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Date: Mon, 17 May 2010 20:20:56 -0300
From: "Gustavo F. Padovan" <gustavo@padovan.org>
To: Elvis =?iso-8859-1?Q?Pf=FCtzenreuter?= <epx@signove.com>
Cc: =?iso-8859-1?Q?Jo=E3o?= Paulo Rechi Vita <jprvita@gmail.com>,
	jcaden@libresoft.es, linux-bluetooth@vger.kernel.org
Subject: Re: HDP proposed API(0.5)
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Hi Elvis,

* Elvis Pf?tzenreuter <epx@signove.com> [2010-05-17 19:02:33 -0300]:

> 
> On 17/05/2010, at 18:38, Gustavo F. Padovan wrote:
> 
> > Hi,
> > 
> > * Jo?o Paulo Rechi Vita <jprvita@gmail.com> [2010-05-17 18:17:58 -0300]:
> > 
> > <snip>
> > 
> >> On Mon, May 17, 2010 at 11:54, Jos? Antonio Santos Cadenas
> >> <jcaden@libresoft.es> wrote:
> >>> 
> >>>                array GetDataChannelFileDescriptor(uint16 mdlid)
> >>> 
> >> 
> >> Wouldn't be better to pass the fd through the LinkConnected call on
> >> Agent? Doing it here allows any process to get the fds.
> > 
> > I was thinking about the MCAP in kernel implementation, my conclusion is
> > that we can solve one of the biggest problem of HDP API if we do it in
> > kernel: the transparency of MCAP reconnection. HDP will open an MCAP
> > socket and pass it to the user(IEEE app). Then if we need a
> > reconnection, MCAP will transparently reconnect changing the L2CAP
> > channel to transmit the data without the need to change the socket
> > opened with the userspace.
> > 
> > Keep the MCAP socket open means that we are keeping the MCL state for
> > further reconnection.
> > 
> > Doing it inside the kernel removes a big amount of complexity of HDP and
> > IEEE profiles, since we won't need any pipe or change of fd. That could
> > be a killer feature to make it in the kernel.
> > 
> > Any comments? Is this reasonable?
> 
> 
> The fd passed to the application could actually be an UNIX socket, so these semantics could be implemented with MCAP in user space. I guess the pipes only appeared in discussion because of the possibility of zero-copy communication.

MCAP in kernel gives zero-copy communication and just one socket to the
HDP API. The better of both UNIX socket and pipes options.
 
> That semantics would be implemented in MCAP part. The HDP profile code itself is (or should be) free from these worries.
> 
> Moving MCAP to kernel hides the complexity from userspace HDP profile, but it continues to exist. Then I worry about more complex debugging, dependency on kernel version (or need to put efforts on backporting). MCAP sits above L2CAP, not besides it.

The complexity will exist anyway, I'm proposing a less complex option.
Is not that complex change the L2CAP channels used by the MCL inside the
kernel. Between all the proposed options to handle reconnection it
looks the less complex one.

We should not care about backporting now. We are working directly
with upstream and upstream is what really matters here. :)

You will depend on kernel version anyway since ERTM is inside the
kernel. About backport if you backport ERTM, then backport MCAP will be
easy.

Also MCAP will keep sitting on top of L2CAP, that won't change.

> 
> I acknowledge that it would work, but honestly I prefer to see things being moved to the outside of the kernel than to the inside :)

The real question here is where MCAP will fit better. And where it adds
less complexity to its users.

-- 
Gustavo F. Padovan
http://padovan.org