I would like to know the source code repository for openGL implementation. Is openGL API implementation entirely up to GPU driver developers or is there some open source implementation.
In most implementations, rasterisation is performed by hardware; the software part is mostly about tracking state and issuing commands to the hardware.
For an implementation which includes software rasterisation, look at Mesa.
The “open” in OpenGL has nothing whatsoever to do with “open source” - OpenGL actually pre-dates the open source movement (1992 vs 1998). What OpenGL is is an open standard. Unfortunately the presence of the word “open”, together with the fact that OpenGL drivers are available for Linux, seems to have led to this confusion: a classic case of adding two and two and getting five.
As a general rule OpenGL is implemented by your GPU vendor. It’s part of the device driver for your hardware, and it’s primary task is to act as a translation layer between your program and your hardware. Your program makes hardware-independent OpenGL calls, the device driver takes those hardware-independent calls and converts them to something hardware-specific, then your GPU does the actual work of drawing something. Don’t get the idea that OpenGL is in some way a software library because it’s not.
Exceptions exist. As already mentioned, Mesa is a pure software implementation. Microsoft ship their own software implementation with Windows. ANGLE is a software wrapper around Direct3D. And sometimes a vendor will provide a software emulated path rather than a hardware accelerated one for features which their hardware doesn’t support but which the OpenGL spec mandates their driver must provide.
If you want to validate your GLSL code, there is also a reference implementation for GLSL.