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Subject: Re: Default to HW mSBC on capable controllers ?
From:   Marcel Holtmann <marcel@holtmann.org>
In-Reply-To: <20201222092100.ru5inf45v55qoa4m@pali>
Date:   Tue, 22 Dec 2020 11:05:19 +0100
Cc:     Luiz Augusto von Dentz <luiz.dentz@gmail.com>,
        Joakim Tjernlund <Joakim.Tjernlund@infinera.com>,
        "linux-bluetooth@vger.kernel.org" <linux-bluetooth@vger.kernel.org>
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To:     =?utf-8?Q?Pali_Roh=C3=A1r?= <pali@kernel.org>
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Hi Pali,

>>>>> There seems to be quite a few USB controllers gaining the BTUSB_WIDEBAND_SPEECH which I guess means HW mSBC but currently there is no way to select this mode.
>>>>> Any idea if one could patch the kernel to default to HW mSBC and user apps like bluealsa/pulseaudio would just use it automatically?
>>>> 
>>>> It is in our plan to support HW offloading, but that doesn't mean all
>>>> platforms will be supported since that depends on the PCM lines being
>>>> connected to BT controller in the first place.
>>> 
>>> Dedicated PCM lines are used in embedded world and maybe also still in
>>> some mobile segment. I remember that e.g. Nokia N900 had this setup. And
>>> it was quite crazy how it was finally configured... but it worked!
>>> 
>>> But this is nothing for classic x86 laptops with USB bluetooth
>>> controllers on classic intel bluetooth+wifi mPCIe cards where SCO
>>> traffic is routed via HCI (over USB). And not via dedicated PCM pins.
>>> Moreover I think there are not any mainstream laptop which have PCM pins
>>> on mPCIe slots usable for such bluetooth mPCIe cards.
>>> 
>>> For classic desktop / laptop it is needed to deal with fact that SCO
>>> audio is routed via HCI (like A2DP) and therefore support for Enhanced
>>> Setup Synchronous Connection HCI command.
>>> 
>>> AFAIK even for routing SCO over PCM when mSBC hw encoder is used,
>>> Enhanced Setup Synchronous Connection HCI command is required.
>> 
>> So you are saying that we should do PCM over HCI and that would
>> actually work (meaning we have enough bandwidth)?
> 
> This is something which needs to be tested. And without full
> implementation (with control of all parameters) we cannot say YES or NO.
> 
> And if you are aware of bandwidth, Enhanced Setup Synchronous Connection
> HCI command allows you to use also software based CVSD codec. Meaning
> that CVSD encoding/decoding can be done by application and therefore
> decreasing amount of data to transfer to bluetooth adapter.
> 
> As I said this command is needed also if you want to use mSBC hw encoder
> over PCM, so I think usage of Enhanced Setup Synchronous Connection HCI
> command always have benefits to implement it (I have unfinished and
> untested implementation).

CVSD is an air codec only. Controller<->Host is PCM in that case. Only with mSBC you have both air codec and Controller<->Host running with mSBC.

Regards

Marcel