Re: [RFC] USB: core: Add wireless_status sysfs attribute

[Bastien Nocera]

On Tue, 2023-01-17 at 17:14 +0100, Greg Kroah-Hartman wrote:
> On Tue, Jan 17, 2023 at 04:17:23PM +0100, Bastien Nocera wrote:
> > Hey,
> > 
> > TLDR: new sysfs attribute that makes it possible to leave receivers
> > for
> > wireless headsets plugged in. At the USB level, or at the base
> > driver
> > level?
> > 
> > Longer version:
> > I started working on implementing support for some wireless
> > headsets
> > that use USB receivers to communicate to the headset itself.
> 
> Would this also include wireless keyboard/mice recievers?

This could also be used by certain keyboard or mice receivers, if the
keyboard or mouse is kept constantly connected and there's a way to
find out whether it's connected or not.

> Why is "wireless" somehow a special attribute that userspace needs to
> know about?

"wireless" isn't the attribute user-space would be interested in,
"wireless status" would be. If the headset's wireless status is
"disconnected", then the headset is unavailable. Then user-space can
route the audio accordingly.

> > The USB receivers have multiple interfaces, and independent drivers
> > for
> > each, as is wont to do for USB devices. There's usually a HID
> > interface
> > to do the custom stuff (LEDs, battery status, connection status,
> > etc.)
> > and a standard audio class interface.
> 
> This probably should be an interface attribute (as Alan points out),
> as
> it's not a device attribute (think about updating the firmware for
> one
> of these, that's on an interface for the reciever you plugged in, not
> on
> the other end of the wireless connection...)

OK, fair enough.

> > Those drivers don't know anything about each other, and getting
> > them to
> > talk to each other would be rather complicated. Additionally the
> > audio
> > interface is still somewhat functional when the headset is
> > disconnected.
> 
> Those drivers shouldn't know about each other, that's up to userspace
> to
> group and control if needed.  No kernel interactions should be
> needed.
> 
> > In the end, I came up with this new sysfs attribute that would make
> > it
> > possible for user-space (PulseAudio or Pipewire) to know whether
> > the
> > receiver is plugged in or not.
> 
> Again, should be an interface attribute, if at all.
> 
> > That allows user-space to not show the battery information for the
> > device (rather than 0 percent), not offer the headset as an output,
> > and
> > potentially automatically switch to it when the headset is powered
> > on.
> 
> Same for a keyboard/mouse, right?

Yes, although I haven't found devices where this would be useful.

I'll reimplement this as an interface attribute.

Thanks very much to you and Alan for the quick replies.

Cheers