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From:   Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Date:   Mon, 10 Jun 2019 15:16:54 +0300
Message-ID: <CABBYNZJKmX-VutDjuT_q_n7VnMXLi8omqK4JU5Du8r3RFCsNwA@mail.gmail.com>
Subject: Re: bluez A2DP socket reliability
To:     =?UTF-8?Q?Pali_Roh=C3=A1r?= <pali.rohar@gmail.com>
Cc:     "linux-bluetooth@vger.kernel.org" <linux-bluetooth@vger.kernel.org>
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Hi Pali,

On Mon, Jun 10, 2019 at 1:51 PM Pali Roh=C3=A1r <pali.rohar@gmail.com> wrot=
e:
>
> On Friday 07 June 2019 18:23:41 Luiz Augusto von Dentz wrote:
> > Hi Pali,
> >
> > On Fri, Jun 7, 2019 at 4:00 PM Pali Roh=C3=A1r <pali.rohar@gmail.com> w=
rote:
> > >
> > > On Sunday 19 May 2019 14:22:23 Pali Roh=C3=A1r wrote:
> > > > On Sunday 19 May 2019 11:13:09 Luiz Augusto von Dentz wrote:
> > > > > Hi Pali,
> > > > >
> > > > > On Sat, May 18, 2019 at 11:12 PM Pali Roh=C3=A1r <pali.rohar@gmai=
l.com> wrote:
> > > > > >
> > > > > > Hello! How is L2CAP layer of bluetooth socket used for A2DP aud=
io
> > > > > > transfer configured in bluez? It is reliable with big/infinite
> > > > > > retransmit count? Or in best-effort manner and some packets may=
 be
> > > > > > dropped? And it is possible to change between these two modes f=
or
> > > > > > application which uses bluez DBUS API? I'm asking because some =
A2DP
> > > > > > audio codecs are designed to deal with packet loss and for thos=
e codecs
> > > > > > it would be probably better to configure L2CAP socket to unreli=
able
> > > > > > mode.
> > > > >
> > > > > We don't use ERTM with AVDTP, both signaling and transport socket=
s are
> > > > > using basic mode which don't support retransmissions, there the
> > > > > concept of flush timeout which iirc we don't currently it.
> > > >
> > > > On bluez.org site there is no information how to use bluez sockets =
and
> > > > the only documentation/tutorial which I found on internet was this:
> > > >
> > > >   https://people.csail.mit.edu/albert/bluez-intro/x559.html
> > > >
> > > > I do not know how up-to-date it is, but seems that by default bluez
> > > > L2CAP sockets are reliable and to change them to unreliable mode it=
 is
> > > > needed to issue OGF_HOST_CTL / OCF_WRITE_AUTOMATIC_FLUSH_TIMEOUT (0=
x28)
> > > > request. As default is zero =3D infinity =3D reliable connection.
> > > >
> > > > I do not understand low level bluetooth details, but is ERTM relate=
d to
> > > > OCF_WRITE_AUTOMATIC_FLUSH_TIMEOUT?
> > > >
> > > > So what are default settings for L2CAP socket used by AVDTP/A2DP
> > > > profiles which are transferred to user application via DBUS?
> > >
> > > Hi! Do you have any idea about OCF_WRITE_AUTOMATIC_FLUSH_TIMEOUT? It =
is
> > > related to ERTM or not?
> >
> > The OCF usually describes an HCI command which may affect the entire
> > ACL connection, ERTM is a L2CAP channel mode that includes
> > retransmissions. The A2DP stream transport doesn't ERTM so no
> > retransmissions shall take place.
>
> Fine, no retransmission is good for A2DP.
>
> And how it is with delivering packets? It is possible that may be
> delivered to other side in wrong order?

It is a single hop with guaranteed ordering at L2CAP channel, you can
control the priority on a per socket manner but afaik PA is already
setting that to high priority which helps when dealing with
congestions due to other traffic like OBEX, etc.

--=20
Luiz Augusto von Dentz