David Gnedt [off-list ref] writes:

Port the bt_coex_mode sysfs interface from wl1251 driver version included
in the Maemo Fremantle kernel to allow bt-coexistence mode configuration.
This enables userspace applications to set one of the modes
WL1251_BT_COEX_OFF, WL1251_BT_COEX_ENABLE and WL1251_BT_COEX_MONOAUDIO.
The default mode is WL1251_BT_COEX_OFF.

Sorry, but this was an awful hack in the fremantle kernel and I really
would not like to see this in the upstream driver. We should do better
than that :) And besides, who would use it? The only user I know is
maemo's wlancond and maemo is dead and buried anyway. RIP

I'm not saying that we don't need this, I just want to understand
where we need this. And also sysfs is not the best choise imho, maybe
debugfs is better?

It should be noted that this driver always enabled bt-coexistence before
and enabled bt-coexistence directly affects the receiving performance,
rendering it unusable in some low-signal situations. Especially monitor
mode is affected very badly with bt-coexistence enabled.

Yes, btcoex is very tricky to get right. Also one problem is that
wl1251's btcoex implementation significantly increases power
consumption.

-- 
Kalle Valo