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

[Pali Rohár]

On Tuesday 18 February 2020 11:06:03 Georg Chini wrote:
> On 18.02.20 10:34, Pali Rohár wrote:
> > 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.
> They are at least nearly working. Yes, they have some flaws
> but with some work on them it can surely be improved.

They have important flaws, after they are integrated they completely
breaks support for headsets which are currently working. This is really
no-go.

> Work
> on the implementation is needed anyway and the old patches
> on patchwork and the current ones on gitlab provide a good
> starting point because a lot of the work is already done.
> > 
> > > > 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 :-(
> I can't believe that.

Me too. But I already spend 3+ months in this area. During implementation
of hsphfpd and debugging more headsets I saw how they works, what they
implements and how to use it.

> Maybe you have to supply some dummy answers
> to certain AT commands, but it should be doable.
> > 
> > > > 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.
> You misunderstand me. You can selectively disable headset
> support in ofono while keeping the other role fully functional.

Disabling headset support means that ofono would not handle bluetooth
headset. So?

> > > > 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.
> 
> As said above, you can simply disable headset support. You
> just have to specify --noplugin=hfp_ag_bluez5 on the ofono
> command line.

Yes, so then ofono would not handle HFP bluetooth profile.

Therefore you loose support for telephony operations in HFP bluetooth
profile via ofono. This is obvious.

> > 
> > > 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).
> Since when is the HSP AG role broken? It worked (at least for me)
> about two years ago. And support for additional features
> can be added. It must be done anyway somewhere, whichever
> solution you want to implement.

It it used if you want to act as "bluetooth headset". So other
"pulseaudio" notebooks can connect you to as they would think you are
"headset". It is not so commonly used.

I already implemented it in hsphfpd and I know how to fix it in
pulseaudio. For me it is now simple, just I did not wanted to do it
until all problems with HSP/HFP are resolved.

> > 
> > > 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.
> I don't think this is true. The additional features are handled via
> the RFCOMM channel and that channel is the same in both cases.

Seems you misunderstood the point. The point is that if you implement
HFP profile in one application and HSP in second application, then you
need to implement common functionality on two different places.

So if HSP profile stay implemented in pulseaudio, you would have to
implement those missing functionality in pulseaudio.
And if HFP profile is implemented in ofono (or other application) then
missing functionality must be implemented in that ofono application.

So you would have implementation of e.g. power supply in two different
locations (for HSP in pulseaudio, for HFP in ofono). And this
functionality is basically same, so you would have to implement same
logic on two different places.

If you would need to extend it, you would have to do it again on two
different places.

In hsphfpd design, I implemented it on one place, in one daemon, which
shares common functions/code.

> Because HSP is a subset of HFP, you can use the same RFCOMM
> handling, in the worst case with a flag to distinguish between HSP
> and HFP.
> > 
> > > > 
> > > > 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.
> 
> The RFCOMM channel is created when the profile is connected,
> so it should be possible to have one RFCOMM to device A in AG role
> and another one to device B in HS role and have ofono handle one
> device while PA handles the other.

Seems you again misunderstood the main problem. Please look at my
hsphppfd email where I already tried to explain it. Via rfcomm socket
are handled all functionality of *one* device. I'm taking about one HFP
device which provides power supply, input event, text display, telephony
and audio functions. And all this can be handled only by application
which owns rfcomm socket, application which implements HFP profile.

> 
> > 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.
> > 
> Yes, but it does not make sense to implement hsphfpd only for PA.

I wrote email about hsphfpd to other teams like bluez-alsa or pipewire
and they can benefit from it. Otherwise they would have to reimplement
and reimplement whole bluetooth related parts again and again. And
extending it for new futures...

> Then you can implement things directly in PA.

But why on the earth you would implement power supply, input event
handling, text displaying function, phone dialing functionality and
other things in audio server = pulseaudio?

<sarcasm> Here I would suggest: lets implement WWW browser in
pulseaudio. For sure that it has a great oportunity to have both text
displaying functionality and browsing WWW pages via pulseaudio. Then we
can continue with init system. Yes, pulseaudio should really implements
own init system as there is still war between systemd and non-systemd
people and now pulseaudio is a good candidate which can finally solve
it. </sarcasm>

-- 
Pali Rohár
pali.rohar at gmail.com