On kernel 6.1-rcX, I have been getting the following dmesg error message
on every boot, resume from suspend and rfkill unblock of the Bluetooth
device:

	Bluetooth: hci0: HCI_REQ-0xfcf0

After some investigation, it turned out to be caused by
commit dd50a864 ("Bluetooth: Delete unreferenced hci_request code")
which modified hci_req_add() in net/bluetooth/hci_request.c to always
print an error message when it is executed. In my case, the function was
executed by msft_set_filter_enable() in net/bluetooth/msft.c, which
provides support for Microsoft vendor opcodes.

As explained by Brian Gix, "the error gets logged because it is using a
deprecated (but still working) mechanism to issue HCI opcodes" [1]. So
this is just a debugging tool to show that a deprecated function is
executed. As such, it should not be included in the mainline kernel.
See for example
commit 771c0353 ("deprecate the '__deprecated' attribute warnings entirely and for good")
Additionally, this error message is cryptic and the user is not able to
do anything about it.

[1]
Link: https://lore.kernel.org/lkml/beb8dcdc3aee4c5c833aa382f35995f17e7961a1.camel@intel.com/

Fixes: dd50a864 ("Bluetooth: Delete unreferenced hci_request code")
Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Cc: Brian Gix <brian.gix@intel.com>
Cc: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: Johan Hedberg <johan.hedberg@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>