517b693351
When alt mode 6 is not available, fallback to the kernel <= 5.7 behavior of always using alt mode 1. Prior to kernel 5.8, btusb would always use alt mode 1 for WBS (Wide Band Speech aka mSBC aka transparent SCO). In commitbaac6276c0("Bluetooth: btusb: handle mSBC audio over USB Endpoints") this was changed to use alt mode 6, which is the recommended mode in the Bluetooth spec (Specifications of the Bluetooth System, v5.0, Vol 4.B §2.2.1). However, many if not most BT USB adapters do not support alt mode 6. In fact, I have been unable to find any which do. In kernel 5.8, this was changed to use alt mode 6, and if not available, use alt mode 0. But mode 0 has a zero byte max packet length and can not possibly work. It is just there as a zero-bandwidth dummy mode to work around a USB flaw that would prevent device enumeration if insufficient bandwidth were available for the lowest isoc mode supported. In effect, WBS was broken for all USB-BT adapters that do not support alt 6, which appears to nearly all of them. Then in commit461f95f04f("Bluetooth: btusb: USB alternate setting 1 for WBS") the 5.7 behavior was restored, but only for Realtek adapters. I've tested a Broadcom BRCM20702A and CSR 8510 adapter, both work with the 5.7 behavior and do not with the 5.8. So get rid of the Realtek specific flag and use the 5.7 behavior for all adapters as a fallback when alt 6 is not available. This was the kernel's behavior prior to 5.8 and I can find no adapters for which it is not correct. And even if there is an adapter for which this does not work, the current behavior would be to fall back to alt 0, which can not possibly work either, and so is no better. Signed-off-by: Trent Piepho <tpiepho@gmail.com> Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.