Building a Linux Powered Multi-copter from Scratch

Introduction

I purchased my first quadcopter about 6 years ago, back in the heyday of MultiWii. I flew it a few times, crashed it a few times, and it was fun for a little while, but my model lacked GPS, WiFi, and stabilized video, so it ended up sitting on a basement shelf for a few years.

Fast forward to fall 2017, where two things came together. One, I was able to get my hands on an Intel Aero RTF. This is a very powerful computing package on "wings." And two, the Beaglebone Blue came to my attention. It didn't take me long to come up a burning question...."I wonder if I can get the Beaglebone to do most of the stuff the Aero does?"

Let's find out!

Here is my syllabus for the next few blog posts. I'll try to match my normal development progression: get something working and then tweak it for performance.

1. Component Details
2. Initial Board Bring-up
3. Flight Control Software
4. Ground Tests
5. Mavlink Improvements
6. Boot Time Improvements

Stay tuned for Component Details...coming up after a short break.

Beaglebone Debian Read Only Filesystem

Overview

To save the eMMC and to allow for faster bootup/shutdowns, I wanted to mount the root partition as read-only. I spent many days researching and trying to find out what areas of the filesystem needed to have write permissions. It seemed like I was going in circles. Unionfs, aufs, mount -bind, overlayfs, tmpfs.....so many options and so many modules that didn't exist in the default BBB Debian kernel. I finally got so fed up that I decided to just start hacking away at the fstab.

Details

It turns out that the basic read-only filesystem is fairly easy to create, as everything that needs to be configured is contained in /etc/fstab. We need to change the root mount to read-only and add a couple of temporary file systems to handle logs and such. Let's start with an ordinary fstab created by the eMMC flasher script:

# /etc/fstab: static file system information.

#

# Auto generated by: beaglebone-black-eMMC-flasher.sh

#

UUID=716e1ca7-bc61-4958-8c0d-665e48102cfd  /  ext4  ro,noatime,errors=remount-ro  0  1

UUID=0DFE-E81B  /boot/uboot  auto  defaults  0  0

debugfs         /sys/kernel/debug  debugfs  defaults          0  0

We could then use the following sed command to add the "ro" tag to the root mount line and save it to a temporary fstab file:

> sudo sed s:ext4\ \ noatime:ext4\ \ ro,noatime: /etc/fstab_ro

Next we need to append some tmpfs declarations to our temporary fstab file:

> sudo nano /etc/fstab_ro

and put the following lines at the end:

tmpfs           /tmp  tmpfs  nodev,nosuid,size=32M  0  0

tmpfs           /srv  tmpfs  nodev,size=512K  0  0

tmpfs           /var/log  tmpfs  defaults,noatime,size=1M  0  0

tmpfs           /var/tmp  tmpfs  defaults,noatime,size=512K  0  0

tmpfs           /var/run  tmpfs  defaults,noatime,size=512K  0  0

Replace the real /etc/fstab with your temporary file:

> sudo cp /etc/fstab /etc/fstab.orig

> sudo mv /etc/fstab_ro /etc/fstab

Reboot and you're done!

You may have to experiment with different size limits, depending on your logging needs, but the sizes above should get you started.

Alternate Method

Systemd does the actually mounting, by parsing the fstab. The order in which the mounts are created is not guaranteed. If you need to create mounts in a particular order you need to create a systemd .mount unit for each mount:

> sudo nano /etc/systemd/system/my.mount

with content similar to the following:

[Unit]

Description=Runtime Directory

Before=local-fs.target



[Mount]

What=/mine

Where=/tmp/myfs

Type=bind

Options=bind

Remount RW

If you need to modify the filesystem, you can remount it rw, for a while, with the following command:

> sudo mount -o remount,rw /

To return to read-only mode, either reboot or run this command:

> sudo mount -o remount,ro /

References

https://groups.google.com/forum/#!topic/beagleboard/9J2r8xn3-Os
http://adis.ca/tag/beaglebone/
http://unix.stackexchange.com/questions/27449/mount-a-filesystem-read-only-and-redirect-writes-to-ram
https://help.ubuntu.com/community/aufsRootFileSystemOnUsbFlash
http://unix.stackexchange.com/questions/81959/how-to-mount-aufs-file-system-on-boot-in-archlinux
http://www.thegeekstuff.com/2013/05/linux-aufs/
https://wiki.debian.org/ReadonlyRoot
https://code.google.com/p/rootaufs/wiki/HowToUse

Qt 5.2.1 Beaglebone Binaries

Overview

So, how did I create these binaries....well I thought it would be fun to see how long it would take the Beaglebone to compile Qt. Here is a brief description of my setup.

