On Fri, 8 May 2015, Lu Baolu wrote: > This patch adds two new entries in hc_driver. With these new entries, > USB core can notify host driver when a USB device is about to suspend > or just resumed. > > The xHCI spec is designed to allow an xHC implementation to cache the > endpoint state. Caching endpoint state allows an xHC to reduce latency > when handling ERDYs and other USB asynchronous events. However holding > this state in xHC consumes resources and power. The xHCI spec designs > some methods through which host controller driver can hint xHC about > how to optimize its operation, e.g. to determine when it holds state > internally or pushes it out to memory, when to power down logic, etc. > > When a USB device is going to suspend, states of all endpoints cached > in the xHC should be pushed out to memory to save power and resources. > Vice versa, when a USB device resumes, those states should be brought > back to the cache. USB core suspends or resumes a USB device by sending > set/clear port feature requests to the parent hub where the USB device > is connected. Unfortunately, these operations are transparent to xHCI > driver unless the USB device is plugged in a root port. This patch > utilizes the callback entries to notify xHCI driver whenever a USB > device suspends or resumes. > > It is harmless if a USB devices under USB 3.0 host controller suspends > or resumes without a notification to hcd driver. However there may be > less opportunities for power savings and there may be increased latency > for restarting an endpoint. The precise impact will be different for > each xHC implementation. It all depends on what an implementation does > with the hints. > > Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx> Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
