Adding Build Dependencies to Clean the Source Tree

Linus Torvalds recently took the lead on adjusting the kernel build dependencies after Masahiro Yamada asked about the situation of compiled – or in some other way processed – files in the kernel source tree. For example, some files in the Git repository had been processed by Lex and Yacc before going into the tree. On one level, this was odd, because traditionally only true source files are included in source repositories. On another level, though, it was useful. If all those source files had to be compiled by the user, there would always be the risk that the user's system would contain the wrong versions of the various Lex, Yacc, GCC, and other utilities needed to compile those files properly.

In his post, Masahiro noted that the kbuild system had already added some code to recompile those processed blobs from source, so, he reasoned, why not make that the default?

In an unusual move, Linus decided to see about implementing this himself. Possibly he just felt like having a little fun with something that wasn't really very dangerous to play with. Whatever the reason, he started tinkering with the kbuild system to see how robust he could make it and still compile all the Lex, Yacc, and other source files from scratch.

For most tools, he found that it was easy to require and detect a minimal version number to be officially supported by a given kernel release, but in the case of gperf, he found that the tool didn't generate any version markers that could be used to detect which version was actually in use on a given system.

This was relevant because apparently gperf behavior had changed in version 3.1, causing it to generate kernel files improperly. Without a good way to test for that version of the tool, kbuild couldn't satisfy itself that it would produce a proper build on a given run.

The usual way to solve such issues would be to use autoconf, which Linus hated with a fiery passion. Instead, he felt that any tool lacking proper version markers should just be removed from the build requirements. In this case, he would remove any gperf dependencies from the Linux kernel.

So, Linux now depends on more tools than it did before, and yet it's highly unlikely that anyone will even notice the change, because all the tools it relies on are standard on every Linux distribution. Still, it's unusual for Linus to be so quick to allow new dependencies like that. Maybe he figured that the dependencies were there in any case; they were just hidden by the fact that the compiled versions of each file were included in the source tree.