On Fri, Dec 18, 2009 at 6:05 PM, Marcel Holtmann <marcel@xxxxxxxxxxxx> wrote: > 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. >> >> My use case is just for A2DP. I turn on flushing with a timeout of say >> 160ms just before starting streaming of A2DP data, and turn it off >> when I finish. This is not a problem with either API proposal. > > I count this as creating socket, setting flushable and then using it. > Then closing it. And especially in A2DP case where the media socket is > brought up and taken down a lot that is a proper usage. However I do > expect that each socket should not change from flushable to > non-flushable in mid term usage. While potentially possible it don't see > its usage at all. > > So we could even force the flushable option into non-changeable after > the socket has been connected. Like changing the MTU afterwards makes no > sense. > >> Where it becomes a problem is if there is a reason to have two >> flush-able L2CAP connections to the same host. With your API proposal, >> the second connection has no way of turning on flushing without >> over-writing the flush timeout set by the first socket. You could >> implement another API to read the current flush timeout, and have the >> second socket read that API, but thats racy. >> >> If this is not a use-case you care about, then ok. But I just want to >> point out that this is a problem that will be baked into the API - and >> will require ugly workarounds in userspace as soon as someone requires >> 2 flushable L2CAP connections to one host. Given the rate at which >> Bluetooth changes and new profiles and use cases are added I would not >> be so quick to dismiss this use case. > > So my idea would actually be that every socket can has its own flush > timeout, but the core than picks the time to actually do the flushing of > packets. Also we can not have one socket change a socket option of > another one. It is a per socket option and not a global one. I think you are confused. This patch does not implement HCI Enhance Flush Command. The flush timeout that I am referring to is passed to the Bluetooth Chipset with the HCI Write Automatic Flush Timeout command. Which is why it is global for the ACL link. > > On other possible way would be to use CMSG details to inform sockets > about flushable packets. We have to see how useful that is. Since the > flushable is only useful for the time in between the packet is hold in > the Bluetooth chip buffers and hasn't been transmitted over the air. > Once the packet is on the air, there is nothing to flush anymore. And > with L2CAP ERTM this all becomes obsolete since we can flush at any time > anyway. The retransmission takes care of any accidental flush. > > Regards > > Marcel > > > -- To unsubscribe from this list: send the line "unsubscribe linux-bluetooth" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html