This code is obsolete - the required functionality is now in the mainline
kernel, and the new approach is to use stock btattach rather than a mutant
version of hciattach.
This is a patch vs kernel.org 3.18 source for Realtek H4/H5 UART bluetooth
as found in the Falls City Intel Compute Stick. It is derived from much
messier code provided by Realtek
(Linux_BT_UART_3.0.20150325_8723BS_BTCOEX_20150119-5844) for a kernel of
roughly 3.2 vintage. We updated the patch for 3.18 and cleaned up its kernel
configuration integration.
Also provided (verbatim) from the original Realtek bundle are the modified
version of hciattach and the run-time firmware it requires.
This patch proved too difficult to update for 4.x kernels, so we omitted it
from later kernel builds. What ultimately ended up going into the mainline
kernel didn't resemble this code at all. Our first kernel release using the
new method was 5.4 (not sure exactly when it was added).