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I have been developing applications for ESP32 and ESP8266 devices on FreeBSD for quite some time. Here is a summary of how to develop code for the embedded devices on FreeBSD.
FreeBSD Wiki has a page about the same topic. However, it includes quite a lot of messy hacks.
To build projects, you need two requirements; SDK (arduino people call it core) and toolchain. An SDK provides standard functions and libraries. A toolchain provides programs to compile and link your code. SDK and toolchain are tightly coupled. You cannot swap one of it; you have to install a toolchain for an SDK.
ESP32 Overview
ESP32 development is easier than ESP8266. Unless required, choose esp-idf. Otherwise, use espressif/arduino-esp32.
esp-idf
Choose this SDK, if you want to write C code, or prefer well-documented SDK and build environment. espressif does not support FreeBSD, but they have been very open to patches.
The SDK depends on the official toolchain, espressif/crosstool-NG. The port in the official FreeBSD ports tree is lagged behind. Besides, it does not allow to install multiple versions.
My updated version can be found at trombik/xtensa-esp32-elf. The port has FLAVOR, idf3 and idf4. Each corresponds to esp-idf version. Choose the one you are using. Note that there is no reason to develop new application with esp-idf version 3.x. The version 4.x is far better. If you are developing your own code, choose idf4.
espressif/arduino-esp32
Choose this SDK, if you want to write C++ code, or to use arduino libraries.
The SDK depends on specific version of the official toolchain. Their release page mentions which esp-idf is used for releases. Use FLAVOR mentioned above to choose version.
ESP8266 Overview
For ESP8266, there are two options: ESP8266 RTOS SDK, or esp-idf, and esp8266/Arduino. ESP8266 RTOS SDK works when it works, but esp8266/Arduino is better. esp-open-rtos is another option, but is not updated very often.
ESP8266 RTOS SDK
Choose this SDK if you want to develop applications compatible with esp-idf for ESP32, or to write pure C code. As I do not like arduino, I usually choose the SDK. The SDK is poorly maintained, but libraries and build procedures are quite similar — but not identical — to esp-idf. You can port code for ESP32 to ESP8266 most of time. Unfortunately, libraries in the ESP8266 RTOS SDK are often out-of-sync, especially networking libraries. Issues and Pull Requests are ignored. The documentation is poor.
The SDK depends on the official toolchain from espressif, espressif/crosstool-NG. I ported the toolchain based on the one written by others, which can be found at: trombik/freebsd-ports-xtensa-lx106-elf.
ESP8266 RTOS SDK released several versions, but you often have to use master branch because fixes in esp-idf are not merged in a timely fashion.
esp8266/Arduino
Choose this SDK, when you want to use libraries for arduino, or to write C++. The documentation is better than ESP8266 RTOS SDK, but less than esp-idf for ESP32.
The SDK depends on a third-party toolchain, esp-quick-toolchain. The toolchain is developed for specific GCC versions. You must use a version of the SDK designed for the GCC version used in the esp8266/Arduino. At the moment, esp8266/Arduino version 3.x.x uses GCC 10.x, esp8266/Arduino version 2.x.x uses GCC 4.8.
I ported the toolchain for FreeBSD — devel/esp-quick-toolchain — which can be found at: trombik/freebsd-ports-esp-quick-toolchain.
The port has FLAVORS so that you can install one toolchain for esp8266/Arduino version 2.x.x, and another for version 3.x.x.
Build system
A build system is a system to compile, link, and upload your code. You have a few options:
arduinoIDEplatformioesphomeidf.pyofesp-idf
arduino IDE is not recommended because the GUI is counter-intuitive, hard to maintain projects, and all other reasons why you should avoid GUI. The FreeBSD ports tree has a port of arduino GUI, but the port is not esp-friendly. The maintainer decided to drop the library manager all together, which makes it quite useless. The upstream has recently released a CLI version, but I have not tried yet. The FreeBSD ports tree has two versions: devel/arduino and devel/arduino18. Confusingly, devel/arduino is very old version, and should not be used. Use devel/arduino18 if you really need to.
platformio works mostly fine on FreeBSD. The major issue is; they maintain their own binaries. As you would guess, they do not publish binaries for FreeBSD. However, with my private ports and symlink hacks, it works. You don’t have to install random binaries from the Internet. See how to use my ports with platformio in the READMEs of the ports.
esphome is sort of block programming for esp32 and esp8266. In most cases, your code does same things, like reading sensor values in a loop, publishing values over MQTT, displaying results on a display, performing OTA, etc. Even when there are many libraries available, it is still repetitive, and boring. esphome do these routines, and you can focus on the core logic. All your code is in a YAML file, but you can implement quite complex logic. esphome is actually using platformio as their build system. If platformio works on your machine, esphome should work, too. esphome is C++ framework, using esp8266/Arduino behind the scene. That means you need to install my devel/esp-quick-toolchain port. esphome is recommended for sensors and home automation devices. esphome is integrated in homeassistant, but not very tightly. You can still write independent devices or sensors without homeassistant.
idy.py is a CLI tool for esp-idf. With my ports, it works on FreeBSD (remember to use esp-idf version 4.x.x). They still support GNU make build system, but it has been deprecated.
Conclusion
Embedded development for ESP32 and ESP8266 is possible on FreeBSD. Prefer esp-idf SDK, which is very well-documented. For ESP8266, your best bet would be esp8266/Arduino version 3.x.x, which is based on esp-idf version 4.x.x. Use platformio, or esphome for building arduino code. Use idf.py for esp-idf.