I powered the Beaglebone with a 5v power supply and booted a Debian rootfs using TFTP and NFS (a future post will explain how to configure this) so I could utilize the large HD of my Linux host. After compiling and installing Qt, the total root filesystem usage is around 3 Gb. The actual compile took about 21 hours, which includes compiling all examples and plugins.

Configuration

I created a simple shell script to modify the Qt source, with the Beaglebone configuration, and run the Qt configuration script:

#!/bin/sh




# Create beaglebone hard float configuration file

cd qt-everywhere-opensource-src-5.2.1/qtbase/mkspecs/devices/

cp -r linux-beagleboard-g++ linux-beaglebone-g++

sed 's/softfp/hard/' <linux-beagleboard-g++/qmake.conf >linux-beaglebone-g++/qmake.conf





# Run configuration


cd ../../../

./configure -v -opensource -confirm-license -prefix /usr/local/qt-5.2 -no-largefile -no-accessibility -qt-zlib -no-gif -qt-libpng -qt-libjpeg -no-nis -no-cups -device linux-beaglebone-g++

This produced the following summary:

   Configure summary




Build type:    linux-g++ (arm, CPU features:)

Platform notes:




            - Also available for Linux: linux-kcc linux-icc linux-cxx

        

qmake vars .......... styles += mac fusion windows DEFINES += QT_NO_MTDEV QT_CFLAGS_DBUS = -I/usr/include/dbus-1.0 -I/usr/lib/arm-linux-gnueabihf/dbus-1.0/include   QT_LIBS_DBUS = -ldbus-1   QT_CFLAGS_GLIB = -pthread -I/usr/include/glib-2.0 -I/usr/lib/arm-linux-gnueabihf/glib-2.0/include   QT_LIBS_GLIB = -pthread -lgthread-2.0 -lrt -lglib-2.0   QT_CFLAGS_PULSEAUDIO = -D_REENTRANT -I/usr/include/glib-2.0 -I/usr/lib/arm-linux-gnueabihf/glib-2.0/include   QT_LIBS_PULSEAUDIO = -lpulse-mainloop-glib -lpulse -lglib-2.0   DEFINES += QT_NO_LIBUDEV QMAKE_CFLAGS_XCB =   QMAKE_LIBS_XCB = -lxcb   QMAKE_CFLAGS_DIRECTFB = -D_REENTRANT -I/usr/include/directfb   QMAKE_LIBS_DIRECTFB = -ldirectfb -lfusion -ldirect -lpthread   sql-drivers =  sql-plugins =  sqlite qmake switches ......... 




Build options:

  Configuration .......... alsa audio-backend c++11 clock-gettime clock-monotonic compile_examples concurrent dbus directfb evdev eventfd full-config getaddrinfo getifaddrs glib iconv inotify ipv6ifname large-config linuxfb medium-config minimal-config mremap neon no-gif no-harfbuzz opengl openssl pcre png posix_fallocate precompile_header pulseaudio qpa qpa reduce_exports reduce_relocations release rpath shared small-config system-freetype xcb xcb-qt xkbcommon-qt xlib zlib 

  Build parts ............ libs tools examples

  Mode ................... release

  Using C++11 ............ yes

  Using PCH .............. yes

  Target compiler supports:

    iWMMXt/Neon .......... no/yes




Qt modules and options:

  Qt D-Bus ............... yes (loading dbus-1 at runtime)

  Qt Concurrent .......... yes

  Qt GUI ................. yes

  Qt Widgets ............. yes

  JavaScriptCore JIT ..... yes (To be decided by JavaScriptCore)

  QML debugging .......... yes

  Use system proxies ..... no




Support enabled for:

  Accessibility .......... no

  ALSA ................... yes

  CUPS ................... no

  FontConfig ............. no

  FreeType ............... system

  Iconv .................. yes

  ICU .................... no

  Image formats: 

    GIF .................. no

    JPEG ................. yes (plugin, using bundled copy)

    PNG .................. yes (in QtGui, using bundled copy)

  Glib ................... yes

  GTK theme .............. no

  Large File ............. no

  mtdev .................. no

  Networking: 

    getaddrinfo .......... yes

    getifaddrs ........... yes

    IPv6 ifname .......... yes

    OpenSSL .............. yes (loading libraries at run-time)

  NIS .................... no

  OpenGL ................. desktop

  OpenVG ................. no

  PCRE ................... yes (bundled copy)

  pkg-config ............. yes 

  PulseAudio ............. yes

  QPA backends: 

