Hi Nick,
> > > > >> >> >> Right now Bluez always requests flushable ACL
> packets (but does not
> > > > >> >> >> set a flush timeout, so effectively they are
> non-flushable):
> > > > >> >> >>
> > > > >> >> >> However it is desirable to use an ACL flush
> timeout on A2DP packets so
> > > > >> >> >> that if the ACL packets block for some reason
> then the LM can flush
> > > > >> >> >> them to make room for newer packets.
> > > > >> >> >>
> > > > >> >> >> Is it reasonable for Bluez to use the 0x00 ACL
> packet boundary flag by
> > > > >> >> >> default (non-flushable packet), and let
> userspace request flushable
> > > > >> >> >> packets on A2DP L2CAP sockets with the socket
> option
> > > > >> >> >> L2CAP_LM_RELIABLE.
> > > > >> >> >
> > > > >> >> > the reliable option has a different meaning. It
> comes back from the old
> > > > >> >> > Bluetooth 1.1 qualification days where we had to
> tests on L2CAP that had
> > > > >> >> > to confirm that we can detect malformed packets
> and report them. These
> > > > >> >> > days it is just fine to drop them.
> > > > >> >>
> > > > >> >> Got it, how about introducing
> > > > >> >>
> > > > >> >> #define L2CAP_LM_FLUSHABLE 0x0040
> > > > >> >
> > > > >> > that l2cap_sock_setsockopt_old() sets this didn't
> give you a hint that
> > > > >> > we might wanna deprecate this socket options ;)
> > > > >> >
> > > > >> > I need to read up on the flushable stuff, but in
> the end it deserves its
> > > > >> > own socket option. Also an ioctl() to actually
> trigger Enhanced flush
> > > > >> > might be needed.
> > > > >> >
> > > > >> >> struct l2cap_pinfo {
> > > > >> >> ...
> > > > >> >> __u8 flushable;
> > > > >> >> }
> > > > >> >
> > > > >> > Sure. In the long run we need to turn this into a
> bitmask. We are just
> > > > >> > wasting memory here.
> > > > >>
> > > > >> Attached is an updated patch, that checks the LMP
> features bitmask
> > > > >> before using the new non-flushable packet type.
> > > > >>
> > > > >> I am still using L2CAP_LM_FLUSHABLE socket option in
> > > > >> l2cap_sock_setsockopt_old(), which I don't think you
> are happy with.
> > > > >> So how about a new option:
> > > > >>
> > > > >> SOL_L2CAP, L2CAP_ACL_FLUSH
> > > > >> which has a default value of 0, and can be set to 1
> to make the ACL
> > > > >> data sent by this L2CAP socket flushable.
> > > >
> > > > Was this proposal ok?
> > >
> > > Even SOL_L2CAP goes away. Use SOL_BLUETOOTH for this.
> > >
> > > > >> In a later commit we would then add
> > > > >> SOL_ACL, ACL_FLUSH_TIMEOUT
> > > > >> That is used to set an automatic flush timeout for
> the ACL link on a
> > > > >> L2CAP socket. Note that SOL_ACL is new.
> > > > >
> > > > > can I stop you right here (without even looking at the
> patch). We do
> > > > > have the generic SOL_BLUETOOTH that you should be
> using. So adding
> > > > > SOL_ACL is not a viable option at all.
> > > >
> > > > This would be in a later patch, and SOL_BLUETOOTH,
> ACL_FLUSH_TIMEOUT
> > > > is fine too, or whatever you prefer.
> > >
> > > Why not just use BT_FLUSHABLE and have it always take a
> timeout option
> > > and then 0 means not flushable. And advantage of having it
> separated?
> >
> > I think keeping them separate makes it clear that the flush
> timeout is
> > global for a given ACL link, whereas the
> flushable/non-flushable
> > boolean is specific to a L2CAP channel. (Which is why I
> suggested
> > introducing a new level SOL_ACL for the ACL_FLUSH_TIMEOUT
> option -
> > since this option applies at the ACL level in the stack).
> >
> > A specific advantage of this is that flushable packets can
> be enabled
> > without over-writing a previous flush timeout that was set
> on a
> > different L2CAP socket on the same ACL link. I guess this
> can also be
> > achieved with getsockopt() but that is racy.
>
>
> I am talking here about Enhanced Flush support and that would
> happen on
> a per ACL handle basis. So it actually almost applies on a per
> L2CAP
> socket level. Only exception is if you establish two or more
> L2CAP
> connections to the same remote device and set them all to
> flushable.
> Then of course all of them will be flushed. So strictly
> speaking it
> might be an ACL link feature, but we don't wanna use it that
> way. And in
> practice you won't have multiple concurrent flushable L2CAP
> connections
> to one remote device anyway.
>
>
> I agree that having 2 flush-able L2CAP channels to the same device
> would probably not be common. But who knows what new profiles the
> Bluetooth SIG will come up with that might also benefit from
> flush-able ACL data. And if a use-case comes up, then your proposed
> API will require programmers to write a racy getsockopt/setsockopt if
> they want to turn on flushing on one l2cap connection without
> affecting the ACL flush timeout set by another connection. Building
> race conditions into an API seems like a sub-optimal design choice.
are you expecting to change this frequently and from different parts of
the code during the lifetime of a socket. I just don't see that
happening at all actually. Either you create a "flushable" socket or you
don't. Fill me in on how you wanna actually use this feature.
> But its not worth arguing over. SOL_BLUETOOTH, BT_FLUSHABLE is fine
> (or BT_FLUSH_TIMEOUT instead).
I would call it BT_FLUSHABLE as of now. Since that is how the
specification calls it. However I do have to refresh my memory with the
actual details. I haven't read that part of the specification in a long
time.
Regards
Marcel
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