• Rafael J. Wysocki's avatar
    PM: Limit race conditions between runtime PM and system sleep (v2) · 1e2ef05b
    Rafael J. Wysocki authored
    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 e8665002
    (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 e8665002
    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
    e8665002 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: default avatarRafael J. Wysocki <rjw@sisk.pl>
    Acked-by: default avatarKevin Hilman <khilman@ti.com>
    1e2ef05b
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