[pulseaudio-discuss] [PATCH v13 10/10] bluetooth: policy: Treat bi-directional A2DP profiles as suitable for VOIP

[Hyperion]

28.10.2019, 16:11, "Tanu Kaskinen" <tanuk at iki.fi>:
> On Sat, 2019-10-26 at 20:23 +0200, Hyperion wrote:
>>  26.10.2019, 14:39, "Tanu Kaskinen" <tanuk at iki.fi>:
>>  > On Sat, 2019-10-19 at 18:42 +0200, Pali Rohár wrote:
>>  > > On Saturday 19 October 2019 19:27:19 Tanu Kaskinen wrote:
>>  > > > On Sat, 2019-10-19 at 18:16 +0200, Pali Rohár wrote:
>>  > > > > On Saturday 19 October 2019 19:07:44 Tanu Kaskinen wrote:
>>  > > > > > On Sat, 2019-10-19 at 17:20 +0200, Pali Rohár wrote:
>>  > > > > > > On Friday 18 October 2019 15:29:43 Tanu Kaskinen wrote:
>>  > > > > > > > On Thu, 2019-10-17 at 15:34 +0200, Hyperion wrote:
>>  > > > > > > > > Regression would mean that some devices can't connect anymore : this
>>  > > > > > > > > won't happen if a workaround is provided, and this workaround won't
>>  > > > > > > > > be used often.
>>  > > > > > > > >
>>  > > > > > > > > Most (99% ?) of the devices will work correctly with my patch (many
>>  > > > > > > > > of them in XQ mode, and some in legacy mode because they will fall
>>  > > > > > > > > back to legacy bitpool during negociation)
>>  > > > > > > > >
>>  > > > > > > > > The remaining (1% ?) : will need a simple boolean swicth in one of
>>  > > > > > > > > the PA config files to restrict negociation to legacy bitpool (a
>>  > > > > > > > > module option ? or daemon.conf ?).
>>  > > > > > > > >
>>  > > > > > > > > I think it's really "simple", efficient, and not dependent of any
>>  > > > > > > > > upcoming Bluez feature.
>>  > > > > > > > >
>>  > > > > > > > > "The complex solution is always the best until one find a simpler one"
>>  > > > > > > >
>>  > > > > > > > I don't know the number of users who use bluetooth headsets with
>>  > > > > > > > PulseAudio, but even just 1% regression rate can mean quite a few
>>  > > > > > > > unhappy users. When your headset suddenly stops working, it's not
>>  > > > > > > > trivial to figure out that you may need to pass a special argument to
>>  > > > > > > > module-bluetooth-discover in order to make it work again.
>>  > > > > > > >
>>  > > > > > > > It would be better to have a module argument to enable the XQ settings.
>>  > > > > > >
>>  > > > > > > Main question, do we really need this special "settings"? Because my
>>  > > > > > > patch series introduce also SBC XQ profile and basically replaces above
>>  > > > > > > module parameter, by runtime configuration.
>>  > > > > > >
>>  > > > > > > For me above solution looks like a hack. It adds some module parameter
>>  > > > > > > for tweaking configuration. And what would happen with that parameter
>>  > > > > > > after we have "proper" support for multiple codecs? Do we need to
>>  > > > > > > maintain backward compatibility? Or would we remove that configuration
>>  > > > > > > and therefore revert to state prior existence of new module parameter
>>  > > > > > > (which is current situation)?
>>  > > > > >
>>  > > > > > After your patches there's still the "automatic bitpool" mode
>>  > > > > > available, right?
>>  > > > >
>>  > > > > Yes, I wanted to have it there for legacy/backward compatibility reasons
>>  > > > > for those devices which could be broken with new settings. That is the
>>  > > > > reason I do not wanted to touch Automatic mode, to have exact same
>>  > > > > behavior as in current (and older) pulseaudio versions.
>>  > > > >
>>  > > > > But if automatic mode is going to be changed, I do not see reason for
>>  > > > > keeping it (the argument for backward compatibility would not apply
>>  > > > > anymore, if it is going to be changed). My patch series with new A2DP
>>  > > > > API can fully replace that automatic mode.
>>  > > >
>>  > > > I don't see how the proposed option changes anything about
>>  > > > compatibility. The option will be disabled by default, so the default
>>  > > > behaviour will be the same as always.
>>  > >
>>  > > And what should happen after support for multiple A2DP codecs (from my
>>  > > patch series) would be there? Basically it obsoletes that config option.
>>  > > As all such settings can be set at runtime.
>>  >
>>  > If the "enable XQ in the automatic bitpool mode" option is made run-
>>  > time configurable, then yes, the module argument will become obsolete.
>>
>>  I don't think so : by default Pali's patch first connects using one of the 2 XQ modes : so
>>  it will fail with devices not "XQ able". Since we are having this discussion only because
>>  we don't want ANY regression (even as rare as the devices that can't do bitpool 38 dual
>>  , and even for users who don't know how to switch SBC modes) : then the default mode
>>  of operations shouldn't allow XQ at all.
>
> I think we're talking about different things here.
>
> A mode for automatic bitpool is needed and will have to be the default.
> XQ bitpool values need to be disabled by default. An option for
> enabling XQ bitpool values in the automatic mode would be useful, so I
> hope we will have that.
>
> When I said that "the module argument will become obsolete" didn't mean
> that the XQ option itself would become obsolete. If the option can be
> configured at run-time, for example with "pactl enable-sbc-xq true" or
> with pavucontrol, then that will not be implemented via a module
> argument. It will be implemented using the message API (which is still
> under review), and the option value will be stored in a database.
> Having a module argument doesn't add any value in this scheme, which is
> why I said the module argument will become obsolete.
>
> This depends on someone actually implementing the interface for setting
> the XQ option, however. I hope someone will promise to do that. It
> could be you, or it could be Pali. I'm not promising to do it myself.
> If nobody promises to implement that, then I'm going to accept a module
> argument, because having something is better than having nothing.

