I am keeping a WIFI driver alive by patching compilation errors for new Kernel versions. I can build it against a source tree, so I do not have to boot the kernel for which I want to fix it.
Unfortunately for this I have to fully compile the entire kernel. I know how to build a small version by using make localmodconfig, but that still takes very long.
Recently, I learned about the prepare target. This allows me to "compile" the module, so I learn about compilation problems. However, it fails in the linking phase, which prevents using make prepare in a Git bisect run. I also had the impression that it requires to clean the source tree from time to time due to spurious problems.
The question is: What is the fastest way to prepare a source tree so I can compile a Wifi module against it?
The target you are looking for is modules_prepare. From the doc:
An alternative is to use the "make" target "modules_prepare." This will make sure the kernel contains the information required. The target exists solely as a simple way to prepare a kernel source tree for building external modules.
NOTE: "modules_prepare" will not build Module.symvers even if CONFIG_MODVERSIONS is set; therefore, a full kernel build needs to be executed to make module versioning work.
If you run make -j modules_prepare (-j is important to execute everything in parallel) it should run pretty fast.
However if you need module versioning, which the kernel you are building for may require, you will also need make -j modules because modules_prepare does not create the Modules.symvers file.
What you need is something like this:
# Prepare kernel source
cd '/path/to/kernel/source'
make localmodconfig
make -j modules_prepare
# May also be needed if you need module versioning,
# in which case modules_prepare is not enough
# make -j modules
# Build your module against it
cd '/path/to/your/module/source'
make -j -C '/path/to/kernel/source' M="$(pwd)" modules
# Clean things up (will delete your .ko module so grab it and
# move it somewhere else first)
make -j -C '/path/to/kernel/source' M="$(pwd)" clean
cd '/path/to/kernel/source'
make distclean
The last cleaning up step is needed if you are in a bisect run before proceeding to the next bisection step, otherwise you may leave behind unwanted object files that might make other builds fail.