kbuild: doc: improve KBUILD_BUILD_TIMESTAMP documentation

This patch adds an example of how to set KBUILD_BUILD_TIMESTAMP to a
specific date. Also, note that the provided timestamp is used for
initramfs mtime fields, which are 32-bit and thus limited to dates
between the Unix epoch and 2106-02-07 06:28:15 UTC. Dates outside this
range will cause errors.

Suggested-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Gang Yan <yangang@kylinos.cn>
Reviewed-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://patch.msgid.link/20251017021209.6586-1-gang.yan@linux.dev
Signed-off-by: Nicolas Schier <nsc@kernel.org>
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Gang Yan 2025-10-17 10:12:09 +08:00 committed by Nicolas Schier
parent af61da281f
commit bfb046f67a
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1 changed files with 8 additions and 2 deletions

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@ -328,8 +328,14 @@ KBUILD_BUILD_TIMESTAMP
---------------------- ----------------------
Setting this to a date string overrides the timestamp used in the Setting this to a date string overrides the timestamp used in the
UTS_VERSION definition (uname -v in the running kernel). The value has to UTS_VERSION definition (uname -v in the running kernel). The value has to
be a string that can be passed to date -d. The default value be a string that can be passed to date -d. E.g.::
is the output of the date command at one point during build.
$ KBUILD_BUILD_TIMESTAMP="Mon Oct 13 00:00:00 UTC 2025" make
The default value is the output of the date command at one point during
build. If provided, this timestamp will also be used for mtime fields
within any initramfs archive. Initramfs mtimes are 32-bit, so dates before
the 1970 Unix epoch, or after 2106-02-07 06:28:15 UTC will fail.
KBUILD_BUILD_USER, KBUILD_BUILD_HOST KBUILD_BUILD_USER, KBUILD_BUILD_HOST
------------------------------------ ------------------------------------