kbuild: generate asm-generic wrappers if mandatory headers are missing

Some time ago, Sam pointed out a certain degree of overwrap between
generic-y and mandatory-y. (https://lkml.org/lkml/2017/7/10/121)

I tweaked the meaning of mandatory-y a little bit; now it defines the
minimum set of ASM headers that all architectures must have.

If arch does not have specific implementation of a mandatory header,
Kbuild will let it fallback to the asm-generic one by automatically
generating a wrapper. This will allow to drop lots of redundant
generic-y defines.

Previously, "mandatory" was used in the context of UAPI, but I guess
this can be extended to kernel space ASM headers.

Suggested-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt
index 8da26c6dd886..bf28c47bfd72 100644
--- a/Documentation/kbuild/makefiles.txt
+++ b/Documentation/kbuild/makefiles.txt
@@ -1296,9 +1296,12 @@ See subsequent chapter for the syntax of the Kbuild file.
 
 --- 7.4 mandatory-y
 
-        mandatory-y is essentially used by include/uapi/asm-generic/Kbuild.asm
-        to define the minimum set of headers that must be exported in
-        include/asm.
+        mandatory-y is essentially used by include/(uapi/)asm-generic/Kbuild.asm
+        to define the minimum set of ASM headers that all architectures must have.
+
+        This works like optional generic-y. If a mandatory header is missing
+        in arch/$(ARCH)/include/(uapi/)/asm, Kbuild will automatically generate
+        a wrapper of the asm-generic one.
 
         The convention is to list one subdir per line and
         preferably in alphabetic order.