Re: [PATCH v4 1/3] usb: notify hcd when USB device suspend or resume

From: Alan Stern
Date: Fri May 08 2015 - 10:35:38 EST

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 <>

Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>

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