Hi @ktand ,
in fact it already exists. Each arduino core define its own define: -DARDUINO_ARCH_{build.arch}
Use ARDUINO_ARCH_STM32

Hi @fpistm.

Unfortunately the Platformio framework that has been modified for STM32GENERIC defines the same ARDUINO_ARCH_STM32. I've created a HAL for STM32F4 for the Marlin project using STM32GENERIC but would like to be able use this core as well. Any idea how I could identify this framework without relying on the ARDUINO_ARCH_STM32 define?

Unfortunately, I would not add new define, official one is ARDUINO_ARCH_STM32
This define is used in several third party libraries for this core and do not want multiply define to identify this core.

@fpistm I realized that STM32GENERIC defines STM32GENERIC so I can use that define to switch on. I will close this PR.