When is the kernel "source" symlink created?

[Goverp]

I'm trying to document the "right way" to build kernels from a normal user rather than root. The basic command is

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cd builddir
make -C /usr/src/linux O=$PWD <target>

which at some point creates a "builddir/source" symlink pointing at the source tree (ie, the target of the /usr/src/linux symlink).

I can't find any documentation as to when said source symlink is created, nor indeed overwritten. Tests show that "make clean" doesn't create it (which was a surprise). "make defconfig" does, so I suspect it's anything that creates something. Which is actually rather annoying.

The trouble is, having created the symlink, it's then permissible to just use "make <target>" without the "-C" parameter; make follows the symlink to the kernel source - note, the versioned /usr/src/linux-5.12.1-gentoo or whatever /usr/src/linux pointed at when the symlink was created. Which may be the right thing to do; it means you can download a new kernel and update the symlink, but make will still address the old version until you do something to update the source symlink.

I can see a couple of use cases:
a) normal kernel building: update gentoo-sources with USE="symlink", and issue "make -C /usr/src/linux -O=builddir clean oldconfig" to start building a new kernel with the latest sources.
b) external kernel building: in this case I'm thinking of cross-compiling a kernel for a Raspberry Pi or some other host. For whatever reason, I may not wish to update to the latest sources, so I'd want to simply "cd builddir; make <target>"

I'm trying to write a single shell script to simplify this, as it's far too easy to make a mess instead of a kernel. Perhaps a new Makefile target "newsource" that takes an optional directory defaulting to /usr/src/linux (or more likely a fake target handled in the script rather than dare to touch a Makefile). Which brings me to the title of this post: when/how should the script cause the "source" symlink to be set? I thought it might be enough to "make -C ... kernelversion", but that reports the version from the "-C" source, while leaving the source symlink pointing where it was, which is useless!

MY gut feel is that this is just a shambles, rather than a documented and well-considered API

(For anyone interested, my documentation thus far is in my user page in the wiki.)

[Naib]

its either made if you have the symlink USE flag (and thus made at emerge) or when the user sets this (via eselect kernel). NOTE this symlink is needed for external modules or for end-user convenience .

[trilithium]

This seems to be the relevant part of /usr/src/linux-5.12.2-gentoo/Makefile:

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PHONY += outputmakefile
# Before starting out-of-tree build, make sure the source tree is clean.
# outputmakefile generates a Makefile in the output directory, if using a
# separate output directory. This allows convenient use of make in the
# output directory.
# At the same time when output Makefile generated, generate .gitignore to
# ignore whole output directory
outputmakefile:
ifdef building_out_of_srctree
	$(Q)if [ -f $(srctree)/.config -o \
		 -d $(srctree)/include/config -o \
		 -d $(srctree)/arch/$(SRCARCH)/include/generated ]; then \
		echo >&2 "***"; \
		echo >&2 "*** The source tree is not clean, please run 'make$(if $(findstring command line, $(origin ARCH)), ARCH=$(ARCH)) mrproper'"; \
		echo >&2 "*** in $(abs_srctree)";\
		echo >&2 "***"; \
		false; \
	fi
	$(Q)ln -fsn $(srctree) source
	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/mkmakefile $(srctree)
	$(Q)test -e .gitignore || \
	{ echo "# this is build directory, ignore it"; echo "*"; } > .gitignore
endif

The outputmakefile target might be useful for what you are trying to do?

[Goverp]

trilitium, thanks! I'd seen that lurking in the Makefile, but hadn't realised I could simply "make -C foo outputmakefile"; that does exactly what I wanted.