87 lines
3.7 KiB
Markdown
87 lines
3.7 KiB
Markdown
# The coreboot build system
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(this document is still incomplete and will be filled in over time)
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## General operation
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The coreboot build system is based on GNU make but extends it significantly
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to the point of providing its own custom language.
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The overhead of learning this new syntax is (hopefully) offset by its lower
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complexity.
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The build system is defined in the toplevel `Makefile` and `toolchain.inc`
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and is supposed to be generic (and is in fact used with a number of other
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projects). Project specific configuration should reside in files called
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`Makefile.inc`.
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In general, the build system provides a number of "classes" that describe
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various parts of the build. These cover the various build targets in coreboot
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such as the stages, subdirectories with more source code, and the general
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addition of files.
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Each class has a name (eg. `romstage`, `subdirs`, `cbfs-files`) and is used
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by filling in a variable of that name followed by `-y` (eg. `romstage-y`,
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`subdirs-y`, `cbfs-files-y`).
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The `-y` suffix allows a simple interaction with our Kconfig build
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configuration system: Kconfig options are available as variables starting
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with a `CONFIG_` prefix and boolean options contain `y`, `n` or are empty.
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This allows `class-$(CONFIG_FOO) += bar` to conditionally add `bar` to
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`class` depending on the choice for `FOO`.
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## classes
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Classes can be defined as required. `subdirs` is handled internally since
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it's parsed per subdirectory to add further directories to the rule set.
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TODO: explain how to create new classes and how to evaluate them.
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### subdirs
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`subdirs` contains subdirectories (relative to the current directory) that
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should also be handled by the build system. The build system expects these
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directories to contain a file called `Makefile.inc`.
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Subdirectories are not read at the point where the `subdirs` statement
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resides but later, after the current directory is handled (and potentially
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others, too).
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### cbfs-files
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This class is used to add files to the final CBFS image. Since several more
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options need to be maintained than can comfortably fit in that single
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variable, additional variables are used.
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`cbfs-files-y` contains the file name used in the CBFS image (called `foo`
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here). Additional options are added in `foo-$(option)` variables. The
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supported options are:
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* `file`: The on-disk file to add as `foo` (required)
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* `type`: The file type. Can be `raw`, `stage`, `payload`, and `flat-binary`
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(required)
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* `compression`: Can be `none` or `lzma` (default: none)
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* `position`: An absolute position constraint for the placement of the file
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(default: none)
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* `align`: Minimum alignment for the file (default: none)
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* `options`: Additional cbfstool options (default: none)
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`position` and `align` are mutually exclusive.
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#### FMAP region support
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With the addition of FMAP flash partitioning support to coreboot, there was a
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need to extend the specification of files to provide more precise control
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which regions should contain which files, and even change some flags based on
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the region.
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Since FMAP policies depend on features using FMAP, that's kept separate from
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the cbfs-files class.
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The `position` and `align` options for file `foo` can be overwritten for a
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region `REGION` using `foo-REGION-position` and `foo-REGION-align`.
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The regions that each file should end in can be defined by overriding a
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function called `regions-for-file` that's called as
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`$(call regions-for-file,$(filename))` and should return a comma-separated
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list of regions, such as `REGION1,REGION2,REGION3`.
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The default implementation just returns `COREBOOT` (the default region) for
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all files.
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vboot provides its own implementation of `regions-for-file` that can be used
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as reference in `src/vboot/Makefile.inc`.
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