# The coreboot build system

(this document is still incomplete and will be filled in over time)

## General operation

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 `Makefile` and `toolchain.mk` 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 `Makefile.mk`.

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. `romstage`, `subdirs`, `cbfs-files`) and is used by filling in a variable of that name followed by `-y` (eg. `romstage-y`, `subdirs-y`, `cbfs-files-y`). The `-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 `y`, `n` or are empty.

This allows `class-$(CONFIG_FOO) += bar` to conditionally add `bar` to `class` depending on the choice for `FOO`.

## classes

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

`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 `Makefile.mk`.

Subdirectories are not read at the point where the `subdirs` statement resides but later, after the current directory is handled (and potentially others, too).

### cbfs-files

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 `foo` (required)

- `type`: The file type. Can be `raw`, `stage`, `payload`, and `flat-binary` (required)

- `compression`: Can be `none` or `lzma` (default: none)

- `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)

`position` and `align` are mutually exclusive.

### Adding Makefile fragments

You can use the `add_intermediate` helper to add new post-processing steps for the final `coreboot.rom` image. For example you can add new files to CBFS by adding something like this to `site-local/Makefile.mk`

    $(call add_intermediate, add_mrc_data)
    	$(CBFSTOOL) $< write -r RW_MRC_CACHE -f site-local/my-mrc-recording.bin

Note that the second line must start with a tab, not spaces.

See also Managing local additions.

#### FMAP region support

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.

The `position` and `align` options for file `foo` can be overwritten for a region `REGION` using `foo-REGION-position` and `foo-REGION-align`.

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 `REGION1,REGION2,REGION3`.

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 `src/vboot/Makefile.mk`.