faehnri.ch

BeagleBone Buildroot

building a Linux image for the BeagleBone Black

I'm going through the Bootlin training for BeagleBone Black with Buildroot. Even though it says it's for the BeagleBone Black board, it's for the wireless version, which doesn't seem available but the regular Black board still is.

So I'm putting up my own notes on it. There's also another tutorial that covers this that I also referenced. I also found the BeagleBone System Reference Manual. The source for the training PDFs is also interesting to see how they made them with LaTeX. I got the board from Adafruit, which I found also has a document on the BeagleBone Black device tree

The Bootlin slides go into details, but it could be used more as a reference. You don't have to use go through that to go through the lab, so you can just do the lab.

First, get their lab materials and extract to the directory you want to keep it. Also make sure you're up to date.

#cd to the directory you want this to live
wget https://bootlin.com/doc/training/buildroot/buildroot-labs.tar.xz
tar xvf buildroot-labs.tar.xz
sudo apt update
sudo apt dist-upgrade
cd buildroot-labs/

Install the required dependencies. It called for just python, but there was only python3 for me, but I did see something called python-is-python3 which if you need something called just python maybe that will work. I also needed to add the ssl library. Also installs the serial communication program picocom we'll use later.

sudo apt install sed make binutils gcc g++ bash patch gzip bzip2 perl tar cpio python3 unzip rsync wget libncurses-dev libssl-dev picocom

Clone buildroot into the lab directory and checkout the version we'll use. Maybe look at tags and take whatever latest you want.

git clone https://git.buildroot.net/buildroot
cd buildroot
git checkout -b bootlin 2022.02

There's already a beaglebone_defconfig, but we'll build our own. We'll configure it with menuconfig.

make menuconfig

Now change the configuration:

If you see U-Boot board name, you might have something set wrong, or a change didn't get saved. Go through these settings again.

Then save and exit. Now you can make it, by splitting the output to an output file while also still printing to the terminal:

make 2>&1 | tee build.log

While that's building, set up the board. I got a USB to TTL serial cable from Adafruit. It's wires are different than whatever they must use in the bootlin tutorial. On the Adafruit one, It has:

Connect ground (black) to the pin closest to power/eth (call that pin 1). The connect the USB TX (green) to the board's RX (pin 4), and the USB RX (white) to the board's TX (pin 5).

Do NOT connect the red power wire. Documentation somewhere said connecting power could fry the serial connection on the board since it doesn't need it.

When you plug in the serial connector you should see it come up as /dev/ttyUSB0. You can also see it appear when you run dmesg.

If you didn't install the serial communication program picocom in the previous step and don't have another one you'd prefer to use, you can do that now.

sudo apt install picocom

You can see only the user root and the group dialout can write to /dev/ttyUSB0. Add yourself to the group dialout.

sudo adduser $USER dialout

If you run groups you'll see you're not yet part of the group dialout. You have to restart you computer for the change to take affect, so wait until you're done building and prepare an SD card in the mean time.

See what devices are on your computer with cat /proc/partitions or lsblk. Then connect your SD card and check devices again to see which is your SD card. You can also see the output of dmesg. Let's say our SD card is at /dev/sdd (But yours might be something else! Verify what it is!)

Unmount your SD card. Then erase the beginning of the SD card to be sure the existing partitions aren't detected. Double check which device is your SD card! /dev/sdd is just the example we're using here! Yours might be different!

sudo dd if=/dev/zero of=/dev/sdd bs=1M count=16

Create two partitions:

-L assigns a volume name to the partition

-E nodiscard disables bad block discarding. While this should be a useful option for cards with bad blocks, skipping this step saves long minutes in SD cards.

Remove the SD card and insert it again, the two partitions should be mounted automatically, in /media/$USER/boot and /media/$USER/rootfs.

Wait for the build to complete if it hasn't already.

Copy the MLO, u-boot.img, zImage and am335x-boneblack.dtb files from output/images/ to the boot partition of the SD card.

cp output/images/MLO output/images/u-boot.img output/images/zImage output/images/am335x-boneblack.dtb /media/eric/boot/
I like to run sync after copying files to disks to be sure it was written.

Extract the rootfs.tar file to the rootfs partition of the SD card.

sudo tar -C /media/$USER/rootfs/ -xf output/images/rootfs.tar

Create extlinux/extlinux.conf in the boot partition with

label buildroot
	kernel /zImage
	devicetree /am335x-boneblack.dtb
	append console=ttyO0,115200 root=/dev/mmcblk0p2 rootwait

Dismount and remove the SD card, insert into the board. Start the serial program.

	picocom -b 115200 /dev/ttyUSB0

You can quit picocom with ctrl-a then ctrl-x.

Hold the S2 button (the one by the USB port) and power the board to tell it to boot from the SD card instead of the eMMC.

Make sure you see the U-Boot version and date looks right. Login as root and with the password you chose, and explore the system.

The article also has this:

Note: if your system doesn’t boot as expected, make sure to reset the U-Boot environment by running the following U-Boot commands:

env default -f -a
saveenv

and reset. This is needed because the U-Boot loaded from the SD card still loads the U-Boot environment from the eMMC.