Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 17 Jan 2023 11:01:42 -0500
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Bastien Nocera <hadess@hadess.net>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Benjamin Tissoires <benjamin.tissoires@redhat.com>,
        linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org
Subject: Re: [RFC] USB: core: Add wireless_status sysfs attribute
Message-ID: <Y8bGZuMAvMRCTDQB@rowland.harvard.edu>
References: <d9f8b9413c10fcf067658979d16a4f5c7abe69e7.camel@hadess.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <d9f8b9413c10fcf067658979d16a4f5c7abe69e7.camel@hadess.net>
Precedence: bulk

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.
> 
> 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.
> 
> 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.
> 
> 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.
> 
> 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.
> 
> The question is whether this should be a USB sysfs attribute, or one at
> the base driver level. Example implementation of the USB sysfs
> attribute itself below.

Do you know of any non-USB devices using the receiver/emitter approach?

> I have a patch for a USB API as well, but I'm having some problems
> creating deferred work on a soft irq.
> 
> Cheers
> 
> ----
> Add a wireless_status sysfs attribute to USB devices to keep track of
> whether a USB device that uses a receiver/emitter combo has its
> emitter connected or disconnected.
> 
> By default, the USB device will declare not to use a receiver/emitter.

How do you plan to tell which devices do use a receiver/emitter?  Is 
this something the drivers already knowo about?

Is it conceivable that a single device might have more than one 
receiver?  If so, should the attribute belong to an interface rather 
than to the USB device?

Alan Stern