[PATCH 2/2] include: is_defined: implement __if_defined helper

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

This is useful when preprocessing non-C code that lacks an if statement
equivalent. For example, external device tree fragments can now specify:

  #include <linux/is_defined.h>

  __if_defined(bcm2711_rpi_4_dts, &emmc2, &sdhost) {

instead of having to use the more verbose #ifdef:

  #ifdef bcm2711_rpi_4_dts
  &emmc2 {
  #else
  &sdhost {
  #endif

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 include/linux/is_defined.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/linux/is_defined.h b/include/linux/is_defined.h
index d68f8e877f9b..bfefca033f70 100644
--- a/include/linux/is_defined.h
+++ b/include/linux/is_defined.h
@@ -34,4 +34,10 @@
 #define ___is_defined(val)		____is_defined(__ARG_PLACEHOLDER_##val)
 #define ____is_defined(arg1_or_junk)	__take_second_arg(arg1_or_junk 1, 0)
 
+#define __if_defined(x, a, b)		___if_defined(__is_defined(x), a, b)
+#define ___if_defined(val, a, b)	____if_defined(val, a, b)
+#define ____if_defined(val, a, b)	____if_defined_##val(a, b)
+#define ____if_defined_1(a, b)		a
+#define ____if_defined_0(a, b)		b
+
 #endif
-- 
2.39.2