From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6222D36C580; Tue, 10 Mar 2026 11:49:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773143342; cv=none; b=vEgy1xkj09euyWn45xI2desRKsKoGEA0tVJT6Yku956hIvtdV6R1zD5R29+STQ/3D+KJJOvk9jyMXictzEmS2TqdrRQuuZSWRV3uMfVZb7dvBaOlEEoQyCvHPklYfXlqxY3y+EjYpp4RIJeBLNsY2FjTkfoaAVDYRkp725P+uaE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773143342; c=relaxed/simple; bh=szOPfSKNmYb03Xy3ZDdouDlpBOIPO8kX5fe7d2upwUg=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:To:Cc; b=H3t++Gmni58SAJ/MFJEw5Bt337AIxfmT2YPpuoz8jOMkYBkhywo3XpohG/7yGvVlf0rrHI1+3grfH5bWSuP0Qqc52S5B1ogSQG33+4tpK+6fIOvmYRq/2dcoT9XVcCLDZrFkvojGIUimrwY75bMgf7uyasIFoa95h1yEzn7qhCo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=nxEPcMbA; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="nxEPcMbA" Received: by smtp.kernel.org (Postfix) with ESMTPSA id ED213C2BC86; Tue, 10 Mar 2026 11:48:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773143342; bh=szOPfSKNmYb03Xy3ZDdouDlpBOIPO8kX5fe7d2upwUg=; h=From:Date:Subject:To:Cc:From; b=nxEPcMbAfvIlEMcH9uvddRQIKTDnRIg6K2oNzDP+OSv7sS5mEEg2QuB6JY1ophvU/ Mjr7GFgeUAMOUmsYfRKsH1pLyI6tjfDf7cCcHmpWUTH9XN+4XsdUyTz+6Wzo2Qebqd x4QHCXmoMjWUdwt+K0RxD0KoaG0QrwSQje1dbpk7y/makYZSp3YrR7AjQLpzVf1NJr /e/8V6Jk0V/116nL79VaVVqS7ifVSpgElRBe82iSn0pKRwjH7OI7mdNtCaJM1CKAra UAykBRFdrVEwapgLm7KWARSq9uwA4kQi5wml4GoKtR5Vlgc2HwuiC+5BEr9k4efB9l 1e9JYRvNmofzA== From: Daniel Gomez Date: Tue, 10 Mar 2026 12:48:43 +0100 Subject: [PATCH] Documentation/kbuild: fix empty string KBUILD_BUILD_TIMESTAMP Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit Message-Id: <20260310-fix-llvm-docs-v1-1-234878874762@samsung.com> X-B4-Tracking: v=1; b=H4sIABoFsGkC/6tWKk4tykwtVrJSqFYqSi3LLM7MzwNyDHUUlJIzE vPSU3UzU4B8JSMDIzMDY0MD3bTMCt2cnLJc3ZT85GJdwzRLw2TD1GRzMwMLJaCegqJUoAKwedG xtbUAtYiwOF8AAAA= X-Change-ID: 20260310-fix-llvm-docs-1f91c1ec7608 To: Nathan Chancellor , Nick Desaulniers , Bill Wendling , Justin Stitt , Nicolas Schier , Jonathan Corbet , Shuah Khan , Alex Shi , Yanteng Si , Dongliang Mu Cc: llvm@lists.linux.dev, linux-kbuild@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Daniel Gomez X-Mailer: b4 0.15-dev X-Developer-Signature: v=1; a=openpgp-sha256; l=3311; i=da.gomez@samsung.com; h=from:subject:message-id; bh=4d6UKFUmuCrPcqriyy404JMUfDZrH81H3xhK0ev9HII=; b=owEBbQKS/ZANAwAIAUCeo8QfGVH7AcsmYgBpsAUpMI7b7AB75BHtPehCbkCcc8WkRD+91AIaA TxMZSH1t7yJAjMEAAEIAB0WIQTvdRrhHw9z4bnGPFNAnqPEHxlR+wUCabAFKQAKCRBAnqPEHxlR +ytOD/0UTfNzVV6zPafCiNBW7m+m7WoM4YsCuVOZEQlW6watfbPegDGjXoH9uPlr3FarRnhgXyY 4bLKfk7Vx0+ecMjBWjf7IDQhjx6br9EJ92/Dd255EgXVwswu6MeOj9x1+eTgeF32qjmcBGG/4zY AkbhBL5en7MUv8bCIN0tDKgz8f639cHgtDCpq8qCQR9y10GuPK+lLIvtT//VZv8PyP0MmEuFeFv 4l7PG8+tNjFXhNQjF855277A8XMBIYiXV8OklisqpH7zZ2AXjp+4yetoacewL5lOwTKcinUkRwr sfkG2x9C/PH6b6rAtgEztB+w+UiWnMIHQUm5u6XjA9Q6rIrbsXUrGlU0j/+TvDi4S2dk3m1l5HA ydtCqXAmXfWB3ILlMxBbiYCuducKocfSId/kMiweFQuNXgpC/CnIC5g3QWkdW4o/53VF0o6rf4n i9Rdrh+lYCpgJRB26EpH7dGXdz+axSqD5Gg3f6k3ca0dJgszBELT+otT3o5JxKm7JtCO299sxVp 1yAe9QFwIjt8f1/6yRuBzMGtMuQzk6KABIy1+d+Y7ayQ/1nOGddYD1mCLuEqENS84VgbBqn2jBr 7MP78PevPDR7gfZhVN2WFDfl5huqJtL2oaeJSFGMKPF/RYSfTE2kjCScIeLpHIszJEELv0nPVvL zM68UofjmxXGxug== X-Developer-Key: i=da.gomez@samsung.com; a=openpgp; fpr=B2A7A9CFDD03B540FF58B27185F56EA4E9E8138F From: Daniel Gomez 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 --- 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