g. PCI). In each case it means a layering violation. * Both the subsystem and the driver can provide .suspend_noirq() callbacks for system suspend that can do whatever the .runtime_suspend() callbacks do just fine, so it really isn't necessary to call pm_runtime_suspend() during system suspend. * The runtime PM's handling of wakeup devices is usually different from the system suspend's one, so .runtime_suspend() may simply be inappropriate for system suspend. * System suspend is supposed to work even if CONFIG_PM_RUNTIME is unset. * The runtime PM workqueue is frozen before system suspend, so if whatever the driver is going to do during system suspend depends on it, that simply won't work. Still, there is a good reason to allow pm_runtime_resume() to succeed during system suspend and resume (for instance, some subsystems and device drivers may legitimately use it to ensure that their devices are in full-power states before suspending them). Moreover, there is no reason to prevent runtime PM callbacks from being executed in parallel with the system suspend/resume .prepare() and .complete() callbacks and the code removed by commit e8665002477f0278f84f898145b1f141ba26ee26 went too far in this respect. On the other hand, runtime PM callbacks, including .runtime_resume(), must not be executed during system suspend's "late" stage of suspending devices and during system resume's "early" device resume stage. Taking all of the above into consideration, make the PM core acquire a runtime PM reference to every device and resume it if there's a runtime PM resume request pending right before executing the subsystem-level .suspend() callback for it. Make the PM core drop references to all devices right after executing the subsystem-level .resume() callbacks for them. Additionally, make the PM core disable the runtime PM framework for all devices during system suspend, after executing the subsystem-level .suspend() callbacks for them, and enable the runtime PM framework for all devices during system resume, right before executing the subsystem-level .resume() callbacks for them. Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl> Acked-by: Kevin Hilman <khilman@ti.com>
Diffstat (limited to 'Documentation/power')
-rw-r--r--Documentation/power/runtime_pm.txt21
1 files changed, 21 insertions, 0 deletions
diff --git a/Documentation/power/runtime_pm.txt b/Documentation/power/runtime_pm.txt
index 0ec3d610fc9..d50dd1ab590 100644
--- a/Documentation/power/runtime_pm.txt
+++ b/Documentation/power/runtime_pm.txt
@@ -583,6 +583,13 @@ this is:
pm_runtime_set_active(dev);
pm_runtime_enable(dev);
+The PM core always increments the run-time usage counter before calling the
+->suspend() callback and decrements it after calling the ->resume() callback.
+Hence disabling run-time PM temporarily like this will not cause any runtime
+suspend attempts to be permanently lost. If the usage count goes to zero
+following the return of the ->resume() callback, the ->runtime_idle() callback
+will be invoked as usual.
+
On some systems, however, system sleep is not entered through a global firmware
or hardware operation. Instead, all hardware components are put into low-power
states directly by the kernel in a coordinated way. Then, the system sleep
@@ -595,6 +602,20 @@ 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.
+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:
+
+ * During system suspend it calls pm_runtime_get_noresume() and
+ pm_runtime_barrier() for every device right before executing the
+ subsystem-level .suspend() callback for it. In addition to that it calls
+ pm_runtime_disable() for every device right after executing the
+ subsystem-level .suspend() callback for it.
+
+ * During system resume it calls pm_runtime_enable() and pm_runtime_put_sync()
+ for every device right before and right after executing the subsystem-level
+ .resume() callback for it, respectively.
+
7. Generic subsystem callbacks
Subsystems may wish to conserve code space by using the set of generic power