Nov 25, 2015

Replacing built-in RTC with i2c battery-backed one on BeagleBone Black from boot

Update 2018-02-16:

There is a better and simplier solution for switching default rtc than one described here - to swap rtc0/rtc1 via aliases in dt overlay.

I.e. using something like this in the dts:

fragment@0 {
  __overlay__ {

    aliases {
      rtc0 = "/ocp/i2c@4802a000/ds3231@68";
      rtc1 = "/ocp/rtc@44e3e000";
Which is also how it's done in repo these days:

BeagleBone Black (BBB) boards have - and use - RTC (Real-Time Clock - device that tracks wall-clock time, including calendar date and time of day) in the SoC, which isn't battery-backed, so looses track of time each time device gets power-cycled.

This represents a problem if keeping track of time is necessary and there's no network access (or a patchy one) to sync this internal RTC when board boots up.

Easy solution to that, of course, is plugging external RTC device, with plenty of cheap chips with various precision available, most common being Dallas/Maxim ICs like DS1307 or DS3231 (a better one of the line) with I2C interface, which are all supported by Linux "ds1307" module.

Enabling connected chip at runtime can be easily done with a command like this:

echo ds1307 0x68 >/sys/bus/i2c/devices/i2c-2/new_device

(see this post on Fortune Datko blog and/or this one on minix-i2c blog for ways to tell reliably which i2c device in /dev corresponds to which bus and pin numbers on BBB headers, and how to check/detect/enumerate connected devices there)

This obviously doesn't enable device straight from the boot though, which is usually accomplished by adding the thing to Device Tree, and earlier with e.g. 3.18.x kernels it had to be done by patching and re-compiling platform dtb file used on boot.

But since 3.19.x kernels (and before 3.9.x), easier way seem to be to use Device Tree Overlays (usually "/lib/firmware/*.dtbo" files, compiled by "dtc" from dts files), which is kinda like patching Device Tree, only done at runtime.

Code for such patch in my case ("i2c2-rtc-ds3231.dts"), with 0x68 address on i2c2 bus and "ds3231" kernel module (alias for "ds1307", but more appropriate for my chip):


/* dtc -O dtb -o /lib/firmware/BB-RTC-02-00A0.dtbo -b0 i2c2-rtc-ds3231.dts */
/* bone_capemgr.enable_partno=BB-RTC-02 */
/* */

/ {
  compatible = "ti,beaglebone", "ti,beaglebone-black", "ti,beaglebone-green";
  part-number = "BB-RTC-02";
  version = "00A0";

  fragment@0 {
    target = <&i2c2>;

    __overlay__ {
      pinctrl-names = "default";
      pinctrl-0 = <&i2c2_pins>;
      status = "okay";
      clock-frequency = <100000>;
      #address-cells = <0x1>;
      #size-cells = <0x0>;

      rtc: rtc@68 {
        compatible = "dallas,ds3231";
        reg = <0x68>;

As per comment in the overlay file, can be compiled ("dtc" comes from same-name package on ArchLinuxARM) to the destination with:

dtc -O dtb -o /lib/firmware/BB-RTC-02-00A0.dtbo -b0 i2c2-rtc-ds3231.dts

Update 2017-09-19: This will always produce a warning for each "fragment" section, safe to ignore.

And then loaded on early boot (as soon as rootfs with "/lib/firmware" gets mounted) with "bone_capemgr.enable_partno=" cmdline addition, and should be put to something like "/boot/uEnv.txt", for example (with dtb path from command above):

setenv bootargs "... bone_capemgr.enable_partno=BB-RTC-02"

Docs in repository have more details and examples on how to write and manage these.

Update 2017-09-19:

Modern ArchLinuxARM always uses initramfs image, which is starting rootfs, where kernel will look up stuff in /lib/firmware, so all *.dtbo files loaded via bone_capemgr.enable_partno= must be included there.

With Arch-default mkinitcpio, it's easy to do via FILES= in "/etc/mkinitcpio.conf" - e.g. FILES="/lib/firmware/BB-RTC-02-00A0.dtbo" - and re-running the usual mkinitcpio -g /boot/initramfs-linux.img command.

