Laura Abbott: Kbuild tricks

Several of the tasks I’ve worked on recently have involved looking at some
of the kernel’s build infrastructure. This is all fairly well
documented
which makes it nice to work with.

The kernel automatically generates some files at build time. This is mostly
set up to be transparent to developers unless they are looking for them.
The majority of these files are headers at include/generated. A good example
of something which needs to be generated is the #define representing the
kernel version (e.g. 4.15.12). The header file include/generated/bounds.h
contains #defines for several enum constants calculated at build time.
Cleverly, most of these files are only actually replaced if the generated
output changes to avoid unnecessary recompile. Most of this work is handled
by the filechk macro.

The C preprocessor is typically used on C files, as one might obviously expect.
It’s not actually limited to C files though. Each architecture has to define
a linker script which meets the architectural requirements. The linker language
is common across architectures so it’s beneficial to have common definitions
for typical sections such as initcalls and rodata. There’s a global
rule
to run the pre-processor on any .lds.S file. Devicetree files also get
preprocessed which avoids a lot of copy and pasting of numerical defines.

The compiler flags are typically set in the top level Makefile and named
as you might expect (CFLAGS, CXXFLAGS etc.). The process of building the
kernel requires building a number of smaller programs. The c flags for
these programs are controlled by a different set of variables (HOSTCFLAGS).
It sounds incredibly obvious but I’ve lost time from my day trying to figure
out why setting options in CFLAGS weren’t being picked up by the host
compiler. For more fun, it’s possible to use environment variables to set
different flags for compiling built-in vs. module code. The moral of
the story is know what you’re setting.

Debugging build infrastructure isn’t always pleasant but the kernel build
system isn’t too bad overall. I’m at least beginning to understand more parts
of it as I find increasingly more obscure things to modify.


Source From: fedoraplanet.org.
Original article title: Laura Abbott: Kbuild tricks.
This full article can be read at: Laura Abbott: Kbuild tricks.

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