Kconfig's conf in Crystal, so that the Linux and U-Boot builds can be configured in the BQ2 seed without flex and bison. It writes the same files as the real conf, byte for byte: .config, include/config/auto.conf and its .cmd, include/generated/autoconf.h, Linux's rustc_cfg, U-Boot's tristate.conf, and the include/config/ dependency files. Make then never needs to build or run conf.

cleave-kconfig [--dialect linux|u-boot|qemu|zephyr] [--srctree DIR] [--functions FILE] \
               (--olddefconfig | --defconfig FILE | --syncconfig) Kconfig

It runs in the object tree, with the environment make gives conf, because Kconfig files read it ($(CC), $(srctree), and so on). The simplest way to get that environment is to put cleave-kconfig where kbuild expects scripts/kconfig/conf.

Dialects

• linux: Linux 6.12's Kconfig (lexer.l, parser.y, menu.c, symbol.c, confdata.c).
• u-boot: the Linux 4.20-era Kconfig that U-Boot 2025.07 carries. It differs in these ways:
  • keywords are keywords only as a line's first word;
  • option lines;
  • choices have their own value and a member list;
  • a menu's dependencies come from its parent's prompt;
  • imply needs the direct dependencies;
  • numbers default to "";
  • automatic submenus can take entries out of a choice;
  • values depend on the order symbols are calculated in;
  • the output formats are older.

This dialect lives in src/cleave/kconfig/uboot.cr.

• qemu: Linux's, except for QEMU's source lines (as of QEMU 11.1.2). Their file names may be unquoted, and are relative to the directory of the file that has them: source hw/Kconfig, then source arm/Kconfig inside it.
• zephyr: Linux's, with what Kconfiglib (Zephyr's scripts/kconfig, v4.4.2) adds:
  • source globs its quoted pattern under srctree as Python's glob does (*, ? and [...] within a path part; no ** recursion or {a,b}; * skips names starting with . unless the part starts with one) and reads the matches in sorted order; rsource is relative to the including file's directory; osource and orsource (and the older gsource and grsource) may match nothing;
  • a quoted string with a $ or \ in it also has $NAME and ${NAME} of set environment variables expanded (Python's os.path.expandvars), and a backslash at the end of a line continues the string on the next;
  • optional choices, which select nothing until the user selects a member;
  • boolean, and def_int, def_hex and def_string;
  • configdefault SYM, whose default lines (the only kind it takes) neither widen the symbol's direct dependencies nor hold beyond them;
  • a definition with no dependencies makes a symbol's direct dependencies y, where Linux's C leaves out an unconditional definition;
  • Zephyr's preprocessor functions (kconfigfunctions.py): normalize_upper, shields_list_contains, substring, add, sub, mul, div, mod, max, min, inc, dec and their _hex forms, and every dt_* function. With no devicetree they answer as Kconfiglib does when it has none: n for a test, 0 or 0x0 for a number, "" for a label or path. A caller gives a devicetree's answers instead: in code, as functions to Config.load; from the command line, with --functions FILE, a JSON object whose keys are a function's name (every call) or a whole call, name(a,b), its arguments as written. Calls are checked against kconfigfunctions.py's argument counts, with its error, and Linux's limit of 15 arguments does not apply;
  • values as Kconfiglib calculates them where it differs from Linux's C:
  • an imply counts only while the symbol's direct dependencies are met, and never takes a default away, so an implied symbol whose dependencies are unmet is not written;
  • an int or hex user value outside the active range is passed over for the defaults (Linux clamps it); a default outside it, or no value, is clamped and written as Python writes the number (0x2a, where Linux copies the bound, 0x2A); the bounds are read in the symbol's own base;
  • a choice takes the user's selection, else its defaults, else its first visible member; a member set to n changes nothing (Linux 6.12 then passes over that member).

Checked against Zephyr v4.4.2's whole tree for qemu_cortex_a53, with the files Zephyr's CMake generates under KCONFIG_BINARY_DIR written by hand: it reads all of it and writes a .config with no warnings. Glint's values from Kconfiglib for the several definitions of PRIVILEGED_STACK_SIZE (arm64 and x86) are a spec.

How it parses

It uses the crystal-robots multipass pattern, like cleave-dtc:

• Lexing. LEXICAL and STRING_LEXICAL in src/cleave/kconfig/parse.cr are tables of regexes, tried in order. The lexer also does what the C lexer does as it reads:
  • expands $(...) macros (src/cleave/kconfig/preprocess.cr);
  • applies variable assignments;
  • splices sourced files in where they are named;
  • skips help texts by their indentation.
• Reduction. Kconfig::GRAMMAR reduces in priority order:
  1. expressions: comparisons, then !, &&, ||;
  2. one attribute line each;
  3. entries, with blocks (if, menu, choice) reducing innermost first.

A syntax error names the first line left unreduced inside the blocks, not the line that opens the block holding it.

• Semantics. src/cleave/kconfig/config.cr applies the parser's actions and then menu.c's finalization and symbol.c's value calculation, method for method.
• Order. Symbols are created in the order their names first appear, and iterated in hash-table order: FNV-32 into 16,384 buckets (Linux) or 9,973 (U-Boot), newest first. Files that list symbols in for_all_symbols order therefore come out in the same order.

Not done

• Interactive modes, savedefconfig, the all*config and randconfig modes.
• Linux's automatic submenus. They change neither values nor output there.
• In U-Boot's automatic submenus, the "superset of dependencies" case. Only entries that depend on the symbol outright are moved under it.
• In the zephyr dialect:
  • a devicetree (only answers given by the caller);
  • Kconfiglib's option lines, which Zephyr's tree does not use;
  • tristate symbols and m (Zephyr has none): Kconfiglib's rules for them are not checked;
  • Kconfiglib's warnings, which are not all given, and none change a value: a select of a symbol whose direct dependencies are unmet, a default of the wrong type, and so on;
  • checked by reading kconfiglib.py, not by running it: no output of Kconfiglib's is compared byte for byte, except Glint's.

Checked

• Fixtures. spec/kconfig_spec.cr compares both dialects against the output of the real conf, on fixtures covering:
  • macros, menus, if and visible if;
  • choices, including a member that becomes a submenu;
  • select, imply, ranges, hex, tristate with modules, and quoted strings.
• Linux v6.12.49-ti-arm64-r56. Armbian's vendor config plus our audio fragment, run through kbuild's olddefconfig and syncconfig. All outputs and all 2,846 dependency files are identical.
• U-Boot 2025.07. am67a_beagley_ai_r5_defconfig and am67a_beagley_ai_a53_defconfig, run through U-Boot's defconfig and syncconfig. All outputs and all include/config/*.h files are identical.