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# sources
(ordered chronologically by access time)
- [Kbuild object variable name rationale][kbuild-obj-var]
- [Kbuild kernel module setup guide][kbuild-module-makefile]
- [`struct file_operations`][c-struct-file-operations] documentation, also
- write(2)
- read(2)
- (Sadly) [the Linux kernel source][kernel] itself, especially
- drivers/char/random.c
- drivers/char/mem.c
- [Linux kernel labs - character device drivers][kernel-labs-chrdev]
- [Core API][core-api]
- [Driver API][driver-api]
- [AM335x Technical reference manual][ti-am335x-rm]
- [BeagleBone Black system reference manual][bbb-srm]
- [I2C device driver guide][linux-i2c]
[kbuild-obj-var]: https://docs.kernel.org/kbuild/makefiles.html#loadable-module-goals-obj-m
[kbuild-module-makefile]: https://www.kernel.org/doc/html/latest/kbuild/modules.html#creating-a-kbuild-file-for-an-external-module
[c-struct-file-operations]: https://www.kernel.org/doc/html/latest/filesystems/vfs.html#struct-file-operations
[kernel]: https://github.com/torvalds/linux
[kernel-labs-chrdev]: https://linux-kernel-labs.github.io/refs/heads/master/labs/device_drivers.html
[core-api]: https://docs.kernel.org/core-api/kernel-api.html
[driver-api]: https://www.kernel.org/doc/html/latest/driver-api/infrastructure.html
[ti-am335x-rm]: https://www.ti.com/lit/ug/spruh73q/spruh73q.pdf
[bbb-srm]: https://cdn-shop.adafruit.com/datasheets/BBB_SRM.pdf
[linux-i2c]: https://www.kernel.org/doc/html/latest/i2c/writing-clients.html
# 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 `chroot`ing 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.
1. 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.
2. 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)
3. 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).
4. Run the following command to to activate the connection.
```
$ nmcli connection up NAME
```
5. (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).
6. 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.
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