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

[Pali Rohár]

On Saturday 26 January 2019 07:39:54 Arun Raghavan wrote:
> On Fri, 25 Jan 2019, at 3:19 PM, 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.
> 
> I strongly agree with this. Separate profiles for each codec is simply not the way to go -- it's horrible for usability.

Why it is horrible?

I think horrible is when you need to introduce a new API into pulseaudio
core for adding ability for switching to codec which supports
microphone.

Same for having hardcoded priority list of codecs somewhere in config
file or in module argument.

It would mean that switching to codec with microphone support cannot be
done in existing GUI tools and it is needed to edit config file or
re-load bluetooth module. This is horrible.

> > 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.
> 
> I don't think adding this to the core is necessarily the best option, but I think this is a separate problem.
> 
> The current patchset should, imo, just take a priority-ordered list of codecs as a modarg and use that (we can choose some default if it is not specified, also ideally based on what codecs are supported on the system -- as I've suggested before, I don't want us to depend on the codec implementation, but I can help deal with that as a separate step).
> 
> So the modarg approach gives us a static configuration option for people who care about this setting immediately, with a sensible default for most of our users who will not. How we can make this runtime configurable can be figured out separately (for example, with Georg's on-going work).
> 
> -- Arun

-- 
Pali Rohár
pali.rohar at gmail.com
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