• U
    PM / Domains: Remove intermediate states from the power off sequence · ba2bbfbf
    Ulf Hansson 提交于
    Genpd's ->runtime_suspend() (assigned to pm_genpd_runtime_suspend())
    doesn't immediately walk the hierarchy of ->runtime_suspend() callbacks.
    Instead, pm_genpd_runtime_suspend() calls pm_genpd_poweroff() which
    postpones that until *all* the devices in the genpd are runtime suspended.
    
    When pm_genpd_poweroff() discovers that the last device in the genpd is
    about to be runtime suspended, it calls __pm_genpd_save_device() for *all*
    the devices in the genpd sequentially. Furthermore,
    __pm_genpd_save_device() invokes the ->start() callback, walks the
    hierarchy of the ->runtime_suspend() callbacks and invokes the ->stop()
    callback. This causes a "thundering herd" problem.
    
    Let's address this issue by having pm_genpd_runtime_suspend() immediately
    walk the hierarchy of the ->runtime_suspend() callbacks, instead of
    postponing that to the power off sequence via pm_genpd_poweroff(). If the
    selected ->runtime_suspend() callback doesn't return an error code, call
    pm_genpd_poweroff() to see if it's feasible to also power off the PM
    domain.
    
    Adopting this change enables us to simplify parts of the code in genpd,
    for example the locking mechanism. Additionally, it gives some positive
    side effects, as described below.
    
    i)
    One device's ->runtime_resume() latency is no longer affected by other
    devices' latencies in a genpd.
    
    The complexity genpd has to support the option to abort the power off
    sequence suffers from latency issues. More precisely, a device that is
    requested to be runtime resumed, may end up waiting for
    __pm_genpd_save_device() to complete its operations for *another* device.
    That's because pm_genpd_poweroff() can't confirm an abort request while it
    waits for __pm_genpd_save_device() to return.
    
    As this patch removes the intermediate states in pm_genpd_poweroff() while
    powering off the PM domain, we no longer need the ability to abort that
    sequence.
    
    ii)
    Make pm_runtime[_status]_suspended() reliable when used with genpd.
    
    Until the last device in a genpd becomes idle, pm_genpd_runtime_suspend()
    will return 0 without actually walking the hierarchy of the
    ->runtime_suspend() callbacks. However, by returning 0 the runtime PM core
    considers the device as runtime_suspended, so
    pm_runtime[_status]_suspended() will return true, even though the device
    isn't (yet) runtime suspended.
    
    After this patch, since pm_genpd_runtime_suspend() immediately walks the
    hierarchy of the ->runtime_suspend() callbacks,
    pm_runtime[_status]_suspended() will accurately reflect the status of the
    device.
    
    iii)
    Enable fine-grained PM through runtime PM callbacks in drivers/subsystems.
    
    There are currently cases were drivers/subsystems implements runtime PM
    callbacks to deploy fine-grained PM (e.g. gate clocks, move pinctrl to
    power-save state, etc.). While using the genpd, pm_genpd_runtime_suspend()
    postpones invoking these callbacks until *all* the devices in the genpd
    are runtime suspended. In essence, one runtime resumed device prevents
    fine-grained PM for other devices within the same genpd.
    
    After this patch, since pm_genpd_runtime_suspend() immediately walks the
    hierarchy of the ->runtime_suspend() callbacks, fine-grained PM is enabled
    throughout all the levels of runtime PM callbacks.
    
    iiii)
    Enable fine-grained PM for IRQ safe devices
    
    Per the definition for an IRQ safe device, its runtime PM callbacks must
    be able to execute in atomic context. In the path while genpd walks the
    hierarchy of the ->runtime_suspend() callbacks for the device, it uses a
    mutex. Therefore, genpd prevents that path to be executed for IRQ safe
    devices.
    
    As this patch changes pm_genpd_runtime_suspend() to immediately walk the
    hierarchy of the ->runtime_suspend() callbacks and without needing to use
    a mutex, fine-grained PM is enabled throughout all the levels of runtime
    PM callbacks for IRQ safe devices.
    
    Unfortunately this patch also comes with a drawback, as described in the
    summary below.
    
    Driver's/subsystem's runtime PM callbacks may be invoked even when the
    genpd hasn't actually powered off the PM domain, potentially introducing
    unnecessary latency.
    
    However, in most cases, saving/restoring register contexts for devices are
    typically fast operations or can be optimized in device specific ways
    (e.g. shadow copies of register contents in memory, device-specific checks
    to see if context has been lost before restoring context, etc.).
    
    Still, in some cases the driver/subsystem may suffer from latency if
    runtime PM is used in a very fine-grained manner (e.g. for each IO request
    or xfer). To prevent that extra overhead, the driver/subsystem may deploy
    the runtime PM autosuspend feature.
    Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>
    Reviewed-by: NKevin Hilman <khilman@linaro.org>
    Tested-by: NGeert Uytterhoeven <geert+renesas@glider.be>
    Tested-by: NLina Iyer <lina.iyer@linaro.org>
    Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>
    ba2bbfbf
domain.c 54.7 KB