If using e.g. i2c-1 instead (with &bb_i2c1_pins), BB-I2C1 should also be included and loaded there, strictly before rtc overlay.

Update 2017-09-19:

bone_capemgr seem to be broken in linux-am33x packages past 4.9, and produces kernel BUG instead of loading any overlays - be sure to try loading that dtbo at runtime (checking dmesg) before putting it into cmdline, as that might make system unbootable.

Workaround is to downgrade kernel to 4.9, e.g. one from beagleboard/linux tree, where it's currently the latest supported release.

That should ensure that this second RTC appears as "/dev/rtc1" (rtc0 is the internal one) on system startup, but unfortunately it still won't be the first one and kernel will already pick up time from internal rtc0 by the time this one gets detected.

Furthermore, systemd-enabled userspace (as in e.g. ArchLinuxARM) interacts with RTC via systemd-timedated and systemd-timesyncd, which both use "/dev/rtc" symlink (and can't be configured to use other devs), which by default udev points to rtc0 as well, and rtc1 - no matter how early it appears - gets completely ignored there as well.

So two issues are with "system clock" that kernel keeps and userspace daemons using wrong RTC, which is default in both cases.

"/dev/rtc" symlink for userspace gets created by udev, according to "/usr/lib/udev/rules.d/50-udev-default.rules", and can be overidden by e.g. "/etc/udev/rules.d/55-i2c-rtc.rules":

SUBSYSTEM=="rtc", KERNEL=="rtc1", SYMLINK+="rtc", OPTIONS+="link_priority=10", TAG+="systemd"

This sets "link_priority" to 10 to override SYMLINK directive for same "rtc" dev node name from "50-udev-default.rules", which has link_priority=-100.

Also, TAG+="systemd" makes systemd track device with its "dev-rtc.device" unit (auto-generated, see systemd.device(5) for more info), which is useful to order userspace daemons depending on that symlink to start strictly after it's there.

"userspace daemons" in question on a basic Arch are systemd-timesyncd and systemd-timedated, of which only systemd-timesyncd starts early on boot, before all other services, including systemd-timedated, and (for early-boot clock-dependant services).

So basically if proper "/dev/rtc" and system clock gets initialized before systemd-timesyncd (or whatever replacement, like ntpd or chrony), correct time and rtc device will be used for all system daemons (which start later) from here on.

Adding that extra step can be done as a separate systemd unit (to avoid messing with shipped systemd-timesyncd.service), e.g. "i2c-rtc.service":

Before=systemd-timesyncd.service ntpd.service chrony.service

DeviceAllow=/dev/rtc rw
ExecStart=/usr/bin/hwclock -f /dev/rtc --hctosys


Update 2017-09-19: -f /dev/rtc must be specified these days, as hwclock seem to use /dev/rtc0 by default, pretty sure it didn't used to.

Note that Before= above should include whatever time-sync daemon is used on the machine, and there's no harm in listing non-existant or unused units there jic.

Most security-related stuff and conditions are picked from systemd-timesyncd unit file, which needs roughly same access permissions as "hwclock" here.

With udev rule and that systemd service (don't forget to "systemctl enable" it), boot sequence goes like this:

  • Kernel inits internal rtc0 and sets system clock to 1970-01-01.
  • Kernel starts systemd.
  • systemd mounts local filesystems and starts i2c-rtc asap.
  • i2c-rtc, due to Wants/After=dev-rtc.device, starts waiting for /dev/rtc to appear.
  • Kernel detects/initializes ds1307 i2c device.
  • udev creates /dev/rtc symlink and tags it for systemd.
  • systemd detects tagging event and activates dev-rtc.device.
  • i2c-rtc starts, adjusting system clock to realistic value from battery-backed rtc.
  • systemd-timesyncd starts, using proper /dev/rtc and correct system clock value.
  • activates, as it is scheduled to, after systemd-timesyncd and i2c-rtc.
  • From there, boot goes on to, and starts all the daemons.

udev rule is what facilitates symlink and tagging, i2c-rtc.service unit is what makes boot sequence wait for that /dev/rtc to appear and adjusts system clock right after that.