It's not obvious to get details of the A2DP protocol without reading the RFC, 
so I'm going to give a few tracks :

- Bluetooth packet size is not constant : BT traffic is splited in radio slots 
that have fixed time lengh : so for a given bandwidth : slots size vary
- Bitpool is dynamically reduced when the bandwidth decrease : for example
when the emitter go away from the receiver. 
- Bitrate is related to bitpool but depends on other parameters (see the attached sheet)
- Bluetooth A2DP emiters (SRC) usually specify a maximum value for Bitpool. 
This value is not the value that will be negociated but rather a 
highest negociation value.
- SBC XQ is standard SBC codec operating at high bitrates and thus reaching 
the transparent audio transport quality of AptX (HD) . It can be achieved either 
in STEREO MODE, with bitpool ~ 76, or in DUAL CHANNEL MODE, with bitpool ~ 38 
per channel (simplified)
- "Legacy automatic" is the barbarous name I gave to the traditional negociated algorithm 
with max bitpool=53 and STEREO MODE prefered to DUAL CHANNEL MODE : it is implemented 
in current PA
- "XQ automatic" is the barbarous name I gave to the negociated algorithm 
implemented in my patch with max bitpool=38 per channel, and DUAL CHANNEL 
prefered to STEREO MODE (simplified)
- "Forced XQ" modes are several algorithms implemented by Pali with fixed 
bitpool values : not negociated : if the receiver can do the fixed bitpool it works, else 
it fails (simplified)

Now the "quaity switch" idea :
The "quaity switch" (aka module param or pactl API call, all good for me) is a way 
to switch between "Legacy automatic" and something else giving better quality. 
The "something else" could be :
- "XQ automatic" + several "Forced XQ" modes (my latest idea, relies on Pali's code + my code +  "quaity switch")
- "XQ automatic" alone (my former idea, relies on my "simple" patch +  "quaity switch")
- several "Forced XQ" modes alone (Pali's idea, relies on Pali's code +"quaity switch")

Pali : please fix the above in case I did some mistakes 
JP

>
> --
> Tanu
>
> https://www.patreon.com/tanuk
> https://liberapay.com/tanuk
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