sources

(ordered chronologically by access time)

Kbuild object variable name rationale
Kbuild kernel module setup guide
struct file_operations documentation, also
- write(2)
- read(2)
(Sadly) the Linux kernel source itself, especially
- drivers/char/random.c
- drivers/char/mem.c
Linux kernel labs - character device drivers
Core API
Driver API
AM335x Technical reference manual
BeagleBone Black system reference manual
I2C device driver guide

tips

Start writing a kernel module that targets the kernel version of the BeagleBone image you're planning on using.
When the kernel documentation references manpages, these actually contain useful information. Manpages are not only for commands, but also include detailed documentation for the syscall interface and other C APIs.

Since most of the manpages referenced by the kernel are from section 2 (on system calls), you should append the number between brackets to the man command, i.e. to read 'write(2)' use the command man 2 write.
Use dmesg with the -w or -W option (see man dmesg(1))
If you somehow manage to corrupt/break your system in any way, the IoT images seem to work fine for chrooting and fixing stuff. (Installing a linux-image- package from chroot prints lots of errors, but seems to work fine afterwards?). I have not tried using the IoT images as an installation base, as we were steered away from using these images due to slow boot times.
Just use the decompiled device tree blobs instead of trying to compile the device tree sources from the linux kernel.
You can check if your modified device tree blob is working by cat-ing the output of /sys/firmware/devicetree/base/led_extern/label and checking if it matches with the label you set in the led_extern section of your device tree source file.
ZSH on the BeagleBone appears to print the prompt over command output if it is not terminated with a newline character. You can work around this by using echo "$(cat ...)" instead.

direct ethernet setup

You can use NetworkManager to both share your computer's existing network connection and start a DHCP server for assigning IP addresses automatically. This allows you to ssh into the BeagleBone, as well as share your internet connection (even eduroam!) over a single direct ethernet connection.

Connect the BeagleBone to your computer using an ethernet cable, and make sure the BeagleBone is on. The ethernet port's link lights should start blinking.

Run nmcli device on your computer, note the name of the connection on the ethernet port:

$ nmcli device             
DEVICE         TYPE      STATE                                  CONNECTION
wlan0          wifi      connected                              eduroam
enp0s25        ethernet  connecting (getting IP configuration)  Wired connection 1
lo             loopback  connected (externally)                 lo

(the automatically generated name Wired connection 1 is the ethernet connection to the BeagleBone)

Run the following command to enable sharing on this port (replace NAME with the connection name you found).
```
$ nmcli connection modify NAME ipv4.method shared ipv4.addresses 192.168.2.2/24
```
This assigns the ip address 192.168.2.2 to your computer, and gives the BeagleBone an address in the 192.168.2.0/24 range (edit this only if it conflicts with another subnet).
Run the following command to to activate the connection.
```
$ nmcli connection up NAME
```
(Optionally) use nmap -sn 192.168.2.0/24 as root to find the IP address the BeagleBone got, or just connect to the BeagleBone using the hostname you configured before (in /etc/hostname on the BeagleBone).
Profit

When you're done doing stuff on the BeagleBone, and want to reverse all the above changes, simply run:

$ nmcli connection delete NAME

NetworkManager will automatically re-configure the ethernet connection for regular use the next time you plug in an ethernet cable.