[PATCH] kconfig: Proposed language extension for multiple builds

[Simon Glass]

Hi Masahiro,

On Sun, 19 Feb 2023 at 14:55, Simon Glass <sjg at chromium.org> wrote:
>
> In the case of Linux, only one build is produced so there is only a
> single configuration. For other projects, such as U-Boot and Zephyr, the
> same code is used to produce multiple builds, each with related (but
> different) options enabled.
>
> This can be handled with the existing kconfig language, but it is quite
> verbose, somewhat tedious and very error-prone, since there is a lot of
> duplication. The result is hard to maintain.
>
> Describe an extension to the Kconfig language to support easier handling
> of this use case.
>
> Signed-off-by: Simon Glass <sjg at chromium.org>
> ---
>
>  Documentation/kbuild/kconfig-language.rst | 134 ++++++++++++++++++++++
>  1 file changed, 134 insertions(+)
>

Do you have any comments on this, please?

Regards,
Simon

> diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
> index 858ed5d80defe..73fb016a5533f 100644
> --- a/Documentation/kbuild/kconfig-language.rst
> +++ b/Documentation/kbuild/kconfig-language.rst
> @@ -228,6 +228,24 @@ applicable everywhere (see syntax).
>    enables the third modular state for all config symbols.
>    At most one symbol may have the "modules" option set.
>
> +- phase declaration: "defphase"
> +  This defines a new build phase. See `Build Phases`_.
> +
> +- default phase: "phasedefault"
> +  This indicates the default build phase. See `Build Phases`_.
> +
> +- add entries for phases: "addphases"
> +  This creates new phase-specific entries based on a template entry and adds
> +  the same attributes to it. See `Build Phases`_.
> +
> +- set entries for phases: "setphases"
> +  This sets the phases which need an entry. This allows creating an entry that
> +  only has a primary phase. See `Build Phases`_.
> +
> +- indicate a phase-specific attribute: "forphases"
> +  This marks an attribute as being applicable only to a particular phase or
> +  group of phases.  See `Build Phases`_.
> +
>  Menu dependencies
>  -----------------
>
> @@ -319,6 +337,119 @@ MODVERSIONS directly depends on MODULES, this means it's only visible if
>  MODULES is different from 'n'. The comment on the other hand is only
>  visible when MODULES is set to 'n'.
>
> +Build Phases
> +------------
> +
> +Some projects use Kconfig to control multiple build phases, each phase
> +resulting in a separate set of object files and executable. This is the
> +case in U-Boot [12]_. Zephyr OS seems to be heading this way too [13]_.
> +
> +Generally the phases are related, so that enabling an entry in the primary
> +phase also enables it by default in the others. But in some cases it may
> +be desirable to use separate conditions for each phase.
> +
> +All phases have a phase name, for example `SPL`. This name is used as a
> +prefix to each entry used in that phase, with an underscore in between.
> +So if FOO is the primary entry, the equivalent entry for the SPL phase
> +is SPL_FOO. The primary phase is marked with a "phasedefault" entry.
> +
> +Phases are declared like any other menu entry except that also have a
> +"defphase" keyword. Phase entries are normally hidden so do not have a
> +prompt::
> +
> +    config PPL
> +        bool
> +        defphase "Primary Program Loader"
> +        phasedefault
> +        help
> +          This is the primary bootloader.
> +
> +    config SPL
> +        bool
> +        defphase "Secondary Program Loader"
> +        help
> +          This is used to set up memory and load the primary bootloader.
> +
> +The default phase (here PPL) is assumed for all entries, in the sense that
> +all entries are present in PPL by default and no prefix is needed on these
> +entries. So FOO means that it applies to PPL. There must be exactly one
> +default phase.
> +
> +The resulting menu entries can be used normally throughout the Kconfig. With
> +this technique, the different build phases can be fully and individually
> +controlled from Kconfig.
> +
> +However it is not ideal. Often the secondary phases have far fewer entries than
> +the primary phase, since they offer fewer features. Even so, each FOO that is
> +needed in a phase must have an SPL_FOO, etc. To avoid an explosion of entries,
> +it is possible to indicate which are enabled, as a shortcut for creating new
> +entries::
> +
> +    config FOO
> +        bool "Enable foo feature"
> +        addphases SPL
> +        depends on %BAR
> +        depends on QUX
> +        forphases SPL depends on FIZZ
> +
> +Note that "%" expands to the phase, so this is equivalent to (ignoring BAR)::
> +
> +    config FOO
> +        bool "Enable foo feature"
> +        depends on BAR
> +        depends on QUX
> +
> +    config SPL_FOO                        # Phase is prepended
> +        bool "Enable foo feature (SPL)"    # Suffix is added
> +        depends on SPL_BAR                 # "%" dependency is expanded
> +        depends on QUX
> +        depends on FIZZ                    # Added only for SPL
> +        depends on SPL                     # Added automatically
> +
> +Attributes declared in the primary symbol FOO (such as "depends on BAR") also
> +apply to the secondary ones.
> +
> +An entry without any 'addphases' attribute applies to all phases. Individual
> +phase entries are not available in that case. If the entry is enabled, then
> +it is enabled for all phases. Only one entry appears in the resulting Kconfig.
> +
> +In the case where an entry should apply only to the primary phase (or a
> +particular set of phases), you can uses "setphases" instead of "addphases"::
> +
> +    config FOO
> +        bool "Enable foo feature"
> +        setphases PPL
> +
> +This means that even if the option is enabled, it will not be active outside
> +the primary-phase build, here named "PPL".
> +
> +Internally, phases are implemented simply by creating new entries. These
> +appear in the Kconfig as per normal. It would be possible for a Kconfig
> +editor to show the entries just for a particular phase, leaving out the
> +entries not applicable to that phase.
> +
> +When phases are used, the Kconfig tool outputs separate auto.conf files for
> +each phase (e.g. auto_spl.conf), so that if SPL_FOO is enabled, then
> +`CONFIG_FOO=y` is present in the file. This makes it easy for the build system
> +to build the correct code, use IS_ENABLED(), etc.
> +
> +To ensure that the correct options are enabled for each build, in addition to
> +the normal CONFIG_FOO option in the file, phase symbols are added too. For
> +example, if FOO is enabled in the SPL phase, then auto_spl.conf contains::
> +
> +    CONFIG_FOO=y
> +    CONFIG_SPL_FOO=y
> +
> +The phase-specific line (CONFIG_SPL_FOO) is seldom needed, but it allows one
> +phase to access the symbols from another phase. For example, if the primary
> +phase needs to receive boot timings from SPL, then support for boot timings must
> +be added in both phases. If the timings are at a fixed address, this can be in a
> +shared symbol (like CONFIG_SPL_TIMING_BASE) that both phases can access.
> +
> +Technical note: the grammar definition of <symbol> in this documeent does not
> +include the "%" prefix at present. It can be used with any attribute. It cannot
> +be used with any top-level items, like "config", "menuconfig", "if" and "menu".
> +
>
>  Kconfig syntax
>  --------------
> @@ -744,3 +875,6 @@ https://kernelnewbies.org/KernelProjects/kconfig-sat
>  .. [9] https://www4.cs.fau.de/Publications/2011/tartler_11_eurosys.pdf
>  .. [10] https://paulgazzillo.com/papers/esecfse21.pdf
>  .. [11] https://github.com/paulgazz/kmax
> +
> +.. [12] https://u-boot.readthedocs.io/en/latest/develop/spl.html
> +.. [13] https://docs.zephyrproject.org/latest/build/sysbuild/index.html
> --
> 2.39.2.637.g21b0678d19-goog
>