netdev - Re: [PATCH v2] r8152: Suspend USB device before shutdown when WoL is enabled

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Date: Wed, 2 Aug 2023 14:56:43 +0000
From: "Gagniuc, Alexandru" <alexandru.gagniuc@...com>
To: Alan Stern <stern@...land.harvard.edu>
CC: "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>, "davem@...emloft.net"
	<davem@...emloft.net>, "edumazet@...gle.com" <edumazet@...gle.com>,
	"kuba@...nel.org" <kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>,
	"hayeswang@...ltek.com" <hayeswang@...ltek.com>, "jflf_kernel@....com"
	<jflf_kernel@....com>, "bjorn@...k.no" <bjorn@...k.no>, "svenva@...omium.org"
	<svenva@...omium.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Zhang, Eniac" <eniac-xw.zhang@...com>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH v2] r8152: Suspend USB device before shutdown when WoL is
 enabled

On Wed, Jul 19, 2023 at 02:36:25PM -0400, Alan Stern wrote:
> On Wed, Jul 19, 2023 at 05:37:56PM +0000, Alexandru Gagniuc wrote:
> > For Wake-on-LAN to work from S5 (shutdown), the USB link must be put
> > in U3 state. If it is not, and the host "disappears", the chip will
> > no longer respond to WoL triggers.
> > 
> > To resolve this, add a notifier block and register it as a reboot
> > notifier. When WoL is enabled, work through the usb_device struct to
> > get to the suspend function. Calling this function puts the link in
> > the correct state for WoL to function.
> 
> How do you know that the link will _remain_ in the correct state?

The objective is to get to xhci_set_link_state() with the USB_SS_PORT_LS_U3
argument. This is achieved through usb_port_suspend() in drivers/usb/host/hub.c,
and the function is implemented in drivers/usb/host/xhci-hub.c.

This is the only path in the kernel that I am aware of for setting the U3 link
state. Given that it is part of the USB subsystem, I am fairly confident it will
show consistent behavior across platforms.

> That is, how do you know that the shutdown processing for the USB host 
> controller won't disable the link entirely, thereby preventing WoL from 
> working?

We are talking to the USB hub in order to set the link state. I don't see how
specifics of the host controller would influence behavior. I do expect a
controller which advertises S4/S5 in /proc/acpi/wakeup to not do anything that
would sabotage this capability. Disabling the link entirely would probalby
violate that promise.

Think of USB-C docks with a power button showing up as a HID class. The scenario
herein would disable the power button. I would take that to be a bug in the host
controller driver if the S4/S5 capability is advertised.

Alex

P.S. I sincerely apologize for the delay in my reply. The corporate email servers here
have "difficulties" with plaintext and  interleaved replies.