Haven't found an up-to-date and end-to-end description with examples anywhere, so here it is. Cheers!

Oct 22, 2015

Arch Linux chroot and sshd from boot on rooted Android with SuperSU

Got myself Android device only recently, and first thing I wanted to do, of course, was to ssh into it.

But quick look at the current F-Droid and Play apps shows that they either based on a quite limited dropbear (though perfectly fine for one-off shell access) or "based on openssh code", where infamous Debian OpenSSL code patch comes to mind, not to mention that most look like ad-ridden proprietary piece of crap.

Plus I'd want a proper package manager, shell (zsh), tools and other stuff in there, not just some baseline bash and busybox, and chroot with regular linux distro is a way to get all that plus standard OpenSSH daemon.

Under the hood, modern Android (5.11 in my case, with CM 12) phone is not much more than Java VM running on top of SELinux-enabled (which I opted to keep for Android stuff) Linux kernel, on top of some multi-core ARMv7 CPU (quadcore in my case, rather identical to one in RPi2).

Steps I took to have sshd running on boot with such device:

  • Flush stock firmware (usually loaded with adware and crapware) and install some pre-rooted firmware image from or

    My phone is a second-hand Samsung Galaxy S3 Neo Duos (GT-I9300I), and I picked resurrection remix CM-based ROM for it, built for this phone from t3123799, with Open GApps arm 5.1 Pico (minimal set of google stuff).

    That ROM (like most of them, it appears) comes with bash, busybox and - most importantly - SuperSU, which runs its unrestricted init, and is crucial to getting chroot-script to start on boot. All three of these are needed.

    Under Windows, there's Odin suite for flashing zip with all the goodies to USB-connected phone booted into "download mode".

    On Linux, there's Heimdall (don't forget to install adb with that, e.g. via pacman -S android-tools android-udev on Arch), which can dd img files for diff phone partitions, but doesn't seem to support "one zip with everything" format that most firmwares come in.

    Instead of figuring out which stuff from zip to upload where with Heimdall, I'd suggest grabbing a must-have TWRP recovery.img (small os that boots in "recovery mode", grabbed one for my phone from t2906840), flashing it with e.g. heimdall flash --RECOVERY recovery.twrp- and then booting into it to install whatever main OS and apps from zip's on microSD card.

    TWRP (or similar CWM one) is really useful for a lot of OS-management stuff like app-packs installation (e.g. Open GApps) or removal, updates, backups, etc, so I'd definitely suggest installing one of these as recovery.img as the first thing on any Android device.

  • Get or make ARM chroot tarball.

    I did this by bootstrapping a stripped-down ARMv7 Arch Linux ARM chroot on a Raspberry Pi 2 that I have around:

    # pacman -Sg base
     ### Look through the list of packages,
     ###  drop all the stuff that won't ever be useful in chroot on the phone,
     ###  e.g. pciutils, usbutils, mdadm, lvm2, reiserfsprogs, xfsprogs, jfsutils, etc
     ### pacstrap chroot with whatever is left
    # mkdir droid-chroot
    # pacstrap -i -d droid-chroot bash bzip2 coreutils diffutils file filesystem \
        findutils gawk gcc-libs gettext glibc grep gzip iproute2 iputils less \
        licenses logrotate man-db man-pages nano pacman perl procps-ng psmisc \
        sed shadow sysfsutils tar texinfo util-linux which
     ### Install whatever was forgotten in pacstrap
    # pacman -r droid-chroot -S --needed atop busybox colordiff dash fping git \
        ipset iptables lz4 openssh patch pv rsync screen xz zsh fcron \
        python2 python2-pip python2-setuptools python2-virtualenv
     ### Make a tar.gz out of it all
    # rm -f droid-chroot/var/cache/pacman/pkg/*
    # tar -czf droid-chroot.tar.gz droid-chroot

    Same can be obviously done with debootstrap or whatever other distro-of-choice bootstrapping tool, but likely has to be done on a compatible architecture - something that can run ARMv7 binaries, like RPi2 in my case (though VM should do too) - to run whatever package hooks upon installs.

