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How can I compile a single driver/file in OpenBSD without compiling the whole kernel?

I have started to write a driver (for the I2C device PCF8574 from NXP) for OpenBSD.

Currently I compile the kernel with the official instruction https://www.openbsd.org/faq/faq5.html from the OpenBSD page.

But when I change the code of the driver and I type make, the whole kernel will be compiled. Not only the changed file. How can I prevent this? How can I compile a single file from the kernel and then link it with the rest of the kernel?

Thanks in advance!

Solution

  • Some parts are always rebuilt but the full kernel is never rebuilt if you use the correct options.

    # cd /usr/src/sys/arch/amd64/conf
# config GENERIC.MP
# cd ../compile/GENERIC.MP
# make clean
# make -j4
# make install

    Change any source file in /usr/src/sys and then:

    # cd /usr/src/sys/arch/amd64/compile/GENERIC.MP
# make config
# make -j4
# make install

    The last make -j4 will only build the changed files + other necessary files.