Re: [PATCH v2] Bluetooth: btusb: Add support USB ALT 3 for WBS

[Pauli Virtanen <pav@iki.fi>]

Hi,

pe, 2021-05-14 kello 11:19 +0800, hildawu@realtek.com kirjoitti:
> From: Hilda Wu <hildawu@realtek.com>
> 
> Because mSBC frames do not need to be aligned to the SCO packet
> boundary. Using USB ALT 3 let HCI payload >= 60 bytes, let mSBC
> data satisfy 60 Bytes avoid payload unaligned situation and fixed
> some headset no voise issue.
> 
> USB Alt 3 supported also need HFP support transparent MTU in 72
> Bytes.
> 
> Signed-off-by: Hilda Wu <hildawu@realtek.com>
> ---
> Changes in v2:
> - Updated signed off name.
> - Adjusted coding style.
> ---

This change seemed to break msbc audio on some non-realtek adapters I
have. Tested Pipewire on BCM20702A1 (0b05:17cb), CSR8510A10 (0a12:0001)
-> no sound output and input appears garbled. Reverting this patch
makes it work again. Indeed these adapters report SCO mtu=64 which is
less than 72. On the other hand, with RTL8761BU (0bda:8771) msbc audio
works fine with this patch out of the box, indeed reading/writing 72
byte packets to/from the sco socket.

ALT 3 on RTL8761BU does appear to address garbled audio with some
headsets. Maybe the altsetting should be determined based on a quirk,
the mtu requirement be checked, or something similar? AFAIK, userspace
can't do anything to fix this.

Best,
Pauli Virtanen

> ---
>  drivers/bluetooth/btusb.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/bluetooth/btusb.c b/drivers/bluetooth/btusb.c
> index 6f253378e893..1e98f985740b 100644
> --- a/drivers/bluetooth/btusb.c
> +++ b/drivers/bluetooth/btusb.c
> @@ -1752,6 +1752,13 @@ static void btusb_work(struct work_struct
> *work)
>                          * which work with WBS at all.
>                          */
>                         new_alts = btusb_find_altsetting(data, 6) ? 6
> :
> 1;
> +                       /* Because mSBC frames do not need to be
> aligned to the
> +                        * SCO packet boundary. If support the Alt 3,
> use the
> +                        * Alt 3 for HCI payload >= 60 Bytes let air
> packet
> +                        * data satisfy 60 bytes.
> +                        */
> +                       if (new_alts == 1 &&
> btusb_find_altsetting(data, 3))
> +                               new_alts = 3;
>                 }
>  
>                 if (btusb_switch_alt_setting(hdev, new_alts) < 0)