    Easier way (that won't require having spare ARM box or vm) would be to take pre-made image for any ARMv7 platform, from list or such, but unless there's something very generic (which e.g. "ArchLinuxARM-armv7-latest.tar.gz" seem to be), there's likely be some platform-specific cruft like kernel, modules, firmware blobs, SoC-specific tools and such... nothing that can't be removed at any later point with package manager or simple "rm", of course.

    While architecture in my case is ARMv7, which is quite common nowadays (2015), other devices can have Intel SoCs with x86 or newer 64-bit ARMv8 CPUs. For x86, bootstrapping can obviously be done on pretty much any desktop/laptop machine, if needed.

  • Get root shell on device and unpack chroot tarball there.

    adb shell (pacman -S android-tools android-udev or other distro equivalent, if missing) should get "system" shell on a USB-connected phone.

    (btw, "adb" access have to be enabled on the phone via some common "tap 7 times on OS version in Settings-About then go to Developer-options" dance, if not already)

    With SuperSU (or similar "su" package) installed, next step would be running "su" there to get unrestricted root, which should work.

    /system and /data should be on ext4 (check mount | grep ext4 for proper list), f2fs or such "proper" filesystem, which is important to have for all the unix permission bits and uid/gid info which e.g. FAT can't handle (loopback img with ext4 can be created in that case, but shouldn't be necessary in case of internal flash storage, which should have proper fs'es already).

    /system is a bad place for anything custom, as it will be completely flushed on most main-OS changes (when updating ROM from zip with TWRP, for instance), and is mounted with "ro" on boot anyway.

    Any subdir in /data seem to work fine, though one obvious pre-existing place - /data/local - is probably a bad idea, as it is used by some Android dev tools already.

    With busybox and proper bash on the phone, unpacking tarball from e.g. microSD card should be easy:

    # mkdir -m700 /data/chroots
    # cd /data/chroots
    # tar -xpf /mnt/sdcard/droid-chroot.tar.gz

    It should already work, too, so...

    # cd droid-chroot
    # mount -o bind /dev dev \
        && mount -o bind /dev/pts dev/pts \
        && mount -t proc proc proc \
        && mount -t sysfs sysfs sys
    # env -i TERM=$TERM SHELL=/bin/zsh HOME=/root $(which chroot) . /bin/zsh

    ...should produce a proper shell in a proper OS, yay! \o/

    Furthermore, to be able to connect there directly, without adb or USB cable, env -i $(which chroot) . /bin/sshd should work too.

    For sshd in particular, one useful thing to do here is:

    # $(which chroot) . /bin/ssh-keygen -A
 populate /etc/ssh with keys, which are required to start sshd.

  • Setup init script to run sshd or whatever init-stuff from that chroot on boot.

    Main trick here is to run it with unrestricted SELinux context (unless SELinux is disabled entirely, I guess).

    This makes /system/etc/init.d using "sysinit_exec" and /data/local/ with "userinit_exec" unsuitable for the task, only something like "init" ("u:r:init:s0") will work.

    SELinux on Android is documented in Android docs, and everything about SELinux in general applies there, of course, but some su-related roles like above "userinit_exec" actually come with CyanogenMod or whatever similar hacks on top of the base Android OS.

    Most relevant info on this stuff comes with SuperSU though (or rather libsuperuser) -

    That doc has info on how to patch policies, to e.g. transition to unrestricted role for chroot init, setup sub-roles for stuff in there (to also use SELinux in a chroot), which contexts are used where, and - most useful in this case - which custom "init" dirs are used at which stages of the boot process.

    Among other useful stuff, it specifies/describes /system/su.d init-dir, from which SuperSU runs scripts/binaries with unrestricted "init" context, and very early in the process too, hence it is most suitable for starting chroot from.

