On Wednesday 16 October 2019 21:06:41 Marcel Holtmann wrote: > Hi Pali, > > >>>>>>>> Currently bluez API, method Acquire() already inform called application > >>>>>>>> what is socket MTU for input and output. So from this information it is > >>>>>>>> possible to detect if device supports EDR 3 or not. > >>>>>>>> > >>>>>>>> But it is too late to have this information. I need to send SBC > >>>>>>>> parameters to bluez first when doing A2DP negotiation, this is early > >>>>>>>> steps before Acquire() is called. > >>>>>>> > >>>>>>> This seems to be the kind of information which is fixed, for the life of the pairing. > >>>>>>> > >>>>>>> What if you assumed the lower speed the first time you connected, determined the > >>>>>>> speed during the first streaming, and then either immediately renegotiate (caching the identifying > >>>>>>> information > >>>>>>> of the SNK), or just cache the information for future connections. > >>>>>>> > >>>>>>> Or the reverse, and assume fast, but immediately adjust down if you aren't getting what you hoped for. > >>>>>>> > >>>>>>> In any case, this would be a "Device Setup" glitch which you could note as a routine part of pairing in the > >>>>>>> documentation. > >>>>>> > >>>>>> Or, Stream "Silence" the first time you connect, in order to determine throughput. It would add 1-2 seconds to > >>>>>> your connection time perhaps, but would be less noticable to the user. > >>>>> > >>>>> This increase connection time, increase complexity of implementation > >>>>> (lot of things can fail) and just complicate lot of things there. Plus > >>>>> adds that glitch which is not user friendly. > >>>>> > >>>>> Also bluetooth devices, like headsets, probably do not expects that > >>>>> somebody is going to do such thing and we can hit other implementation > >>>>> problems... > >>>>> > >>>>> And moreover it is just big hack and workaround for that problem. Not a > >>>>> reasonable solution. > >>>>> > >>>>> In btmon I can see it, so kernel already knows that information. Why it > >>>>> cannot tell it to userspace and bluetooth daemon to client application? > >>>>> > >>>>> Client application (e.g. pulseaudio) should really know if is going to > >>>>> talk with bluetooth device with EDR 2 or EDR 3. > >>>>> > >>>>>>> > >>>>>>>> Therefore I'm asking for some way how to get information if device > >>>>>>>> supports EDR 2 or EDR 3. This is basically requirement for proper > >>>>>>>> implementation of SBC in high quality mode. So if there are not such API > >>>>>>>> yet, could it be exported from kernel to userspace and bluetoothd > >>>>>>>> daemon? > >>>>>>>> > >>>>>>>> See these two articles for more details about SBC and its high quality: > >>>>>>>> > >>>>>>>> https://habr.com/en/post/456182/ > >>>>>>>> http://soundexpert.org/articles/-/blogs/audio-quality-of-sbc-xq-bluetooth-audio-codec > >>>>>>>> > >>>>>>>>>> Is there any bluez API for it? > >>>>>>>>>> > >>>> > >>>> There quite a few assumption here that are not really how it is > >>>> implemented in BlueZ: > >>>> > >>>> 1. The L2CAP MTU is not based on the ACL link (should be relatively > >>>> easy to change) > >>>> 2. L2CAP MTU is not required to be symmetric. > >>>> 3. Since the ACL link is shared for all channels we shouldn't really > >>>> assume all throughput will be available > >>>> 4. PA flow control is not just filling up packets and sending them > >>>> when they are full to maximize speed, instead it send packets when > >>>> necessary to maintain a constant speed so the MTU may not be fully > >>>> used, in fact trying to maximize the MTU may result in higher latency > >>>> since packets would be sent less frequently. > >>>> > >>>> With this in mind I think the only thing we should look into is to > >>>> adjust the default L2CAP MTU to match the underline ACL Link, so it > >>>> maximizes throughput, the remote side may choose a different MTU which > >>>> will have to follow though. > >>> > >>> Hi Luiz! The main problem is not MTU size, as I wrote I cannot use it > >>> normally for distinguish for usage of SBC XQ or not. Instead of MTU I > >>> rather need to know if device supports EDR 2 or EDR 3. > >> > >> I was trying to implement this on the kernel to match the MTU size of > >> L2CAP with ACL but the packet type current in use by the connection is > >> not exposed in the connection complete, or at least I couldn't any > >> reference to it, we could possibly expose the packet types via socket > >> option as well but changing it at runtime is probably not a good idea. > > > > Hello, when I run btmon, I see following information very early before > > creating A2DP connection: > > > > HCI Event: Read Remote Supported Features (0x0b) plen 11 > > Status: Success (0x00) > > Handle: 35 > > Features: 0xff 0xff 0x8f 0xfe 0x9b 0xff 0x59 0x87 > > ... > > Enhanced Data Rate ACL 2 Mbps mode > > Enhanced Data Rate ACL 3 Mbps mode > > > > I do not need to change MTU size, I would just like to know EDR features > > capabilities. Cannot be those information exported somehow from kernel? > > I guess that kernel should have these information if it can send it to > > btmon. > > so I have not seen a single device that only supports one of these two modes. They all support both and if possible you want to use 3 Mpbs anyway since that safes power. Hi! But there are devices which support none of the two modes? Anyway, could be these "features" exported as dbus property from bluez? -- Pali Rohár pali.rohar@xxxxxxxxx
Attachment:
signature.asc
Description: PGP signature
