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 7E23334B663; Wed, 11 Mar 2026 08:06:51 +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=1773216411; cv=none; b=kcLDcuAxm9RiCRliP+e2ZPWBCw86C25KS4fp16gvdyoeUbvN2PV5SXbUKxpDnQe/GG072v4V9XYJO32vBFTxHXIoFqiU7pRwWHEwlOxqIOtuulRWTdx12Q//pnEmBc+f0LzUK1dFtVkiYSMZ5vnTMe6lCpuBqbYVItwJyfLgY7A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773216411; c=relaxed/simple; bh=dVEH6w6A2ru6wvApMC4aSElWkR/TFzmlhS1C9lEPXbI=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=luBsKiBpLWtwWfJUKPeMeMGqvQCx/xxHfgzS2ynHFmn2WIoi8Euvezc+Bzj6wxXJuxld3an3AaaTb6rWbx1/a4cMcksNq2CTo71K56OR5F6gWD4NmxaWfEpdk1QgsKxhSToCjXrVlo6tw7tXcpUZdI2B030Oxy+SVVD/B9f11N8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=f0YS7QPV; 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="f0YS7QPV" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0E249C4CEF7; Wed, 11 Mar 2026 08:06:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773216411; bh=dVEH6w6A2ru6wvApMC4aSElWkR/TFzmlhS1C9lEPXbI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=f0YS7QPVkOH607lM76aLFMmSumjt0Of0A0d4z+MQ7kBskP6mZZaJpSKjbxJHsjj/1 wFTO3exIeXQWhdfCt2e3Q+tcHR69ooaxaXv0OXWmAtSQC/YruMn7Dni+AYjAlOyPc6 Lbryy0ceS1snZPxx2UsF5Nxrdkp4ntuFoqzsv88M0+SbfD5ljo1b4Q3qIUiLG0R1Pz HlXou7vXIQn+nL2S8ONlpTkyHTnXAq43mu025S9Z9bMFU+b7OSXHYulPOY/u98iCb9 uZNYABgBGn0KGD0cvDhN+cML/aMAe/bKbASvnqdljde098YLluGDc0qElXIv1zbF1+ TVTN2BKE1jOHw== Date: Wed, 11 Mar 2026 01:06:45 -0700 From: Nathan Chancellor To: Daniel Gomez Cc: Nick Desaulniers , Bill Wendling , Justin Stitt , Nicolas Schier , Jonathan Corbet , Shuah Khan , Alex Shi , Yanteng Si , Dongliang Mu , llvm@lists.linux.dev, linux-kbuild@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Daniel Gomez Subject: Re: [PATCH] Documentation/kbuild: fix empty string KBUILD_BUILD_TIMESTAMP Message-ID: <20260311080645.GC1996626@ax162> References: <20260310-fix-llvm-docs-v1-1-234878874762@samsung.com> 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-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260310-fix-llvm-docs-v1-1-234878874762@samsung.com> On Tue, Mar 10, 2026 at 12:48:43PM +0100, Daniel Gomez wrote: > 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):: 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 >