[pulseaudio-discuss] Bluetooth HSP and HFP support in pulseaudio

Sat Feb 15 21:33:10 UTC 2020

Hello!

More then two months ago I started discussion how to handle currently
unsupported parts of Bluetooth HSP and HFP profiles on Linux via
pulseaudio.

Main problems are:

1) These profiles are bound with telephony stack and without having half
   of telephony stack it is not possible to handle stable and working
   HFP profile. Telephony stack is needed for parsing AT commands and
   handling state machine.

2) Only one application can own RFCOMM socket over which are transmitting
   AT commands.

3) Application which own socket needs to implement all features of HSP
   and HFP profiles. Therefore if users want to read battery status,
   this application needs to implement it. If users want to handle
   headset buttons, this application needs to implement it. And if users
   want to do telephony operations, this application needs to implement
   whole telephony stack.

4) Wideband audio depends on HFP profile. Therefor 3), 2) and 1) must be
   solved if we want wideband high quality audio support for voice
   calls.

To solve these problems I proposed a new hsphfpd daemon which would
implement HSP and HFP profiles, therefore a new daemon which would own
rfcomm socket and would proxies AT commands (which could not resolve by
its own) to target applications. So telephony operations could be
implemented by one software (e.g. ofono), battery/power related by
another (e.g. upower) and audio by another (e.g. pulseaudio).

This design was rejected by ofono developers as they do not want to use
such proxy daemon. ofono already implements some parts of HFP profile
(but not HSP) and therefore is in the position of the "owner" of rfcomm
socket, like my design of hsphfpd. ofono already provides some API for
audio applications, but this API is not very suitable. I asked about
missing features and APIs which are designed and provided by hsphfpd,
but after a longer discussion ofono developer said that there are no
plans in ofono to implement missing features and APIs of HFP profile
which are currently missing in ofono. Also ofono's implementation of HFP
profile requires in computer to have connected and working cellular
modem, without it bluetooth HFP profile for bluetooth headsets does not
work. Pulseaudio has on wiki written some steps how to workaround this
limitation by usage of modem simulator, but ofono developers wrote that
this is hack and should not be used at all. And HSP profile is not
supported at all.

So conclusion from ofono discussion is: They do not want to support my
proposed solution via hsphpfd. And also they do not plan to implement
missing features of HFP profile to their HFP implementations, like usage
of bluetooth headset without connected cellular modem into computers,
support for HSP profile, support for custom HSP and HFP audio codecs,
support for battery and input buttons, etc...

So ofono is fully unusable for any HSP or HFP features of bluetooth
headsets on regular desktop or laptop computer with Linux.

If Linux desktop / laptop with pulseaudio want to support HFP profile
there are following options:

1) As written above, implement full HFP profile, therefore telephony
   stack in pulseaudio and handle all users features in pulseaudio
   (input devices, power devices, telephony features) including audio
   features (wide band support, custom codec support). In this setup
   pulseaudio would be incompatible with ofono and ofono must be stopped
   on that computer to prevent ofono from taking rfcom socket.

2) Delegate all non-audio features of HSP and HFP profiles from 1) to
   hsphfpd daemon and implement in pulseaudio only audio related
   features via DBus API provided by hsphfpd daemon. In this setup
   hsphfpd would own rfcom socket and via DBus API would communicate
   with other applications (e.g. pulseaudio, upower). This setup is
   incompatible with ofono, as ofono developers wrote that they do not
   want to use this design and because ofono implements own handling of
   HFP profiles, ofono daemon would need to be stopped on such machine
   to prevent ofono from taking rfcom socket. So telephony functions would
   not be supported until somebody write alternative telephony software
   which would connect to hsphfpd as ofono devs do not want to use
   hsphfpd.

3) In pulseaudio drop support for all desktop and laptop computers which
   do not have connected cellular modem compatible with ofono. In this
   way we could use ofono's HFP implementation for some basic audio
   stuff. But no additional features (like battery status or input
   buttons) would be provided. Also no custom codecs, etc.

4) In pulseaudio do not implement proper and full HFP profile support at
   all. Just say to users, that if they want to use bluetooth HFP
   headset, they have to change operating system from Linux to some
   other which implement it.

5) Like 4) but be silent and do not say anything to users. Do not answer
   to question from users about bluetooth HSP/HFP. Just do not do
   anything.

So as you can see there is no reasonable solution. Bluetooth rfcomm
socket would be owned either by ofono (and then there would be no
support for computers without cellular modem) or by other application
e.g. pulseaudio, hsphfpd, ... (and then ofono needs to be stopped and
telephony functions would not be probably supported in near future).

And now I would like to hear from you, pulseaudio developers/maintainers,
which option 1) - 5) you choose to solve problem with Bluetooth HSP and
HFP profiles, specially for wide band support, battery level support,
input event support, telephony support and etc.. So features which are
provided and supported by now all common Bluetooth headsets.

I'm willing to implement option 2). I have already implemented prototype
implementation of hsphfpd and it is already working. So missing part is
support from pulseaudio side. I can implement it and push pulseaudio
code via pull request or patch to mailing list. For pulseaudio it means
implementing just audio parts of HSP nad HFP profiles. Not telephony or
battery/power functions. If somebody is interesting in this option, help
me with this (either pulseaudio part of hsphfpd daemon itself), please
let me know.

On other options 1), 3), 4) or 5) I'm going to participate as I do not
think they bring any value to Linux desktop. And just cause another
problems.

So please, pulseaudio developers/maintainers, write what you think and
which option you choose and who would implement that option. Remember,
that silence means you automatically chose option 5) which would be rude
to all pulseaudio users.

Please note that this is not problem only for pulseaudio, but also for
any other audio software which want to support HSP/HFP on Linux.

-- 
Pali Rohár
pali.rohar at gmail.com
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