I would like to propose a unified logging library in TF-M. The primary aim of this is to combine the fragmented logging we have across TF-M into a single logging library, with a single top-level API. Currently, different components (BL1, SPM, secure partition, etc.) have different logging APIs and these also have different underlying implementations. Some call directly into the UART output string function and others call printf. Sometimes using one logging API in a different component leads to build failures, or nothing ending up on the UART at all.

The primary aim here is for users to be able to use the same API throughout TF-M and for it to always work. This API will naturally be split into different logging levels, with build configuration controlled whether or not the string is actually output. Note that the underlying implementation can be different for different components - there can be hooks within the library which allow components to specify how they want to output the string. Initially, these can be used to maintain the existing underlying implementation. In the longer term, however, it would be useful to unify the underlying implementation also whether that be by using printf or with our own custom printf format parser.