What is the best open source IDE for Arm software developer ?

The free DS-5 Community Edition might be a great tool, but being intended to "accelerate the development of native (C/C++) embedded Linux and Android applications" might not be very useful for quanhu who mentions mbed online and Keil, so apparently looks for solutions to develop bare metal applications.

As a conclusion, under the Eclipse umbrella there are many integrated tools and even more separate plug-ins to support either Linux based embedded and/or bare metal applications development (GNU Arm Eclipse supports both, in 32 and 64-bits variants, but is intended mainly to bare metal Cortex-M applications).

The free DS-5 Community Edition might be a great tool, but being intended to "accelerate the development of native (C/C++) embedded Linux and Android applications" might not be very useful for quanhu who mentions mbed online and Keil, so apparently looks for solutions to develop bare metal applications.

As a conclusion, under the Eclipse umbrella there are many integrated tools and even more separate plug-ins to support either Linux based embedded and/or bare metal applications development (GNU Arm Eclipse supports both, in 32 and 64-bits variants, but is intended mainly to bare metal Cortex-M applications).