* [PATCH] Documentation/kbuild: fix empty string KBUILD_BUILD_TIMESTAMP
@ 2026-03-10 11:48 Daniel Gomez
2026-03-11 8:06 ` Nathan Chancellor
0 siblings, 1 reply; 2+ messages in thread
From: Daniel Gomez @ 2026-03-10 11:48 UTC (permalink / raw)
To: Nathan Chancellor, Nick Desaulniers, Bill Wendling, Justin Stitt,
Nicolas Schier, Jonathan Corbet, Shuah Khan, Alex Shi, Yanteng Si,
Dongliang Mu
Cc: llvm, linux-kbuild, linux-doc, linux-kernel, Daniel Gomez
From: Daniel Gomez <da.gomez@samsung.com>
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@samsung.com>
---
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@samsung.com>
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] Documentation/kbuild: fix empty string KBUILD_BUILD_TIMESTAMP
2026-03-10 11:48 [PATCH] Documentation/kbuild: fix empty string KBUILD_BUILD_TIMESTAMP Daniel Gomez
@ 2026-03-11 8:06 ` Nathan Chancellor
0 siblings, 0 replies; 2+ messages in thread
From: Nathan Chancellor @ 2026-03-11 8:06 UTC (permalink / raw)
To: Daniel Gomez
Cc: Nick Desaulniers, Bill Wendling, Justin Stitt, Nicolas Schier,
Jonathan Corbet, Shuah Khan, Alex Shi, Yanteng Si, Dongliang Mu,
llvm, linux-kbuild, linux-doc, linux-kernel, Daniel Gomez
On Tue, Mar 10, 2026 at 12:48:43PM +0100, Daniel Gomez wrote:
> From: Daniel Gomez <da.gomez@samsung.com>
>
> 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@samsung.com>
> ---
> 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)::
To be entirely honest, I don't think this is quite valid anymore, at
least not with fairly modern ccache versions.
$ ccache --version | head -1
ccache version 4.12.3
$ ccache -C
$ make -skj"$(nproc)" ARCH=arm64 CC="ccache clang" HOSTCC="ccache clang" LLVM=1 mrproper defconfig all
$ ccache -z
Statistics zeroed
$ make -skj"$(nproc)" ARCH=arm64 CC="ccache clang" HOSTCC="ccache clang" LLVM=1 clean all
$ ccache -s
Cacheable calls: 10497 / 10554 (99.46%)
Hits: 10496 / 10497 (99.99%)
Direct: 10488 / 10496 (99.92%)
Preprocessed: 8 / 10496 ( 0.08%)
Misses: 1 / 10497 ( 0.01%)
Uncacheable calls: 57 / 10554 ( 0.54%)
Local storage:
Cache size (GB): 1.2 / 100.0 ( 1.21%)
Hits: 10496 / 10497 (99.99%)
Misses: 1 / 10497 ( 0.01%)
I think we should just remove mention of KBUILD_BUILD_TIMESTAMP all together.
Ccache
------
``ccache`` can be used with ``clang`` to improve subsequent builds::
make LLVM=1 CC="ccache clang"
> - 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@samsung.com>
>
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