kconfig: Add support for conditional values

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 CONFIG_WIBBLE_ADDR if CONFIG_WIBBLE is defined and will
produce a build error if not.. 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@chromium.org>

2 files changed, 41 insertions, 1 deletions

diff --git a/include/linux/kconfig.h b/include/linux/kconfig.h
index 52586a7557..d20da615b9 100644
--- a/include/linux/kconfig.h
+++ b/include/linux/kconfig.h
@@ -58,6 +58,30 @@
 #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. When used
+ * in assembly context, this likely produces a assembly error, or hopefully at
+ * least something recognisable.
+ */
+long invalid_use_of_IF_ENABLED_INT(void);
+#endif
+
+/* Evaluates to int_option if option is defined, otherwise a build error */
+#define IF_ENABLED_INT(option, int_option) \
+	config_opt_enabled(option, int_option, invalid_use_of_IF_ENABLED_INT())
+
+/*
  * Count number of arguments to a variadic macro. Currently only need
  * it for 1, 2 or 3 arguments.
  */
@@ -111,5 +135,21 @@
 #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. When used
+ * in assembly context, this likely produces a assembly error, or hopefully at
+ * least something recognisable.
+ */
+long invalid_use_of_CONFIG_IF_ENABLED_INT(void);
+#endif
+
+/*
+ * Evaluates to SPL_/TPL_int_option if SPL_/TPL_/option is not defined,
+ * otherwise build error
+ */
+#define CONFIG_IF_ENABLED_INT(option, int_option) \
+	CONFIG_IS_ENABLED(option, (CONFIG_VAL(int_option)), \
+		(invalid_use_of_CONFIG_IF_ENABLED_INT()))
 
 #endif /* __LINUX_KCONFIG_H */
diff --git a/scripts/Makefile.autoconf b/scripts/Makefile.autoconf
index 5ed9abc8e1..0b3ffa08bf 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;					\