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From: "Pali Rohár" <pali.rohar@gmail.com>
To: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Cc: "Gix, Brian" <brian.gix@intel.com>,
	"iam@valdikss.org.ru" <iam@valdikss.org.ru>,
	"linux-bluetooth@vger.kernel.org"
	<linux-bluetooth@vger.kernel.org>
Subject: Re: Determinate EDR speed
Date: Sun, 13 Oct 2019 09:36:03 +0200
Message-ID: <20191013073603.6jnczqr4fk7lpyxq@pali> (raw)
In-Reply-To: <CABBYNZL8u2kUnPqiA56DtZyAPZEAUQBaBOC-wuqUQmOW6ZJ=Og@mail.gmail.com>

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On Saturday 12 October 2019 10:23:58 Luiz Augusto von Dentz wrote:
> Hi Pali,
> 
> On Fri, Oct 11, 2019 at 11:07 PM Pali Rohár <pali.rohar@gmail.com> wrote:
> >
> > On Friday 11 October 2019 19:05:56 Gix, Brian wrote:
> > > On Fri, 2019-10-11 at 19:00 +0000, Gix, Brian wrote:
> > > > Hi Pali,
> > > >
> > > > On Fri, 2019-10-11 at 20:35 +0200, Pali Rohár wrote:
> > > > > Currently bluez API, method Acquire() already inform called application
> > > > > what is socket MTU for input and output. So from this information it is
> > > > > possible to detect if device supports EDR 3 or not.
> > > > >
> > > > > But it is too late to have this information. I need to send SBC
> > > > > parameters to bluez first when doing A2DP negotiation, this is early
> > > > > steps before Acquire() is called.
> > > >
> > > > This seems to be the kind of information which is fixed, for the life of the pairing.
> > > >
> > > > What if you assumed the lower speed the first time you connected, determined the
> > > > speed during the first streaming, and then either immediately renegotiate (caching the identifying
> > > > information
> > > > of the SNK), or just cache the information for future connections.
> > > >
> > > > Or the reverse, and assume fast, but immediately adjust down if you aren't getting what you hoped for.
> > > >
> > > > In any case, this would be a "Device Setup" glitch which you could note as a routine part of pairing in the
> > > > documentation.
> > >
> > > Or, Stream "Silence" the first time you connect, in order to determine throughput.  It would add 1-2 seconds to
> > > your connection time perhaps, but would be less noticable to the user.
> >
> > This increase connection time, increase complexity of implementation
> > (lot of things can fail) and just complicate lot of things there. Plus
> > adds that glitch which is not user friendly.
> >
> > Also bluetooth devices, like headsets, probably do not expects that
> > somebody is going to do such thing and we can hit other implementation
> > problems...
> >
> > And moreover it is just big hack and workaround for that problem. Not a
> > reasonable solution.
> >
> > In btmon I can see it, so kernel already knows that information. Why it
> > cannot tell it to userspace and bluetooth daemon to client application?
> >
> > Client application (e.g. pulseaudio) should really know if is going to
> > talk with bluetooth device with EDR 2 or EDR 3.
> >
> > > >
> > > > > Therefore I'm asking for some way how to get information if device
> > > > > supports EDR 2 or EDR 3. This is basically requirement for proper
> > > > > implementation of SBC in high quality mode. So if there are not such API
> > > > > yet, could it be exported from kernel to userspace and bluetoothd
> > > > > daemon?
> > > > >
> > > > > See these two articles for more details about SBC and its high quality:
> > > > >
> > > > > https://habr.com/en/post/456182/
> > > > > http://soundexpert.org/articles/-/blogs/audio-quality-of-sbc-xq-bluetooth-audio-codec
> > > > >
> > > > > > > Is there any bluez API for it?
> > > > > > >
> 
> There quite a few assumption here that are not really how it is
> implemented in BlueZ:
> 
> 1. The L2CAP MTU is not based on the ACL link (should be relatively
> easy to change)
> 2. L2CAP MTU is not required to be symmetric.
> 3. Since the ACL link is shared for all channels we shouldn't really
> assume all throughput will be available
> 4. PA flow control is not just filling up packets and sending them
> when they are full to maximize speed, instead it send packets when
> necessary to maintain a constant speed so the MTU may not be fully
> used, in fact trying to maximize the MTU may result in higher latency
> since packets would be sent less frequently.
> 
> With this in mind I think the only thing we should look into is to
> adjust the default L2CAP MTU to match the underline ACL Link, so it
> maximizes throughput, the remote side may choose a different MTU which
> will have to follow though.

Hi Luiz! The main problem is not MTU size, as I wrote I cannot use it
normally for distinguish for usage of SBC XQ or not. Instead of MTU I
rather need to know if device supports EDR 2 or EDR 3.

-- 
Pali Rohár
pali.rohar@gmail.com

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  reply index

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-11  8:27 Pali Rohár
2019-10-11 18:15 ` Gix, Brian
2019-10-11 18:35   ` Pali Rohár
2019-10-11 19:00     ` Gix, Brian
2019-10-11 19:05       ` Gix, Brian
2019-10-11 20:04         ` Pali Rohár
2019-10-12  7:23           ` Luiz Augusto von Dentz
2019-10-13  7:36             ` Pali Rohár [this message]
2019-10-13  8:45               ` Luiz Augusto von Dentz
2019-10-13  9:39                 ` Pali Rohár
2019-10-13  9:49                   ` Luiz Augusto von Dentz
2019-10-16 19:06                   ` Marcel Holtmann
2019-10-16 19:13                     ` Pali Rohár
2019-10-19 19:47                       ` Marcel Holtmann
2019-10-16 19:05           ` Marcel Holtmann

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