    So, again, from root (after "su") shell:

    # mount -o remount,rw /system
    # mkdir -m700 /system/su.d
    # cat >/system/su.d/ <<EOF
    exec /data/local/chroots.bash
    # chmod 700 /system/su.d/
    # cat >/data/local/chroots.bash <<EOF
    export PATH=/sbin:/vendor/bin:/system/sbin:/system/bin:/system/xbin
    [[ $(du -m "$log" | awk '{print $1}') -gt 20 ]] && mv "$log"{,.old}
    exec >>"$log" 2>&1
    echo " --- Started $(TZ=UTC date) --- "
    log -p i -t chroots "Starting chroot: droid-chroot"
    /data/chroots/ &
    log -p i -t chroots "Finished chroots init"
    echo " --- Finished $(TZ=UTC date) --- "
    # chmod 700 /data/local/chroots.bash
    # cd /data/chroots
    # mkdir -p droid-chroot/mnt/storage
    # ln -s droid-chroot/
    # cat >droid-chroot/ <<EOF
    set -e -o pipefail
    usage() {
      bin=$(basename $0)
      echo >&2 "Usage: $bin [ stop | chroot ]"
      exit ${1:-0}
    [[ "$#" -gt 1 || "$1" = -h || "$1" = --help ]] && usage
    cd /data/chroots/droid-chroot
    sshd_pid=$(cat run/ 2>/dev/null ||:)
    mountpoint -q dev || mount -o bind /dev dev
    mountpoint -q dev/pts || mount -o bind /dev/pts dev/pts
    mountpoint -q proc || mount -t proc proc proc
    mountpoint -q sys || mount -t sysfs sysfs sys
    mountpoint -q tmp || mount -o nosuid,nodev,size=20%,mode=1777 -t tmpfs tmpfs tmp
    mountpoint -q run || mount -o nosuid,nodev,size=20% -t tmpfs tmpfs run
    mountpoint -q mnt/storage || mount -o bind /data/media/0 mnt/storage
    case "$1" in
        [[ -z "$sshd_pid" ]] || kill "$sshd_pid"
        exit 0 ;;
        exec env -i\
          TERM="$TERM" SHELL=/bin/zsh HOME=/root\
          /system/xbin/chroot . /bin/zsh ;;
      *) [[ -z "$1" ]] || usage 1 ;;
    [[ -n "$sshd_pid" ]]\
      && kill -0 "$sshd_pid" 2>/dev/null\
      || exec env -i /system/xbin/chroot . /bin/sshd
    # chmod 700 droid-chroot/

    To unpack all that wall-of-shell a bit:

    • Very simple /system/su.d/ is created, so that it can easily be replaced if/when /system gets flushed by some update, and also so that it won't need to be edited (needing rw remount) ever.

    • /data/local/chroots.bash is an actual init script for whatever chroots, with Android logging stuff (accessible via e.g. adb logcat, useful to check if script was ever started) and simplier more reliable (and rotated) log in /data/local/chroots.log.

    • /data/chroots/ is a symlink to init script in /data/chroots/droid-chroot/, so that this script can be easily edited from inside of the chroot itself.

    • /data/chroots/droid-chroot/ is the script that mounts all the stuff needed for the chroot and starts sshd there.

      Can also be run from adb root shell to do the same thing, with "stop" arg to kill that sshd, or with "chroot" arg to do all the mounts and chroot into the thing from whatever current sh.

      Basically everything to with that chroot from now on can/should be done through that script.

    "cat" commands can obviously be replaced with "nano" and copy-paste there, or copying same (or similar) scripts from card or whatever other paths (to avoid pasting them into shell, which might be less convenient than Ctrl+S in $EDITOR).

  • Reboot, test sshd, should work.

Anything other than sshd can also be added to that init script, to make some full-featured dns + web + mail + torrents server setup start in chroot.

With more than a few daemons, it'd probably be a good idea to start just one "daemon babysitter" app from there, such as runit, daemontools or whatever. Maybe even systemd will work, though unlikely, given how it needs udev, lots of kernel features and apis initialized in its own way, and such.

Obvious caveat for running a full-fledged linux separately from main OS is that it should probably be managed through local webui's or from some local terminal app, and won't care much about power management and playing nice with Android stuff.

Android shouldn't play nice with such parasite OS either, cutting network or suspending device when it feels convenient, without any regard for conventional apps running there, though can be easily configured not to.

As I'm unlikely to want this device as a phone ever (who needs these, anyway?), turning it into something more like wireless RPi2 with a connected management terminal (represented by Android userspace) sounds like the only good use for it so far.

