ipc-sock/l2cap sock/g_io_fd

ipc-sock/l2cap sock/g_io_fd

[Pavan Savoy]

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Marcel,

I am sort of lost among the various sockets that are in the bluez/audio, and trying to get a clear picture on what is what.

find attached the short image on my understanding of the audio structure (currently used by apps which use liba2dp and not the pcm plugins..- namely android..)

Is my understanding  correct ?
What does the various sockets represent, as in stream.fd, server.fd - and the fd extracted from the session_io.

I sort of need to know which is which, as in which is communication over local IPC BT_IPC_SOCKET_NAME, which actually corresponds to the l2cap connection which is established in sink.c ?

How does it get into the sesssion->io ? Please help me understand these various sock fd's.

Thanks in advance..


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Re: ipc-sock/l2cap sock/g_io_fd

[Johan Hedberg]

Hi Pavan,

On Tue, Jul 13, 2010, Pavan Savoy wrote:
> Marcel,

I don't know why you expect specifically Marcel to have some clue about
the audio stuff. He hasn't been much involved with the development of it
so I don't excpect you to get a reply from him ;)

> I am sort of lost among the various sockets that are in the
> bluez/audio, and trying to get a clear picture on what is what.
> 
> find attached the short image on my understanding of the audio
> structure (currently used by apps which use liba2dp and not the pcm
> plugins..- namely android..)
> 
> Is my understanding  correct ?
> What does the various sockets represent, as in stream.fd, server.fd -
> and the fd extracted from the session_io.
> 
> I sort of need to know which is which, as in which is communication
> over local IPC BT_IPC_SOCKET_NAME, which actually corresponds to the
> l2cap connection which is established in sink.c ?
> 
> How does it get into the sesssion->io ? Please help me understand
> these various sock fd's.

So I'd say there are three key sockets with A2DP. You've got the
signalling channel for A2DP (I think that's what you mean when you say
session->io). That socket stays always inside bluetoothd. Then there's a
local unix socket which is used to communicate with an external audio
process (alsa, gstreamer, pulseaudio, whatever). I suspect that'd match
the server.fd which you mentioned. Then, for every A2DP media transport
channel we transfer the socket over to the external audio process which
then takes care of writing (and reading in the case of SCO) of it.
That's what stream.fd is referring to.

With BlueZ 5.0 we're gonna scrap the unix socket completely since D-Bus
nowdays has support for file descriptor transfer from one process to
another. There's already a prototype patch for it but it hasn't been
merged upstream yet. Hopefully getting rid of the extra homebrewed IPC
should help simplify the picture a little bit.

Johan

Re: ipc-sock/l2cap sock/g_io_fd

[Pavan Savoy]

=0A--- On Tue, 13/7/10, Johan Hedberg <johan.hedberg@gmail.com> wrote:=0A=
=0A> From: Johan Hedberg <johan.hedberg@gmail.com>=0A> Subject: Re: ipc-soc=
k/l2cap sock/g_io_fd=0A> To: "Pavan Savoy" <pavan_savoy@yahoo.co.in>=0A> Cc=
: linux-bluetooth@vger.kernel.org, "Marcel Holtmann" <marcel@holtmann.org>=
=0A> Date: Tuesday, 13 July, 2010, 6:27 PM=0A> Hi Pavan,=0A> =0A> On Tue, J=
ul 13, 2010, Pavan Savoy wrote:=0A> > Marcel,=0A> =0A> I don't know why you=
 expect specifically Marcel to have=0A> some clue about=0A> the audio stuff=
. He hasn't been much involved with the=0A> development of it=0A> so I don'=
t excpect you to get a reply from him ;)=0A=0AIsn't Marcel =3D GOD for blue=
tooth on linux ?=0AAlthough I did put it to the list.=0A=0A> > I am sort of=
 lost among the various sockets that are=0A> in the=0A> > bluez/audio, and =
trying to get a clear picture on what=0A> is what.=0A> > =0A> > find attach=
ed the short image on my understanding of=0A> the audio=0A> > structure (cu=
rrently used by apps which use liba2dp=0A> and not the pcm=0A> > plugins..-=
 namely android..)=0A> > =0A> > Is my understanding=A0 correct ?=0A> > What=
 does the various sockets represent, as in=0A> stream.fd, server.fd -=0A> >=
 and the fd extracted from the session_io.=0A> > =0A> > I sort of need to k=
now which is which, as in which is=0A> communication=0A> > over local IPC B=
T_IPC_SOCKET_NAME, which actually=0A> corresponds to the=0A> > l2cap connec=
tion which is established in sink.c ?=0A> > =0A> > How does it get into the=
 sesssion->io ? Please help=0A> me understand=0A> > these various sock fd's=
.=0A> =0A> So I'd say there are three key sockets with A2DP. You've=0A> got=
 the=0A> signalling channel for A2DP (I think that's what you mean=0A> when=
 you say=0A> session->io). That socket stays always inside=0A> bluetoothd. =
Then there's a=0A> local unix socket which is used to communicate with an=
=0A> external audio=0A> process (alsa, gstreamer, pulseaudio, whatever). I =
suspect=0A> that'd match=0A> the server.fd which you mentioned. Then, for e=
very A2DP=0A> media transport=0A> channel we transfer the socket over to th=
e external audio=0A> process which=0A> then takes care of writing (and read=
ing in the case of SCO)=0A> of it.=0A> That's what stream.fd is referring t=
o.=0A=0AVery nice to know, thanks a lot.=0A=0A> With BlueZ 5.0 we're gonna =
scrap the unix socket completely=0A> since D-Bus=0A> nowdays has support fo=
r file descriptor transfer from one=0A> process to=0A> another. There's alr=
eady a prototype patch for it but it=0A> hasn't been=0A> merged upstream ye=
t. Hopefully getting rid of the extra=0A> homebrewed IPC=0A> should help si=
mplify the picture a little bit.=0A=0AYes, So as I understand, the func "bt=
_audio_service_get_data_fd" helps us pass around the stream fd to another p=
rocess, which can use it for writing, the CMSG thingy ??=0A=0AWhere exactly=
 is it filled up to do a recvmsg ? As in somewhere I should have found a me=
mcpy(CMSG_DATA, &stream.fd, sizeof(int)) ?=0A=0AGreat, fd passing around ov=
er D-Bus, This info, solves my other problem too :) Will look it up ..=0A=
=0A> Johan=0A> =0A=0A