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Subject: Re: Bose QC 35 Battery/ANC Support
To:     Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Cc:     "linux-bluetooth@vger.kernel.org" <linux-bluetooth@vger.kernel.org>
References: <333e38b3-da26-e3c6-76fb-1ee1caf90299@tcd.ie>
 <CABBYNZLFV6THotUuzATG0YJFgDh9cFoU575QqLD+Q2+EHoiPTA@mail.gmail.com>
From:   Peter Mullen <omaolaip@tcd.ie>
Message-ID: <6030b6bc-73f1-6fbd-e5c5-58e2ce4a5d26@tcd.ie>
Date:   Tue, 13 Oct 2020 22:06:29 -0700
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Hi Luiz,

On 2020-10-12 9:43 a.m., Luiz Augusto von Dentz wrote:
> Hi Peter,
> 
> On Sun, Oct 11, 2020 at 1:04 PM Peter Mullen <omaolaip@tcd.ie> wrote:
>>
>> Hi all,
>>
>> I've been working on a plugin that adds support for battery level
>> reporting and Active Noise Cancelling (ANC) control for the Bose QC 35
>> headphones. The patch is nearly ready to go, but because of the
>> non-standard way it's implemented I figured it might be necessary to get
>> some preliminary feedback on it prior to submission (or to see if it's
>> worth submitting).
>>
>> As a brief overview of device control, the QC35 has a set of additional
>> controls operating over rfcomm channel 8 where messages are passed back
>> and forth between devices. The messages consist of a 3-byte opcode, a
>> 1-byte payload length, and N-bytes of payload. There's some more
>> information on the specifics here:
>> https://blog.davidventura.com.ar/reverse-engineering-the-bose-qc35-bluetooth-protocol.html
>>
>> As this is a non-standard "profile", there's no profile UUID assigned to
>> it which makes adding a profile somewhat more complicated. My solution
>> here was to add a profile under the iPod Accessory Protocol (iAP)
>> profile UUID which the QC35 lists, and to then filter out devices in the
>> profile probe callback based on manufacturer ID, device class, and
>> product ID. This isn't ideal as the probe will be called for any device
>> listing the iAP UUID, so suggestions for alternative approaches (if
>> necessary) are appreciated.
> 
> We could in possibly add pid/vid to btd_profile that way it can be
> probed by pid/vid in addition to UUID, that way one can make a driver
> that is specific to a vendor or a product.
Had a go at that this evening as a proof-of-concept; so far it's working
well, so this could be viable. I'll finish it off and test it out properly.

>> For session state control, I've hooked into the AVDTP state-change
>> callback. Again, possibly non-standard but this seemed to be the best
>> way to trigger the initiatiation/tearing down of the rfcomm connection.
> 
> I would recommend hooking to the btd_service state, like for example
> the policy plugin is doing.
I was able to get the plugin working based on the btd_service state cb,
but had a couple of concerns with it, mainly because we'll be receiving
events for all services on all devices, so the filtering of unused
events adds a bit of overhead.
I was also able to use the AVCTP state callback without issue, which may
be better suited than the AVDTP one.

>> For ANC integration, I've added a new dbus interface under
>> "org.bluez.Anc1". The interface contains a read-only "Range" property,
>> which indicates the number of discrete values the ANC can be set to, and
>> a read-write property "Level" which gets/sets the level. This interface
>> layout was chosen to be device agnostic, so that ANC support could
>> potentially be added for more devices in future.
> 
> I'd probably add Bose to the name of the interface if it is bose specific.
While the plugin is Bose specific, I had tried to keep the interface
itself generic so that integration with the DE or similar won't need any
changes if ANC support is added for other devices later.

--
Peter Mullen