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.
