USB: core: Add wireless_status sysfs attribute

Add a wireless_status sysfs attribute to USB devices to keep track of
whether a USB device that's comprised of a receiver dongle and an emitter
device over a, most of the time proprietary, wireless link has its emitter
connected or disconnected.

This will be used by user-space OS components to determine whether the
battery-powered part of the device is wirelessly connected or not,
allowing, for example:
- upower to hide the battery for devices where the device is turned off
  but the receiver plugged in, rather than showing 0%, or other values
  that could be confusing to users
- Pipewire to hide a headset from the list of possible inputs or outputs
  or route audio appropriately if the headset is suddenly turned off, or
  turned on
- libinput to determine whether a keyboard or mouse is present when its
  receiver is plugged in.

This is done at the USB interface level as:
- the interface on which the wireless status is detected is sometimes
  not the same as where it could be consumed (eg. the audio interface
  on a headset dongle will still appear even if the headset is turned
  off), and we cannot have synchronisation of status across subsystems.
- this behaviour is not specific to HID devices, even if the protocols
  used to determine whether or not the remote device is connected can
  be HID.

This is not an attribute that is meant to replace protocol specific
APIs, such as the ones available for WWAN, WLAN/Wi-Fi, or Bluetooth
or any other sort of networking, but solely for wireless devices with
an ad-hoc “lose it and your device is e-waste” receiver dongle.

The USB interface will only be exporting the wireless_status sysfs
attribute if it gets set through the API exported in the next commit.

Signed-off-by: Bastien Nocera <hadess@hadess.net>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/20230302105555.51417-4-hadess@hadess.net
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-bus-usb b/Documentation/ABI/testing/sysfs-bus-usb
index 545c2dd97ed0..cb172db41b34 100644
--- a/Documentation/ABI/testing/sysfs-bus-usb
+++ b/Documentation/ABI/testing/sysfs-bus-usb
@@ -166,6 +166,23 @@ Description:
 		The file will be present for all speeds of USB devices, and will
 		always read "no" for USB 1.1 and USB 2.0 devices.
 
+What:		/sys/bus/usb/devices/<INTERFACE>/wireless_status
+Date:		February 2023
+Contact:	Bastien Nocera <hadess@hadess.net>
+Description:
+		Some USB devices use a USB receiver dongle to communicate
+		wirelessly with their device using proprietary protocols. This
+		attribute allows user-space to know whether the device is
+		connected to its receiver dongle, and, for example, consider
+		the device to be absent when choosing whether to show the
+		device's battery, show a headset in a list of outputs, or show
+		an on-screen keyboard if the only wireless keyboard is
+		turned off.
+		This attribute is not to be used to replace protocol specific
+		statuses available in WWAN, WLAN/Wi-Fi, Bluetooth, etc.
+		If the device does not use a receiver dongle with a wireless
+		device, then this attribute will not exist.
+
 What:		/sys/bus/usb/devices/.../<hub_interface>/port<X>
 Date:		August 2012
 Contact:	Lan Tianyu <tianyu.lan@intel.com>