    DirectFB ............. yes

    EGLFS ................ no

    KMS .................. no

    LinuxFB .............. yes

    XCB .................. yes (bundled copy)

      MIT-SHM ............ yes

      Xcb-Xlib ........... no

      Xcursor ............ yes (loaded at runtime)

      Xfixes ............. yes (loaded at runtime)

      Xi ................. yes (loaded at runtime)

      Xi2 ................ no

      Xinerama ........... yes (loaded at runtime)

      Xrandr ............. yes (loaded at runtime)

      Xrender ............ no

      XKB ................ yes

      XShape ............. yes

      XSync .............. yes

      XVideo ............. yes

  Session management ..... yes

  SQL drivers: 

    DB2 .................. no

    InterBase ............ no

    MySQL ................ no

    OCI .................. no

    ODBC ................. no

    PostgreSQL ........... no

    SQLite 2 ............. no

    SQLite ............... yes (plugin, using bundled copy)

    TDS .................. no

  udev ................... no

  xkbcommon .............. yes (bundled copy)

  zlib ................... yes (bundled copy)




NOTE: libxkbcommon 0.2.0 (or higher) not found on the system, will use 

the bundled version from 3rd party directory.

NOTE: Qt is using double for qreal on this system. This is binary incompatible against Qt 5.1.

Configure with '-qreal float' to create a build that is binary compatible with 5.1.

If you've tried cross compiling Qt, from my other posts, you will notice a lot more modules are enabled in this build.

I've tested a few examples and they all work, but I have not tested everything (haven't been able to test touch screen stuff yet). The examples default to using X11, if you want to use the linuxfb, add '-platform linuxfb' to the command line options. For example:

> analogclock -platform linuxfb

Here are two versions of binaries, one that includes examples (78 Mb) and one that does not (36 Mb).

MD5SUM:
9395fc70f576e58b68df7d732a002abb  qt-5.2-no-examples.tgz
f698f038166c4b2f65cec40056e23db2  qt-5.2.tgz

Extract to /usr/local, as the tarball contains the qt-5.2 folder in it.

If you want to try your hand at compiling applications on the Beaglebone, these packages have a native qmake included. Don't forget to add /usr/local/qt-5.2/bin to your path.

Have fun!

element14 BB-View and the Debian Beta

This is a post I made at the element14 Community pages:

I thought some other might like to know how to get the BB-View working under the latest Debian beta images.

Unfortunately, you are going to have to recompile the Kernel. The video works fine without a recompile (the red/blue swap is easy to fix), but the touchscreen control connections are not the same as the standard TI 4-wire interface. I tried to patch the ti_am335x_tsc.c file, so the changes would be limited to a dts recompile, but I haven't been able to get that working yet (I'm pretty sure it is a state machine / bit assignment issue). But anyway, here are the steps that you need to perform.....

Overview

1) Download the necessary files

     a) Robert Nelson's handy tools for compiling the Beaglebone kernel

     b) BB-View source for Angstrom

2) Build the default kernel

3) Patch the Kernel and perform a fast recompile

4) Copy the new kernel to the Beaglebone

5) Edit the xorg.conf file to correct the Red/Blue color swap

6) Revel in the 4 days you just saved 

Let's Get Started

I am using Ubuntu 12.04 LTS 64 bit running in a VirtualBox VM to compile the kernel:

Make a new folder called bb-view, this is where our build will happen.

> mkdir bb-view

> cd bb-view

Install git, if you haven't already:

> apt-get install git

Clone Robert Nelson's linux-dev project locally. Be prepared for a large download (~100 Mb for the cross-compiler and ~700 Mb for the kernel source).

> git clone https://github.com/RobertCNelson/linux-dev.git

Change into the new linux-dev folder and select branch/tag that matches the Debian version.

> cd linux-dev

> git checkout 3.8.13-bone37 -b tmp

Now we need to build the base image, so our cape drivers folder gets populated. The script will tell you if you need to do or install anything else. 

> ./build_kernel.sh

Come back in an hour or so.....(You might want to download the Angstrom source from element14 now)

Patching the Kernel

Extract two files from the Angstrom source:

> unzip angstrom-source.zip

> tar -zxf bb-black-kernel-3.8.13-bb-view.tar.bz2

> cp ./kernel/kernel/drivers/input/touchscreen/ti_am335x_tsc.c  ~/bb-view/linux-dev/KERNEL/drivers/input/touchscreen/

> cp ./kernel/kernel/firmware/capes/BB-VIEW-LCD7-01-00A0.dts  ~/bb-view/linux-dev/KERNEL/firmware/capes/

Now we need to let the compiler know that we want to add the firmware to the build:

> nano   ~/bb-view/linux-dev/KERNEL/firmware/Makefile

Add the following line somewhere near line 192 (CTRL-C will display current cursor position):

BB-VIEW-LCD7-01-00A0.dtbo \

Don't forget the trailing backslash....it is important. Now save and exit by doing CTRL-O, Enter, CTRL-X.

