Return-Path: MIME-Version: 1.0 In-Reply-To: <1261180228.4041.111.camel@localhost.localdomain> References: <35c90d960912081950t135e3f10m8848e54fde1e596f@mail.gmail.com> <1260335175.2901.20.camel@violet> <35c90d960912082213s26fb0ebse75ce85d43213d9@mail.gmail.com> <1260482634.2901.70.camel@violet> <35c90d960912161359u2b3f9b2fi875288896a7a8478@mail.gmail.com> <1261006596.4041.39.camel@localhost.localdomain> <35c90d960912161548p6cdcc1f0i7d74b31a4bc145b6@mail.gmail.com> <1261177540.4041.106.camel@localhost.localdomain> <35c90d960912181523n1067f87cw1d585a17ba57475a@mail.gmail.com> <1261180228.4041.111.camel@localhost.localdomain> From: Nick Pelly Date: Fri, 18 Dec 2009 16:16:51 -0800 Message-ID: <35c90d960912181616r691fcecau1e1ca8322b2fc4f5@mail.gmail.com> Subject: Re: RFC: Allow Bluez to select flushable or non-flushable ACL packets with L2CAP_LM_RELIABLE To: Marcel Holtmann Cc: linux-bluetooth@vger.kernel.org Content-Type: text/plain; charset=ISO-8859-1 List-ID: Hi Marcel, On Fri, Dec 18, 2009 at 3:50 PM, Marcel Holtmann wrot= e: > > 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 A2D= P 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 boun= dary 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 L= 2CAP 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 h= int that > > > > >> > we might wanna deprecate this socket options ;) > > > > >> > > > > > >> > I need to read up on the flushable stuff, but in the end it de= serves its > > > > >> > own socket option. Also an ioctl() to actually trigger Enhance= d flush > > > > >> > might be needed. > > > > >> > > > > > >> >> struct l2cap_pinfo { > > > > >> >> =A0 =A0... > > > > >> >> =A0 =A0__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 bitma= sk > > > > >> 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 w= ith. > > > > >> 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 addin= g > > > > > SOL_ACL is not a viable option at all. > > > > > > > > This would be in a later patch, and SOL_BLUETOOTH, ACL_FLUSH_TIMEOU= T > > > > is fine too, or whatever you prefer. > > > > > > Why not just use BT_FLUSHABLE and have it always take a timeout optio= n > > > 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=A0flush-able=A0L2CAP 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=A0ACL data. And when this happens, with your proposed API, programmers will not be able to avoid a race condition between getsockopt() then setsockopt() if they want to turn on flushing without changing the ACL flush timeout. But its not worth arguing over.=A0SOL_BLUETOOTH, BT_FLUSHABLE is fine (or BT_FLUSH_TIMEOUT instead). I won't be doing development for a few weeks over the holidays, so I can not update this patch for a while. Nick