btattach: Add auto attach/detach

[Loic Poulain <loic.poulain@intel.com>]

Hi Marcel,

I wonder if adding a dynamic attach/detach to btattach could be a good idea.

Since most HCI UART drivers power on/off the Bluetooth controller on 
proto/ldisc open/close.
It could be interesting to attach/detach the line discipline depending 
Bluetooth usage.

A way to do that could be to track rfkill events via the /dev/rfkill device.
I can propose two solutions:

- btattach monitors a specific rfkill node whose index is a passed as 
argument
and ldisc is attached/detached accordingly.

- An other solution could be to create a rfkill node from btattach so 
that if
something blocks/unblocks the node, HCI ldisc is detached/attached 
automatically.
Problem is that we can't create a rfkill node from user-space, but I 
think we can
easily add this support to the rfkill device driver.

On my laptop, the Thinkpad acpi driver exports a rfkill nodes which 
disconnects/reconnects
the USB embedded Bluetooth controller. This is what I would like to 
reproduce for UART here.

Regards,
Loic

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Intel Open Source Technology Center
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