[PATCH v3 2/3] kconfig: Add support for conditional values

[Simon Glass]

At present if an optional Kconfig value needs to be used it must be
bracketed by #ifdef. For example, with this Kconfig setup:

config WIBBLE
	bool "Support wibbles, the world needs more wibbles"

config WIBBLE_ADDR
	hex "Address of the wibble"
	depends on WIBBLE

then the following code must be used:

 #ifdef CONFIG_WIBBLE
 static void handle_wibble(void)
 {
 	int val = CONFIG_WIBBLE_ADDR;

	...
 }
 #endif

 static void init_machine()
 {
 ...
 #ifdef CONFIG_WIBBLE
	handle_wibble();
 #endif
 }

Add a new IF_ENABLED_INT() to help with this. So now it is possible to
write, without #ifdefs:

 static void handle_wibble(void)
 {
        int val = IF_ENABLED_INT(CONFIG_WIBBLE, CONFIG_WIBBLE_ADDR);

	...
 }

 static void init_machine()
 {
 ...
 if (IS_ENABLED(CONFIG_WIBBLE))
	handle_wibble();
 }

The value will be 0 if CONFIG_WIBBLE is not defined, and
CONFIG_WIBBLE_ADDR if it is. This allows us to reduce the use of #ifdef in
the code, ensuring that the compiler still checks the code even if it is
not ultimately used for a particular build.

Add a CONFIG_IF_ENABLED_INT() version as well.

If an attempt is made to use a value that does not exist (i.e. when the
conditional is not enabled), an error about a non-existing function is
generated, e.g.:

common/bloblist.c:447: undefined reference to `invalid_use_of_IF_ENABLED_INT'

Signed-off-by: Simon Glass <sjg at chromium.org>
---

Changes in v3:
- Add a way to detect use of a non-existent value

 include/linux/kconfig.h   | 32 ++++++++++++++++++++++++++++++++
 scripts/Makefile.autoconf |  2 +-
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/include/linux/kconfig.h b/include/linux/kconfig.h
index a1d1a298426..e59e920c108 100644
--- a/include/linux/kconfig.h
+++ b/include/linux/kconfig.h
@@ -59,6 +59,26 @@
  */
 #define CONFIG_VAL(option)  config_val(option)
 
+/*
+ * This uses a similar mechanism to config_enabled() above. If cfg is enabled,
+ * it resolves to the value of opt_cfg, otherwise it resolves to def_val
+ */
+#define config_opt_enabled(cfg, opt_cfg, def_val) _config_opt_enabled(cfg, opt_cfg, def_val)
+#define _config_opt_enabled(cfg_val, opt_value, def_val) \
+	__config_opt_enabled(__ARG_PLACEHOLDER_##cfg_val, opt_value, def_val)
+#define __config_opt_enabled(arg1_or_junk, arg2, def_val) \
+	___config_opt_enabled(arg1_or_junk arg2, def_val)
+#define ___config_opt_enabled(__ignored, val, ...) val
+
+#ifndef __ASSEMBLY__
+/* detect usage of a the value when the conditional is not enabled */
+long invalid_use_of_IF_ENABLED_INT(void);
+
+/* Evaluates to 0 if option is not defined, int_option if it is defined */
+#define IF_ENABLED_INT(option, int_option) \
+	config_opt_enabled(option, int_option, invalid_use_of_IF_ENABLED_INT())
+#endif
+
 /*
  * Count number of arguments to a variadic macro. Currently only need
  * it for 1, 2 or 3 arguments.
@@ -113,5 +133,17 @@
 #define CONFIG_IS_ENABLED(option, ...)					\
 	__concat(__CONFIG_IS_ENABLED_, __count_args(option, ##__VA_ARGS__)) (option, ##__VA_ARGS__)
 
+#ifndef __ASSEMBLY__
+/* detect usage of a the value when the conditional is not enabled */
+long invalid_use_of_CONFIG_IF_ENABLED_INT(void);
+
+/*
+ * Evaluates to 0 if SPL_/TPL_/option is not defined, SPL_/TPL_int_option if it
+ * is defined
+ */
+#define CONFIG_IF_ENABLED_INT(option, int_option) \
+	CONFIG_IS_ENABLED(option, (CONFIG_VAL(int_option)), \
+		(invalid_use_of_CONFIG_IF_ENABLED_INT()))
+#endif
 
 #endif /* __LINUX_KCONFIG_H */
diff --git a/scripts/Makefile.autoconf b/scripts/Makefile.autoconf
index 5ed9abc8e14..0b3ffa08bfa 100644
--- a/scripts/Makefile.autoconf
+++ b/scripts/Makefile.autoconf
@@ -68,7 +68,7 @@ quiet_cmd_u_boot_cfg = CFG     $@
       cmd_u_boot_cfg = \
 	$(CPP) $(c_flags) $2 -DDO_DEPS_ONLY -dM $(srctree)/include/common.h > $@.tmp && { \
 		grep 'define CONFIG_' $@.tmp | \
-			sed '/define CONFIG_IS_ENABLED(/d;/define CONFIG_VAL(/d;' > $@; \
+			sed '/define CONFIG_IS_ENABLED(/d;/define CONFIG_IF_ENABLED_INT(/d;/define CONFIG_VAL(/d;' > $@; \
 		rm $@.tmp;						\
 	} || {								\
 		rm $@.tmp; false;					\
-- 
2.34.1.703.g22d0c6ccf7-goog