Update 2016-05-16: Added note on ssh-keygen and rm for pacman package cache after pacstrap.

Mar 28, 2015

Bluetooth PAN Network Setup with BlueZ 5.X

It probably won't be a surprise to anyone that Bluetooth has profiles to carry regular network traffic, and BlueZ has support for these since forever, but setup process has changed quite a bit between 2.X - 4.X - 5.X BlueZ versions, so here's my summary of it with 5.29 (latest from fives atm).

First step is to get BlueZ itself, and make sure it's 5.X (at least for the purposes of this post), as some enerprisey distros probably still have 4.X if not earlier series, which, again, have different tools and interfaces.
Should be easy enough to do with bluetoothctl --version, and if BlueZ doesn't have "bluetoothctl", it's definitely some earlier one.

Hardware in my case was two linux machines with similar USB dongles, one of them was RPi (wanted to setup wireless link for that), but that shouldn't really matter.

Aforementioned bluetoothctl allows to easily pair both dongles from cli interactively (and with nice colors!), that's what should be done first and serves as an authentication, as far as I can tell.

--- machine-1
% bluetoothctl
[NEW] Controller 00:02:72:XX:XX:XX malediction [default]
[bluetooth]# power on
Changing power on succeeded
[CHG] Controller 00:02:72:XX:XX:XX Powered: yes
[bluetooth]# discoverable on
Changing discoverable on succeeded
[CHG] Controller 00:02:72:XX:XX:XX Discoverable: yes
[bluetooth]# agent on

--- machine-2 (snipped)
% bluetoothctl
[NEW] Controller 00:02:72:YY:YY:YY rpbox [default]
[bluetooth]# power on
[bluetooth]# scan on
[bluetooth]# agent on
[bluetooth]# pair 00:02:72:XX:XX:XX
[bluetooth]# trust 00:02:72:XX:XX:XX

Not sure if the "trust" bit is really necessary, and what it does - probably allows to setup agent-less connections or something.

As I needed to connect small ARM board to what amounts to an access point, "NAP" was the BT-PAN mode of choice for me, but there are also "ad-hoc" modes in BT like PANU and GN, which seem to only need a different topology (who connects to who) and pass different UUID parameter to BlueZ over dbus.

For setting up a PAN network with BlueZ 5.X, essentially just two dbus calls are needed (described in "doc/network-api.txt"), and basic cli tools to do these are bundled in "test/" dir with BlueZ sources.

Given that these aren't very suited for day-to-day use (hence the "test" dir) and are fairly trivial, did rewrite them as a single script, more suited for my purposes - bt-pan.

Update 2015-12-10: There's also "bneptest" tool, which comes as a part of e.g. "bluez-utils" package on Arch, which seem to do same thing with its "-s" and "-c" options, just haven't found it at the time (or maybe it's a more recent addition).

General idea is that one side (access point in NAP topology) sets up a bridge and calls "org.bluez.NetworkServer1.Register()", while the other ("client") does "org.bluez.Network1.Connect()".

On the server side (which also applies to GN mode, I think), bluetoothd expects a bridge interface to be setup and configured, to which it adds individual "bnepX" interfaces created for each client by itself.

Such bridge gets created with "brctl" (from bridge-utils), and should be assigned the server IP and such, then server itself can be started, passing name of that bridge to BlueZ over dbus in "org.bluez.NetworkServer1.Register()" call:



[[ -n "$(brctl show $br 2>&1 1>/dev/null)" ]] && {
  brctl addbr $br
  brctl setfd $br 0
  brctl stp $br off
  ip addr add dev $br
  ip link set $br up

exec bt-pan --debug server $br

(as mentioned above, bt-pan script is from fgtk github repo)

Update 2015-12-10: This is "net.bnep" script, as referred-to in "net-bnep.service" unit just below.

Update 2015-12-10: These days, systemd can easily create and configure bridge, forwarding and all the interfaces involved, even running built-in DHCP server there - see "man systemd.netdev" and "man", for how to do that, esp. examples at the end of both.

