Received-SPF: pass (google.com: domain of linux-bluetooth-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Thu, 22 Jul 2021 15:29:57 +0200
From:   =?utf-8?B?TWljaGHFgiBLxJlwaWXFhA==?= <kernel@kempniu.pl>
To:     Pauli Virtanen <pav@iki.fi>
Cc:     linux-bluetooth <linux-bluetooth@vger.kernel.org>,
        Hilda Wu <hildawu@realtek.com>
Subject: Re: [PATCH] Bluetooth: btusb: check SCO MTU before enabling USB ALT
 3 for WBS
Message-ID: <YPly1RR8V33T3odj@larwa.hq.kempniu.pl>
References: <d43dffdc43a40782ec6d5d6c24b1638005992a8f.camel@iki.fi>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <d43dffdc43a40782ec6d5d6c24b1638005992a8f.camel@iki.fi>
Precedence: bulk

Hi Pauli,

Thank you for your work on this patch!

> Some USB BT adapters don't satisfy the MTU requirement mentioned in
> commit e848dbd364ac ("Bluetooth: btusb: Add support USB ALT 3 for WBS")
> and have ALT 3 setting that produces no/garbled audio. Check that the MTU
> condition is satisfied, and fall back to ALT 1 if not.
> 
> Tested with USB adapters (mtu<72, produce sound only with ALT1)
> BCM20702A1 0b05:17cb, CSR8510A10 0a12:0001, and (mtu>=72, ALT3)
> RTL8761BU 0bda:8771, Intel AX200 8087:0029 (after disabling ALT6).

Commit e848dbd364aca44c9d23c04bef964fab79e2b34f causes garbled mSBC
audio on my Bluetooth adapter (8087:0a2b), so I gave your patch a try.

Unfortunately, the patch in its current shape does not seem to be enough
to fix things for this adapter, which reports an SCO MTU of 96 bytes and
produces garbled mSBC audio with ALT 3.

For the record, reverting e848dbd364aca44c9d23c04bef964fab79e2b34f on
top of v5.14-rc2 makes mSBC work again with my adapter.

I am no Bluetooth expert, but I will be happy to perform any additional
debugging and/or testing.

-- 
Best regards,
Michał Kępień