linux-kernel - Re: [PATCH v7 5/5] usb: host: enable USB offload during system sleep

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Message-ID: <e8de4837-4c4a-4acd-bb33-6811d7381d20@rowland.harvard.edu>
Date: Thu, 9 Jan 2025 10:08:41 -0500
From: Alan Stern <stern@...land.harvard.edu>
To: Guan-Yu Lin <guanyulin@...gle.com>
Cc: gregkh@...uxfoundation.org, Thinh.Nguyen@...opsys.com,
	mathias.nyman@...el.com, perex@...ex.cz, tiwai@...e.com,
	sumit.garg@...aro.org, kekrby@...il.com, oneukum@...e.com,
	ricardo@...liere.net, lijiayi@...inos.cn, quic_jjohnson@...cinc.com,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-sound@...r.kernel.org
Subject: Re: [PATCH v7 5/5] usb: host: enable USB offload during system sleep

On Thu, Jan 09, 2025 at 03:55:09AM +0000, Guan-Yu Lin wrote:
> Sharing a USB controller with another entity via xhci-sideband driver
> creates power management complexities. To prevent the USB controller
> from being inadvertently deactivated while in use by the other entity, a
> usage-count based mechanism is implemented. This allows the system to
> manage power effectively, ensuring the controller remains available
> whenever needed.
> In order to maintain full functionality of an offloaded USB devices,
> several changes are made within the suspend flow of such devices:
> - skip usb_suspend_device() so that the port/hub are still active for
>   USB transfers via offloaded path.
> - not flushing the endpoints which are used by USB interfaces marked
>   with needs_remote_wakeup. Namely, skip usb_suspend_interface() and
>   usb_hcd_flush_endpoint() on associated USB interfaces. This reserves a
>   pending interrupt urb during system suspend for handling the interrupt
>   transfer, which is necessary since remote wakeup doesn't apply in the
>   offloaded USB devices when controller is still active.

Does this reasoning apply to interrupt-OUT as well as interrupt-IN?

Or looking at it another way: Since the device's endpoints are being 
used by the coprocessor, should the system flush any of them at all?

Alan Stern