Just running this script will then setup a proper "bluetooth access point", and if done from systemd, should probably be a part of and get stopped along with bluetoothd (as it doesn't make any sense running without it):




Update 2015-12-10: Put this into e.g. /etc/systemd/system/net-bnep.service and enable to start with "" (see "man systemd.special") by running systemctl enable net-bnep.service.

On the client side, it's even simplier - BlueZ will just create a "bnepX" device and won't need any bridge, as it is just a single connection:


ExecStart=/usr/local/bin/bt-pan client --wait 00:02:72:XX:XX:XX


Update 2015-12-10: Can be /etc/systemd/system/net-bnep-client.service, don't forget to enable it (creates symlink in ""), same as for other unit above (which should be running on the other machine).

Update 2015-12-10: Created "bnepX" device is also trivial to setup with systemd on the client side, see e.g. "Example 2" at the end of "man".

On top of "bnepX" device on the client, some dhcp client should probably be running, which systemd-networkd will probably handle by default on systemd-enabled linuxes, and some dhcpd on the server-side (I used udhcpd from busybox for that).

Enabling units on both machines make them setup AP and connect on boot, or as soon as BT donges get plugged-in/detected.

Fairly trivial setup for a wireless one, especially wrt authentication, and seem to work reliably so far.

Update 2015-12-10: Tried to clarify a few things above for people not very familiar with systemd, where noted. See systemd docs for more info on all this.

In case something doesn't work in such a rosy scenario, which kinda happens often, first place to look at is probably debug info of bluetoothd itself, which can be enabled with systemd via systemctl edit bluetooth and adding a [Service] section with override like ExecStart=/usr/lib/bluetooth/bluetoothd -d, then doing daemon-reload and restart of the unit.

This should already produce a ton of debug output, but I generally find something like bluetoothd[363]: src/device.c:device_bonding_failed() status 14 and bluetoothd[363]: plugins/policy.c:disconnect_cb() reason 3 in there, which is not super-helpful by itself.

"btmon" tool which also comes with BlueZ provides a much more useful output with all the stuff decoded from the air, even colorized for convenience (though you won't see it here):

> ACL Data RX: Handle 11 flags 0x02 dlen 20               [hci0] 17.791382
      L2CAP: Information Response (0x0b) ident 2 len 12
        Type: Fixed channels supported (0x0003)
        Result: Success (0x0000)
        Channels: 0x0000000000000006
          L2CAP Signaling (BR/EDR)
          Connectionless reception
> HCI Event: Number of Completed Packets (0x13) plen 5    [hci0] 17.793368
        Num handles: 1
        Handle: 11
        Count: 2
> ACL Data RX: Handle 11 flags 0x02 dlen 12               [hci0] 17.794006
      L2CAP: Connection Request (0x02) ident 3 len 4
        PSM: 15 (0x000f)
        Source CID: 64
< ACL Data TX: Handle 11 flags 0x00 dlen 16               [hci0] 17.794240
      L2CAP: Connection Response (0x03) ident 3 len 8
        Destination CID: 64
        Source CID: 64
        Result: Connection pending (0x0001)
        Status: Authorization pending (0x0002)
> HCI Event: Number of Completed Packets (0x13) plen 5    [hci0] 17.939360
        Num handles: 1
        Handle: 11
        Count: 1
< ACL Data TX: Handle 11 flags 0x00 dlen 16               [hci0] 19.137875
      L2CAP: Connection Response (0x03) ident 3 len 8
        Destination CID: 64
        Source CID: 64
        Result: Connection refused - security block (0x0003)
        Status: No further information available (0x0000)
> HCI Event: Number of Completed Packets (0x13) plen 5    [hci0] 19.314509
        Num handles: 1
        Handle: 11
        Count: 1
> HCI Event: Disconnect Complete (0x05) plen 4            [hci0] 21.302722
        Status: Success (0x00)
        Handle: 11
        Reason: Remote User Terminated Connection (0x13)
@ Device Disconnected: 00:02:72:XX:XX:XX (0) reason 3

That at least makes it clear what's the decoded error message is, on which protocol layer and which requests it follows - enough stuff to dig into.

