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From: Marcel Holtmann <marcel@holtmann.org>
To: "Pali Rohár" <pali.rohar@gmail.com>
Cc: Luiz Augusto von Dentz <luiz.dentz@gmail.com>,
	Brian Gix <brian.gix@intel.com>,
	"iam@valdikss.org.ru" <iam@valdikss.org.ru>,
	"linux-bluetooth@vger.kernel.org"
	<linux-bluetooth@vger.kernel.org>
Subject: Re: Determinate EDR speed
Date: Wed, 16 Oct 2019 21:06:41 +0200
Message-ID: <FE2F255A-6762-485B-9166-8348CFB18168@holtmann.org> (raw)
In-Reply-To: <20191013093947.cbqczgnt3uo3f5wi@pali>

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.

Regards

Marcel


  parent reply index

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-11  8:27 Pali Rohár
2019-10-11 18:15 ` Gix, Brian
2019-10-11 18:35   ` Pali Rohár
2019-10-11 19:00     ` Gix, Brian
2019-10-11 19:05       ` Gix, Brian
2019-10-11 20:04         ` Pali Rohár
2019-10-12  7:23           ` Luiz Augusto von Dentz
2019-10-13  7:36             ` Pali Rohár
2019-10-13  8:45               ` Luiz Augusto von Dentz
2019-10-13  9:39                 ` Pali Rohár
2019-10-13  9:49                   ` Luiz Augusto von Dentz
2019-10-16 19:06                   ` Marcel Holtmann [this message]
2019-10-16 19:13                     ` Pali Rohár
2019-10-19 19:47                       ` Marcel Holtmann
2019-10-16 19:05           ` Marcel Holtmann

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