Re: [PATCH v3 6/6] a2dp: Reword LastUsed

["Pali Rohár" <pali.rohar@gmail.com>]

On Tuesday 07 May 2019 20:57:40 Pali Rohár wrote:
> On Tuesday 07 May 2019 13:13:17 Luiz Augusto von Dentz wrote:
> > Hi Pali,
> > 
> > On Tue, May 7, 2019 at 11:52 AM Pali Rohár <pali.rohar@gmail.com> wrote:
> > >
> > > On Monday 06 May 2019 15:02:25 Pali Rohár wrote:
> > > > On Monday 06 May 2019 15:46:03 Luiz Augusto von Dentz wrote:
> > > > > Hi Pali,
> > > > >
> > > > > I hope this fixes the problems you have been seeing, it should at
> > > > > least take care of the issues with wrong order of SelectConfiguration
> > > > > and restoring the exact same endpoint used last time.
> > > >
> > > > Hi Luiz! Great, this should make bluez to be more deterministic.
> > >
> > > Hi! Now I looked at it and in cache file I see:
> > >
> > > LastUsed=04:01
> > >
> > > What would happen when A2DP application (e.g. pulseaudio) register DBus
> > > endpoints in different order? Or e.g. do not register some endpoints due
> > > to missing codec librayr (aptX).
> > >
> > > I guess that in this case LastUsed information stops working...
> > 
> > It would most likely fail at SelectConfiguration and then try with the
> > other endpoints.
> 
> Yes, that is truth. My point is just about validity of LastUsed value.
> 
> > >
> > > Should not be there stored rather dbus endpoint name identifier?
> > 
> > Originally I tried to avoid having the local endpoints because of this
> > problem, but now that SelectConfiguration can fail it shouldn't really
> > be a problem. At least with the seid if you have a system that didn't
> > changed the order or number of endpoints it will keep working as
> > expected,
> 
> But this may happen. And such thing is allowed. Any application,
> including unprivileged can register own new endpoint to bluez. It is not
> specific to pulseaudio. And in my opinion central bluetooth daemon which
> expose such functionality should be robust and be prepared that
> application on "other side of dbus IPC" does not have to be well
> behaved.
> 
> > if we choose to encode the D-Bus connection, etc, as soon as
> > PA is restarted, the system is rebooted, etc, the D-Bus connection may
> > have changed making the stored values in LastUsed invalid.
> 
> That is truth.
> 
> My idea was to encode just dbus path of local dbus endpoint. We can say
> (in IPC API) that client application should preserve dbus path for one
> same endpoint between dbus daemon / computer restarts. Because otherwise
> functionality of "choose last used endpoint" would not work.
> 
> This is less strict requirement and current one (that registration order
> of all existing applications in system must be same across reboots),
> less error prone and still should be easily implemented.

So what about this my idea? Is there anything wrong with such thing?

> > Anyway
> > chances are the LastUsed is only invalidated if you update PA, in
> 
> Or once we include support for dynamic codec loading (based on encoder
> library presence in system), installation of any irrelevant application
> may bring a new supported codec and therefore a new endpoint. So codecs
> can become in any order...
> 
> Or another example, when different application (not PA) register also
> some endpoint.
> 
> > which case there could be new endpoints or a change in their order and
> > package can also provide a script to clear the LastUsed if that
> > happens, but then again LastUsed setting does not actually store the
> > configuration just the endpoint so SelectConfiguration has control
> > over the configuration.
> 
> Yes, whole thing is only about initial codec selection. In the worst
> case user would have pre-selected different codec as which was last
> used.
> 
> But I think that storing dbus path of endpoint as described above should
> be more reliable solution.
> 
> Applications (e.g. PA) talking to bluetooth daemon does not register or
> request directly SEID. Instead they register their dbus path and dbus
> connection and bluetooth daemon later allocates SEID for that path.
> 

-- 
Pali Rohár
pali.rohar@gmail.com