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serdev: add controller runtime PM support
Add support for controller runtime power management to serdev core. This is needed to allow slave drivers to manage the runtime PM state of the underlying serial controller when its driver, in turn, implements more aggressive runtime power management (e.g. using autosuspend). For some applications, for example, where loss off initial data after a remote-wakeup event is acceptable or where rx is not used at all, aggressive serial controller runtime PM may be used without further involvement of the slave driver. But when this is not the case, the slave driver must be able to indicate when incoming data is expected in order to avoid data loss. To facilitate the common case, where the serial controller power state is active whenever the port is open (which is the case with just about every serial driver), and where data loss is not acceptable and cannot even be prevented by explicit controller runtime power management, an RPM reference is taken in serdev open and put again at close. This reference can later be balanced by any serdev driver which wants and/or can handle aggressive controller runtime PM. Note that the .ignore_children flag is set for the serdev controller to allow the underlying hardware to idle when no I/O is expected, regardless of the slave device RPM state. Acked-by:Tony Lindgren <tony@atomide.com> Reviewed-by: