[pulseaudio-discuss] [PATCH v5 0/7] New API for Bluetooth A2DP codecs

Pali Rohár pali.rohar at gmail.com
Fri Jan 25 15:41:26 UTC 2019

On Friday 25 January 2019 17:11:00 Tanu Kaskinen wrote:
> On Fri, 2019-01-25 at 11:02 +0100, Pali Rohár wrote:
> > On Friday 25 January 2019 11:49:32 Tanu Kaskinen wrote:
> > > On Sat, 2019-01-19 at 18:11 +0100, Pali Rohár wrote:
> > > > This is 5th version of my patch series for modular A2DP codec API and
> > > > aptX support. The only change for v5 is support for switching codecs.
> > > > 
> > > > This patch series provides new modular API for Bluetooth A2DP codecs,
> > > > clean up module-bluez5-device and bluez5-util to be codec independent
> > > > and convert SBC codec into this new API for A2DP codecs. Also it adds
> > > > support for aptX, aptX HD and FastStream A2DP codecs.
> > > > 
> > > > New codec API is designed in way, that for adding new codec is not
> > > > needed to touch bluez5-util nor module-bluez5-device files. Whole
> > > > codec registration is done in a2dp-codec-util.c file, without need
> > > > to update any header file.
> > > > 
> > > > Some A2DP codecs are bidirectional and support backchannel for
> > > > microphone voice. This new A2DP codec API fully supports this feature
> > > > and module-bluez5-device was extended to support microphone voice source
> > > > for codecs which declares such support. FastStream is such codec.
> > > > 
> > > > For every A2DP codec is created card profile. When using bluez patches
> > > > from https://marc.info/?l=linux-bluetooth&m=154696260401673&w=2 then
> > > > only those profiles codec profiles are created which are supported
> > > > by remote headset/endpoint.
> > > 
> > > As discussed before, I don't think the codec configuration should be
> > > exposed via card profiles.
> > > There is need for being able to say "switch
> > > to A2DP" without specifying the codec.
> > 
> > So which codec should be chosen when you do not specify it? That is the
> > most important question if codec is going to be hidden.
> Well, which codec do you choose by default when connecting a new
> device?

Not choosing any. When creating a connection, codec selection is up to
the remote device and bluez, on what they decide.

> Surely you assign some internal priority for the codecs.
> If the user has selected a specific codec earlier then that codec
> should be selected.

That is again not under pulseaudio control. When initializing connection
it is up to the bluez and remote device.

Bluez has now API for switching A2DP codec, by doing disconnect and
initializing a new connection with chosen endpoint (codec). And this is
what happen when profile is going to be switched in pulseaudio.

Yes, we can probably implement per-device priority in way that after
bluez initialize connection, we use API for codec switching to one which
we want. But this is, I think rather complicated. And if it is really
needed we can try to implement it later once everything other is

> > And if codec is exported as card profile, is not operation "switch to
> > A2DP" same as "switch to A2DP profile with higher priority"?
> Do you mean "the highest priority" rather than "higher priority"? Is
> your idea that when e.g. Gnome audio settings wants to enable bluetooth
> output,

What do you mean by "wants to enable bluetooth output"?

Because you cannot initialize A2DP connection from pulseaudio (or at
least pulseaudio does not support it). Initializing connection is done
from bluez client (on KDE it is bluedevil). And pulseaudio allows to
choose A2DP profile only if connection is already established by bluez.

> it should look at all output profiles on the card and pick the
> one with the highest priority? I suppose that would work, and quite
> possibly that's what it already does. That requires the profile
> priorities to be dynamic, however, so that when the user selects a
> profile, it becomes the highest priority codec.

Currently codecs have priority number which is exported to pulseaudio
profiles. Also priority number depends on other in which pulseaudio dbus
endpoints for bluez are registered. bluez internally used them for
deciding which codec to choose -- but it is global for all remote
device, not decide specific.

> How do you decide which A2DP profile to activate in module-bluetooth-
> policy?

At which time? module-bluetooth-policy should just switch to A2DP
profile which belong to endpoint which bluez activated.

> > > It's unclear how priority of the codecs (and their parameter
> > > permutations) should be configured, but it seems quite clear that a
> > > "set codec" operation on a specific card would be useful. If the "set
> > > codec" operation is separate from "set profile" operation, then you no
> > > doubt will ask how to implement the client API for this new operation,
> > > and I don't have a very good answer.
> > > 
> > > Georg Chini has been working on a new messaging API, which would be a
> > > good fit for this, but that's stalled (I don't remember if it's waiting
> > > for review or a new version of the patches). If you don't want to be
> > > blocked by that, the alternative is to add new "get bluetooth card
> > > info" and "set bluetooth card a2dp codec" commands to the core protocol
> > > (the card info command would be for enumerating the available codecs),
> > > or to add the commands via a new "protocol extension" similar to what
> > > module-stream-restore, module-device-restore and module-device-manager
> > > do. I would be fine with either approach.
> > > 
> > > Adding new commands to the core protocol would be somewhat simpler, but
> > > I'm not sure other maintainers are ok with adding bluetooth specific
> > > functionality to the core protocol.
> > 
> > All this seems like bluetooth specific. Extending core protocol API for
> > bluetooth specific API means that all applications needs to be extended
> > to support it.
> > 
> > So for choosing A2DP codec, it would be needed to extend KDE sound
> > system settings application, Gnome configuration, and also other...
> > 
> > I think this is too complicated and needs cooperation with other
> > projects and team.
> Well, I disagree about it being too complicated, and there's no need
> for any particular cooperation effort. UIs that want to change the
> codec just need to be updated to take advantage of the new feature. No
> functionality is degraded even if they don't adapt.

Yes, no functionality is degraded, but no functionality is also added.
And extending UI, logic behind it and so... needs cooperation with other
projects and teams. Without it there is zero benefit for pulseaudio for
ability to switch codecs -- as nobody would use it.

> > With profiles it is simple as it is already supported in desktop
> > environments.
> That's certainly a benefit. The problem is just that forcing the user
> to make a choice between codecs isn't very nice if the user doesn't
> know the practical difference between the provided options.

User is not forced. Bluez and remote device choose codec for him. And if
user is not happy with selection then can change it to something
different which "plays better".

We already have module bluetooth policy which switch between HFP and
A2DP profiles based on active VOIP application. So user already has
microphone support when calling. And after hanging call policy module
should switch back to previous A2DP profile (codec) without user

So I do not think it is such catastrophic that user need to choose
something. User has already pre-selected some A2DP variant. And based on
fact that lot of headsets remember last used codec per device, it is not
such a big problem.

Pali Rohár
pali.rohar at gmail.com

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