shenghaoyang/blueEnable_ALARM_non_mainline.md

## blueEnable_ALARM_non_mainline.md

      
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    #1 Update: Recent Rapsberry Pi (downstream) kernel dt-overlays now exposes a config option "krnbt" that negates the need for all this.
Relevant PRs: raspberrypi/linux#3682
Enabling bluetooth

[Pi 3B (Non-Plus) / Pi Zero W ONLY]
As of kernel package, linux-raspberrypi-4.14.59-1, support for notifying the kernel of the
Broadcom bluetooth device through the device tree has been enabled.
There is no longer a need for btattach, which makes the lives of everyone
easier. There is also no need to fall back to hciattach for better performance, as the
kernel correctly restores a higher baud rate after writing out required firmware
to the bluetooth controller at a lower baud rate.
This requires a device tree node to be added, and that can be accomplished with a
simple overlay (bcmbt-overlay.dts):
/*
 * The code contained herein is licensed under the GNU General Public
 * License. You may obtain a copy of the GNU General Public License
 * Version 2 or later at the following location:
 *
 * http://www.gnu.org/copyleft/gpl.html
 *
 * Device tree overlay for Raspberry Pi devices with bluetooth to
 * inform the kernel that a bluetooth controller can be accessed
 * over serial using the main UART (pl011 UART, not mini-UART)
 * via HCI. Assumes the main UART node is termed uart0.
 * 
 * Modified from mainline device tree source for Pi Zero W,
 * bcm2835-rpi-zero-w.dts, 
 * Copyright (C) 2017 Stefan Wahren <stefan.wahren@i2se.com>
 *
 * Tested on a Raspberry Pi Zero W.
 *
 * Copyright (C) 2018 Shenghao Yang <me@shenghaoyang.info>
 *
 */

/dts-v1/;
/plugin/;

/ {
        compatible = "brcm,bcm2708";
        fragment@0 {
                target-path = "uart0";
                __overlay__ {
                        bluetooth {
                                compatible = "brcm,bcm43438-bt";
                                max-speed = <2000000>; /* You can experiment with this */
                                /* Other elements omitted, pins may be different on different PI models */
                        };
                };
        };
};


(NOTE: This overlay assumes a clean configuration, with the pl011 UART routed to the
bluetooth controller. This is untested on a configuration with the mini-UART routed
to the bluetooth controller. YMMV)
To compile this overlay, you'll require the dtc package. After which, you can execute,
in the directory where you've placed the file,
dtc -I dts -O dtb bcmbt-overlay.dts -o bcmbt.dtbo
Afterwards, copy the generated bcmbt.dtbo file to /boot/overlays/, and enable the
overlay in config.txt, by adding this line at the end of the file:
...
dtoverlay=bcmbt

If all goes well, reboot and you should see the following kernel messages:
[   12.440976] Bluetooth: hci0: BCM: chip id 94
[   12.441473] Bluetooth: hci0: BCM: features 0x2e
[   12.443187] Bluetooth: hci0: BCM43430A1
[   12.443216] Bluetooth: hci0: BCM43430A1 (001.002.009) build 0000
[   13.143103] Bluetooth: hci0: BCM (001.002.009) build 0360

Afterwards, you can proceed with installing all the userspace software required
for Bluetooth to operate! (a-la bluez, bluez-utils, ...)
Footnotes:
I'm not sure why upstream raspberrypi/linux is not using this strategy...
perhaps users will freak out over the loss of ttyAMA0?
Testing:
I've tested streaming music from a phone to the Pi and relaying that over to another
computer through the UAC2 gadget functionality, which has also been recently enabled.
It ran for around 20 minutes before I stopped it, which seems decently stable for me.
All of this was done on a Pi Zero W using the dts as above. This should work on a
Pi 3, but I've not tested that, yet. The Pi 3 has to be running the non-mainline
kernel, of course...., because the mainline dts already has this node built-in :)