The compiler accepts MHLO, Tosa or linalg containing MLIR functions. While we aim for comprehensive support of those inputs, each is presently a subset, and we are finishing them based on e2e models, primarily via the frontends for Tensorflow, TFLite, PyTorch and Jax. Aside from simple things, few are hand coding IR of any complexity to feed the compiler, and we consider this layer to be fairly mutable and are not ready to write down a firm spec for it (with the exception of tosa, which has a spec).
The core compiler is meant to be fairly hackable and may satisfy needs outside of the existing frontends but supporting them would be use case/priority driven. It isn't quite clear from your question what you are trying to do.