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

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Message-ID: <fa2a3e71-be88-44d9-a955-94af236cf2c7@rowland.harvard.edu>
Date: Wed, 2 Aug 2023 11:23:46 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: "Gagniuc, Alexandru" <alexandru.gagniuc@...com>
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, Aug 02, 2023 at 02:56:43PM +0000, Gagniuc, Alexandru wrote:
> 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 does not answer my question.  I agree that making this change will 
put the link into the U3 state.  But I don't have any reason to think 
that some other software won't later put the link into some other state.

> > 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.

Specifics of the host controller probably won't influence behavior.  
However, specifics of the _software_ can make a big difference.

>  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.

Not if the kernel _tells_ the controller to disable the link.

> 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.

Indeed.  And I am asking how you can be sure the host controller driver 
(or some other part of the software stack) doesn't have this bug.

Alan Stern