BlueZ also includes a crapton of cool tools for all sorts of diagnostics and manipulation, which - alas - seem to be missing on some distros, but can be built along with the package using --enable-tools --enable-experimental configure-options (all under "tools" dir).

I had to resort to these tricks briefly when trying to setup PANU/GN-mode connections, but as I didn't really need these, gave up fairly soon on that "Connection refused - security block" error (from that "policy.c" plugin) - no idea why BlueZ throws it in this context and google doesn't seem to help much, maybe polkit thing, idk.

Didn't need these modes though, so whatever.

Jan 30, 2015

Enabling i2c1 on BeagleBone Black without Device Tree overlays

Important: This way was pretty much made obsolete by Device Tree overlays, which have returned (as expected) in 3.19 kernels - it'd probably be easier to use these in most cases.

BeagleBone Black board has three i2c buses, two of which are available by default on Linux kernel with patches from RCN (Robert Nelson).

There are plenty of links on how to enable i2c1 on old-ish (by now) 3.8-series kernels, which had "Device Tree overlays" patches, but these do not apply to 3.9-3.18, though it looks like they might make a comeback in the future (LWN).
Probably it's just a bit too specific task to ask an easy answer for.

Overlays in pre-3.9 allowed to write a "patch" for a Device Tree, compile it and then load it at runtime, which is not possible without these, but perfectly possible and kinda-easy to do by compiling a dtb and loading it on boot.

It'd be great if there was a prebuilt dtb file in Linux with just i2c1 enabled (not just for some cape, bundled with other settings), but unfortunately, as of patched 3.18.4, there doesn't seem to be one, hence the following patching process.

For that, getting the kernel sources (whichever were used to build the kernel, ideally) is necessary.

In Arch Linux ARM (which I tend to use with such boards), this can be done by grabbing the PKGBUILD dir, editing the "PKGBUILD" file, uncommenting the "return 1" under "stop here - this is useful to configure the kernel" comment and running makepkg -sf (from "base-devel" package set on arch) there.
That will just unpack the kernel sources, put the appropriate .config file there and run make prepare on them.

With kernel sources unpacked, the file that you'd want to patch is "arch/arm/boot/dts/am335x-boneblack.dts" (or whichever other dtb you're loading via uboot):

--- am335x-boneblack.dts.bak    2015-01-29 18:20:29.547909768 +0500
+++ am335x-boneblack.dts        2015-01-30 20:56:43.129213998 +0500
@@ -23,6 +23,14 @@

 &ocp {
+       /* i2c */
+       P9_17_pinmux {
+               status = "disabled";
+       };
+       P9_18_pinmux {
+               status = "disabled";
+       };
        /* clkout2 */
        P9_41_pinmux {
                status = "disabled";
@@ -33,6 +41,13 @@

+&i2c1 {
+       status = "okay";
+       pinctrl-names = "default";
+       pinctrl-0 = <&i2c1_pins>;
+       clock-frequency = <100000>;
 &mmc1 {
        vmmc-supply = <&vmmcsd_fixed>;

Then "make dtbs" can be used to build dtb files only, and not the whole kernel (which would take a while on BBB).

Resulting *.dtb (e.g. "am335x-boneblack.dtb" for "am335x-boneblack.dts", in the same dir) can be put into "dtbs" on boot partition and loaded from uEnv.txt (or whatever uboot configs are included from there).

Reboot, and i2cdetect -l should show i2c-1:

# i2cdetect -l
i2c-0   i2c             OMAP I2C adapter                    I2C adapter
i2c-1   i2c             OMAP I2C adapter                    I2C adapter
i2c-2   i2c             OMAP I2C adapter                    I2C adapter

As I've already mentioned before, this might not be the optimal way to enable the thing in kernels 3.19 and beyond, if "device tree overlays" patches will land there - it should be possible to just load some patch on-the-fly there, without all the extra hassle described above.

Update 2015-03-19: Device Tree overlays landed in 3.19 indeed, but if migrating to use these is too much hassle for now, here's a patch for 3.19.1-bone4 am335x-bone-common.dtsi to enable i2c1 and i2c2 on boot (applies in the same way, make dtbs, copy am335x-boneblack.dtb to /boot/dtbs).

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