Re: [PATCH V4] PM / Domains: Remove intermediate states from the power off sequence

[Kevin Hilman <khilman@kernel.org>]

Ulf Hansson <ulf.hansson@linaro.org> writes:

> 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: Ulf Hansson <ulf.hansson@linaro.org>

Reviewed-by: Kevin Hilman <khilman@linaro.org>

I believe Geert and Lina have both tested earlier versions, but would be
nice to see their Tested-by for this one.

Kevin