[gnome-bluetooth/wip/hadess/default-adapter-work: 14/17] README: Explain how to disable a Bluetooth adapter

[Bastien Nocera <hadess src gnome org>]

commit cdf9625d00b87d1e95f440fd4bebede3b850db30
Author: Bastien Nocera <hadess hadess net>
Date:   Tue Dec 14 15:21:47 2021 +0100

    README: Explain how to disable a Bluetooth adapter
    
    Explain how to disable a builtin Bluetooth adapter as gnome-bluetooth
    only supports one in its UI.

 README.md | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)
---
diff --git a/README.md b/README.md
index 39114212..715c5ad1 100644
--- a/README.md
+++ b/README.md
@@ -10,6 +10,48 @@ Requirements
 - bluez 5.51 or newer
 - rfkill sub-system enabled in the kernel, and [accessible](https://github.com/systemd/systemd/pull/21605)
 
+Multiple Bluetooth adapters
+---------------------------
+
+The gnome-bluetooth user interface and API have no support for handling
+multiple Bluetooth adapters. Earlier versions of the bluez backend software
+had support for setting a "default adapter" but that is not the case
+any more.
+
+As the goal is usually to disable an internal Bluetooth adapter in favour of
+a more featureful removable one, there are a couple of possibilities
+to do this, depending on the hardware:
+
+- Disable the internal Bluetooth adapter in the system's BIOS or firmware
+
+- Disable the internal adapter through a mechanical "RF kill" switch
+  available on some laptops
+
+- Unplug the USB cable from the wireless card in the case of combo Bluetooth/Wi-Fi
+  desktop cards
+
+- Enable the hardware-specific software kill switch on laptops. First find out
+  whether your hardware has one:
+
+```sh
+rfkill | grep bluetooth | grep -v hci
+5 bluetooth hp-bluetooth  unblocked unblocked
+```
+
+  Then block it with `rfkill block <ID>` where `<ID>` is the identifier in the
+  command above. systemd will remember this across reboots.
+
+- Disable a specific USB adapter through udev by creating a
+  `/etc/udev/rules.d/81-bluetooth-hci.rules` device containing:
+
+```
+SUBSYSTEM=="usb", ATTRS{idVendor}=="0a5c", ATTRS{idProduct}=="21b4", ATTR{authorized}="0"
+```
+
+- If the adapter still needs to be plugged in so it can be used as a passthrough,
+for virtualisation or gaming, we ship [a small script that makes unbinding the Bluetooth
+driver easier](contrib/unbind-bluetooth-driver.sh)
+
 Copyright
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