kbuild: introduce hostprogs-always-y and userprogs-always-y

To build host programs, you need to add the program names to 'hostprogs'
to use the necessary build rule, but it is not enough to build them
because there is no dependency.

There are two types of host programs: built as the prerequisite of
another (e.g. gen_crc32table in lib/Makefile), or always built when
Kbuild visits the Makefile (e.g. genksyms in scripts/genksyms/Makefile).

The latter is typical in Makefiles under scripts/, which contains host
programs globally used during the kernel build. To build them, you need
to add them to both 'hostprogs' and 'always-y'.

This commit adds hostprogs-always-y as a shorthand.

The same applies to user programs. net/bpfilter/Makefile builds
bpfilter_umh on demand, hence always-y is unneeded. In contrast,
programs under samples/ are added to both 'userprogs' and 'always-y'
so they are always built when Kbuild visits the Makefiles.

userprogs-always-y works as a shorthand.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>

1 files changed, 30 insertions, 1 deletions

diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst
index 14d8e7d23c04..b81b8913a5a3 100644
--- a/Documentation/kbuild/makefiles.rst
+++ b/Documentation/kbuild/makefiles.rst
@@ -749,6 +749,10 @@ Both possibilities are described in the following.
 		hostprogs     := lxdialog
 		always-y      := $(hostprogs)
 
+	Kbuild provides the following shorthand for this:
+
+		hostprogs-always-y := lxdialog
+
 	This will tell kbuild to build lxdialog even if not referenced in
 	any rule.
 
@@ -831,7 +835,32 @@ The syntax is quite similar. The difference is to use "userprogs" instead of
 5.4 When userspace programs are actually built
 ----------------------------------------------
 
-	Same as "When host programs are actually built".
+	Kbuild builds userspace programs only when told to do so.
+	There are two ways to do this.
+
+	(1) Add it as the prerequisite of another file
+
+	Example::
+
+		#net/bpfilter/Makefile
+		userprogs := bpfilter_umh
+		$(obj)/bpfilter_umh_blob.o: $(obj)/bpfilter_umh
+
+	$(obj)/bpfilter_umh is built before $(obj)/bpfilter_umh_blob.o
+
+	(2) Use always-y
+
+	Example::
+
+		userprogs := binderfs_example
+		always-y := $(userprogs)
+
+	Kbuild provides the following shorthand for this:
+
+		userprogs-always-y := binderfs_example
+
+	This will tell Kbuild to build binderfs_example when it visits this
+	Makefile.
 
 6 Kbuild clean infrastructure
 =============================