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

[Pali Rohár]

Hello!

On Tuesday 18 February 2020 09:42:38 Georg Chini wrote:
> On 15.02.20 22:33, Pali Rohár wrote:
> > 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.
> There are several patch sets on gitlab and on the mailing list that
> prove that you don't need half the telephony stack. Yes, you need
> some of it but I think you overestimate what is really needed.

These patches does not work and completely break support for some
headsets which are currently working fine. Examples of such headsets are
from Creative Labs company. I already wrote it to pull request on
gitlab.

So no, this is not a solution, breaking support for headsets which are
currently working fine.

> > 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.
> Again I don't agree. There is no need to handle the whole telephony
> stack if you only want headset support.

Unfortunately for some headsets it is needed :-(

> > 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.
> 
> This is not true. You can disable the ofono headset support selectively
> in ofono, so ofono could still handle telephony while PA handles headsets.

Ok, so if this is truth that you can disable ofono <--> bluetooth
support then you can still use your cellular modem to work with ofono
(which is great!), but obviously you loose telephony support on HFP
bluetooth profile. And therefore without telephony support on HFP you
cannot use HFP nor HFP audio. So problem is still there, but it allows
you to have started both ofono and other tool which would implement
telephony stack for HFP.

> > 
> > 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).
> 
> Well, the reasonable solution is to implement HFP headset support
> in PA and let ofono do the telephony bits.

This is only possible with cooperation with ofono as application owns
rfcom bluetooth socket must (currently ofono) must export all needed
APIs for other application. But ofono developers said that this is not
currently planned.

Yes, it is really reasonable solution, but it is not possible right now.

> HSP is already handled
> in PA and ofono does not implement it, so I see no issue there.

Issue is that PA's implementation does not support additional features
(like custom codecs or event buttons or power status). And AG role is
broken (there is broken implementation of handling master listening
bluetooth socket).

> In addition to pure audio, PA can handle headset related features
> like battery status, button press and display messages.

Yes, this is possible, but these custom features are same in HSP an HFP
profiles. It means you would need to implement them on two different
places (where would be handled HSP profile and where HFP). And I do not
like this solution.

> 
> > 
> > 
> > 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.
> > 
> Overall I think the problem is not so big as you describe it.
> I do not see an issue in the co-existence of ofono for telephony
> and PA for headsets. In PA, we only need to implement those
> additional features which are commonly used. Yes, we will not
> be able (and will not want) to support everything but is that really
> a problem?

I think you do not see the main problem. You cannot do telephony
functions in one application and audio related in another and
power/button/display in another. Only application which own rfcomm
bluetooth socket can do these operations and therefore one application.
And you cannot share rfcom socket to more applications as it is
stateful. So what you wrote is not possible to implement. So this is a
big problem.

To allow splitting functionality between more applications I designed
hsphfpd daemon which acts like proxy for rfcomm bluetooth socket and
export needed functionality via DBus APIs to more applications, like you
described above. Please look at my email about hsphfpd where I written
more details about it.

-- 
Pali Rohár
pali.rohar at gmail.com