(this document is still incomplete and will be filled in over time)
The coreboot build system is based on GNU make but extends it significantly to the point of providing its own custom language. The overhead of learning this new syntax is (hopefully) offset by its lower complexity.
The build system is defined in the toplevel
toolchain.inc and is supposed to be generic (and is in fact used with a number of other projects). Project specific configuration should reside in files called
In general, the build system provides a number of "classes" that describe various parts of the build. These cover the various build targets in coreboot such as the stages, subdirectories with more source code, and the general addition of files.
Each class has a name (eg.
cbfs-files) and is used by filling in a variable of that name followed by
-y suffix allows a simple interaction with our Kconfig build configuration system: Kconfig options are available as variables starting with a
CONFIG_ prefix and boolean options contain
n or are empty.
class-$(CONFIG_FOO) += bar to conditionally add
class depending on the choice for
Classes can be defined as required.
subdirs is handled internally since it's parsed per subdirectory to add further directories to the rule set.
TODO: explain how to create new classes and how to evaluate them.
subdirs contains subdirectories (relative to the current directory) that should also be handled by the build system. The build system expects these directories to contain a file called
Subdirectories are not read at the point where the
subdirs statement resides but later, after the current directory is handled (and potentially others, too).
This class is used to add files to the final CBFS image. Since several more options need to be maintained than can comfortably fit in that single variable, additional variables are used.
cbfs-files-y contains the file name used in the CBFS image (called
foo here). Additional options are added in
foo-$(option) variables. The supported options are:
file: The on-disk file to add as
type: The file type. Can be
compression: Can be
position: An absolute position constraint for the placement of the file (default: none)
align: Minimum alignment for the file (default: none)
options: Additional cbfstool options (default: none)
align are mutually exclusive.
With the addition of FMAP flash partitioning support to coreboot, there was a need to extend the specification of files to provide more precise control which regions should contain which files, and even change some flags based on the region.
Since FMAP policies depend on features using FMAP, that's kept separate from the cbfs-files class.
align options for file
foo can be overwritten for a region
The regions that each file should end in can be defined by overriding a function called
regions-for-file that's called as
$(call regions-for-file,$(filename)) and should return a comma-separated list of regions, such as
The default implementation just returns
COREBOOT (the default region) for all files.
vboot provides its own implementation of
regions-for-file that can be used as reference in