[PATCH] Documentation/kbuild: fix empty string KBUILD_BUILD_TIMESTAMP

[Daniel Gomez]

From: Daniel Gomez <da.gomez@xxxxxxxxxxx>

The ccache example in llvm.rst uses KBUILD_BUILD_TIMESTAMP='' but the
text says "should be set to a deterministic value". An empty string is
not deterministic because init/Makefile uses $(or) which treats empty
as false and falls through to $(shell LC_ALL=C date), embedding the
current time:

	cat /tmp/test-or.mk
	build-timestamp-auto = fallback
	build-timestamp = $(or $(KBUILD_BUILD_TIMESTAMP),$(build-timestamp-auto))
	all:
	        @echo "[$(build-timestamp)]"

	make --file=/tmp/test-or.mk
	[fallback]

	KBUILD_BUILD_TIMESTAMP='' make --file=/tmp/test-or.mk
	[fallback]

	KBUILD_BUILD_TIMESTAMP="Sun Aug 25 20:57:08 UTC 1991" \
		make --file=/tmp/test-or.mk
	[Sun Aug 25 20:57:08 UTC 1991]

The same applies to a real kernel build:

	make O=../build LLVM=1 CC="ccache clang" tinyconfig
	make O=../build LLVM=1 CC="ccache clang" \
		KBUILD_BUILD_TIMESTAMP='' --jobs=$(nproc)
	cat ../build/include/generated/utsversion.h
	#define UTS_VERSION "#2 Wed Mar  4 12:13:43 CET 2026"

Replace the empty string with an actual date, matching the pattern
in kbuild.rst and reproducible-builds.rst. Use the first public Linux
announcement as example date.

	make O=../build LLVM=1 CC="ccache clang" tinyconfig
	make O=../build LLVM=1 CC="ccache clang" \
		KBUILD_BUILD_TIMESTAMP="Sun Aug 25 20:57:08 UTC 1991" \
		--jobs=$(nproc)
	cat ../build/include/generated/utsversion.h
	#define UTS_VERSION "#4 Sun Aug 25 20:57:08 UTC 1991"

Signed-off-by: Daniel Gomez <da.gomez@xxxxxxxxxxx>
---
Do not use an empty KBUILD_BUILD_TIMESTAMP Makefile variable.
---
 Documentation/kbuild/llvm.rst                    | 2 +-
 Documentation/translations/zh_CN/kbuild/llvm.rst | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
index bc8a283bc44b..ef46533d785f 100644
--- a/Documentation/kbuild/llvm.rst
+++ b/Documentation/kbuild/llvm.rst
@@ -128,7 +128,7 @@ Ccache
 KBUILD_BUILD_TIMESTAMP_ should be set to a deterministic value between builds
 in order to avoid 100% cache misses, see Reproducible_builds_ for more info)::
 
-	KBUILD_BUILD_TIMESTAMP='' make LLVM=1 CC="ccache clang"
+	KBUILD_BUILD_TIMESTAMP="Sun Aug 25 20:57:08 UTC 1991" make LLVM=1 CC="ccache clang"
 
 .. _KBUILD_BUILD_TIMESTAMP: kbuild.html#kbuild-build-timestamp
 .. _Reproducible_builds: reproducible-builds.html#timestamps
diff --git a/Documentation/translations/zh_CN/kbuild/llvm.rst b/Documentation/translations/zh_CN/kbuild/llvm.rst
index f87e0181d8e7..c68fd395f64c 100644
--- a/Documentation/translations/zh_CN/kbuild/llvm.rst
+++ b/Documentation/translations/zh_CN/kbuild/llvm.rst
@@ -116,7 +116,7 @@ Ccache
 KBUILD_BUILD_TIMESTAMP_ 应设置为同一确定值，以避免 100% 的缓存未命中，
 详见 Reproducible_builds_ 获取更多信息）::
 
-	KBUILD_BUILD_TIMESTAMP='' make LLVM=1 CC="ccache clang"
+	KBUILD_BUILD_TIMESTAMP="Sun Aug 25 20:57:08 UTC 1991" make LLVM=1 CC="ccache clang"
 
 .. _KBUILD_BUILD_TIMESTAMP: kbuild.html#kbuild-build-timestamp
 .. _Reproducible_builds: reproducible-builds.html#timestamps

---
base-commit: 11439c4635edd669ae435eec308f4ab8a0804808
change-id: 20260310-fix-llvm-docs-1f91c1ec7608

Best regards,
--  
Daniel Gomez <da.gomez@xxxxxxxxxxx>