Use ctags. This will save a huge amount of time as compared to doing searches and jumping around between directories.
Use ag. This is the "silver searcher". I spent a lot of time doing "grep -R xyz ." With "ag", you just type "ag xyz" which is much easier. It runs much faster, and it provides much more convenient output.
Rely on breadcrumbs. Namely object files. Build U-Boot for your target and then you will know what source files actually get used in the build, because you will see board.o sitting alongside board.c The build history can also be instructive, but keeping the object files scattered around in the source tree is a great help. Typing "make clean" gets rid of them. Don't do that.
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