Change back to the linux-dev root folder:

> cd   ~/bb-view/linux-dev

Now issue the kernel rebuild command, this won't take very long at all:

> ./tools/rebuild.sh

Copy Kernel to BBB

The following instructions assume that you already have the Debian beta installed and booted up on the BBB.

The easiest way is to copy the files over the network, via the 'scp' command:

> scp   ~/bb-view/linux-dev/deploy/3.8.13-bone37.zImage   debian@192.168.7.2:/home/debian

Replace debian (both instances) with the username you are running on the BBB and 192.168.7.2 with the IP address of the BBB.

Now log into the BBB and copy the kernel image to the boot partition:

> ssh debian@192.168.7.2

bbb>  sudo cp 3.8.13-bone37.zImage   /boot/uboot/zImage

We need to do one more thing before the LCD will work. Since the cape doesn't have an EEPROM, we need to black-list the HDMI drivers and force-load the BB-VIEW drivers in the boot command file:

bbb>  sudo  nano  /boot/uboot/uEnv.txt

Find the "optargs" line and edit it to be:

optargs=capemgr.disable_partno=BB-BONELT-HDMI,BB-BONELT-HDMIN  capemgr.enable_partno=BB-VIEW-LCD7-01

Notice that the preceding "#" has been removed. You don't want to disable BB-BONE-EMMC-2G, or you won't be able to boot from eMMC.

Now save and exit by doing CTRL-O, Enter, CTRL-X.

Reboot the BBB and the LCD should now work....except the blue and red are reversed.

Fixing Red/Blue Color Swap

This is due to errata in the TI AM335x processor when switching between 16 and 24 bit video modes. To fix this, we must first find the name of our screen:

> ssh debian@192.168.7.2

bbb> cat  /var/log/Xorg.0.log  |  grep  screen

Mine was "Builtin Default fbdev Screen 0"

Now we edit our X configuration file:

bbb> sudo nano  /usr/share/X11/xorg.conf.d/10-evdev.conf

and add a "Screen" section at the end of the file:

Section "Screen"

        Identifier "Builtin Default fbdev Screen 0"

        Monitor "Configured Monitor"

        Device "Configured Video Device"

        DefaultDepth 24

EndSection

Save and exit by doing CTRL-O, Enter, CTRL-X.

Reboot and enjoy!

ArchLinux on BeagleBone and Linux 3.8

I've discovered the joys of embedded programming with the new Linux kernel 3.8. It is bleeding edge and not for the faint of heart. The way that I/O works has been totally changed, so anything that relied on the /sys/kernel/debug/omap_mux/ folder is broken.

This has to do with switching over to Device Tree Overlays for processor configurations. The README.md file at https://github.com/jadonk/validation-scripts/tree/master/test-capemgr has a good explanation, but that doesn't make it any easier to get it up and running.

I've cloned the validation-scripts repository from https://github.com/jadonk/validation-scripts.git. I tried to run do_pinctrl_test.sh from test-capemgr folder, but it needs dtc. No problem, right, we'll just install it....wrong! The current version of dtc in ALARM is not patched to support the -@ option (see https://patchwork.kernel.org/patch/1934471/), so you get a "dtc: invalid option -- '@'" error message.

I'm not as hardcore as the guys at http://hipstercircuits.com/adding-beaglebone-cape-support-to-a-kernel-with-device-tree-in-ubuntu/ so I am calling it a night. I will look into how to get a patched version of dtc, but if it involves too much, I can see myself jumping ship and reverting to an older kernel.

FYI: Here are a couple of issues that I ran into, when trying to install ArchLinux, and how to fix them:

I followed the instructions at http://archlinuxarm.org/platforms/armv7/beaglebone for configuring and setting up the SD card. When I followed the links provided though, I ran into 2 issues:

1. I could get a u-boot command prompt, but it could not load the kernel image from the SD card. This was solved by copying /boot/uImage to the DOS partition.
2. After looking for upgrades ('pacman -Syu'), the kernel update gave a warning that my u-boot was probably out of date. After a reboot, sure enough the "image is wrong type" error message appeared in u-boot. I corrected this by downloading and installing http://os.archlinuxarm.org/os/omap/BeagleBone-bootloader.tar.gz