The premise is that my ignorance on BT is complete. After some
investigation, it is my understanding that RFCOMM channels are to RFCOMM
communication a bit like ports in TCP and UDP, with the main difference
that while there are really a lot of TCP/UDP ports and some of them are
reserved as "well known ports", the number of channels in RFCOMM is
extremely limited, so that there cannot be well known ports.
Furthermore, I understand that no one should "hardwire" RFCOMM channels
and that channels should be released whenever not strictly needed.

If this understanding from myself is correct, there may be issues both in LibO 
and in PA.
>From my understanding:

- LibO and the Impress remote should not hardwire RFCOMM channel 3. I
understand that they should dynamically allocate a channel and make it
discoverable via SDP.

- Impress should not keep binded to an RFCOMM channel all the time.
There should be a visible option to switch on and off the binding
(probably there should be a couple of impress remote related options in
the "Slide Show -> Slide Show Settings" window, including one to fire up
the BT rfcomm binding).

- PA should probably similarly not hardwire channel 3 for its HSP role,
but here the situation seems more complex, because it may be the case
that PA needs to strive for compatibility with devices that do not play
to the rules.

- PA should probably have a DBUS interface to switch on and off the
RFCOMM binding, so that to use an application needing RFCOMM channel 3,
one does not need to take down the whole of PA.

I have opened a bug on PA too. See it at
https://gitlab.freedesktop.org/pulseaudio/pulseaudio/issues/827.  That
report is bad, because when I made it my understanding of BT was even
poorer than it is now.

In any case, it seems to me that the current issue could be fixed
completely on the LibO side, by dynamically allocating the channel and
by letting the app running on the phone discover it via SDP.

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https://bugs.launchpad.net/bugs/1798400

Title:
  [upstream] Regression: cannot use impress remote over bluetooth with
  ubuntu bionic

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