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
