Re: [PATCH 1/1] add support to set KBUILD_BUILD_TIMESTAMP for custom kernels

[Jan Kiszka <jan.kiszka@siemens.com>]

On 21.03.22 18:33, Moessbauer, Felix (T CED SES-DE) wrote:
> Hi Jan,
> 
>> -----Original Message-----
>> From: Kiszka, Jan (T CED) <jan.kiszka@siemens.com>
>> Sent: Monday, March 21, 2022 6:18 PM
>> To: Moessbauer, Felix (T CED SES-DE) <felix.moessbauer@siemens.com>; isar-
>> users@googlegroups.com
>> Subject: Re: [PATCH 1/1] add support to set KBUILD_BUILD_TIMESTAMP for
>> custom kernels
>>
>> On 21.03.22 15:40, Felix Moessbauer wrote:
>>> This patch adds the infrastructure to overwrite the
>>> KBUILD_BUILD_TIMESTAMP environment variable for custom kernel builds.
>>> By that, we improve the support for reproducible kernel builds.
>>>
>>
>> Agreed.
>>
>>> Setting this to a fixed string further helps to profit from ccache
>>> caching on repeated builds.
>>
>> Hmm, but the practical impact of this variable should have been limited /wrt
>> ccache. I was seeing significant build-time reduction before the commit already.
> 
> Yes, but of course depends on configuration.
> 
>>
>>>
>>> Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
>>> ---
>>>  meta/recipes-kernel/linux/files/debian/isar/build.tmpl  | 1 +
>>> meta/recipes-kernel/linux/files/debian/isar/common.tmpl | 2 ++
>>>  meta/recipes-kernel/linux/linux-custom.inc              | 3 +++
>>>  3 files changed, 6 insertions(+)
>>>
>>> diff --git a/meta/recipes-kernel/linux/files/debian/isar/build.tmpl
>>> b/meta/recipes-kernel/linux/files/debian/isar/build.tmpl
>>> index b7749cec..32d2eb23 100644
>>> --- a/meta/recipes-kernel/linux/files/debian/isar/build.tmpl
>>> +++ b/meta/recipes-kernel/linux/files/debian/isar/build.tmpl
>>> @@ -34,6 +34,7 @@ print_settings() {
>>>  # ---------------
>>>  # ARCH=${ARCH}
>>>  # CROSS_COMPILE=${CROSS_COMPILE}
>>> +# KBUILD_BUILD_TIMESTAMP=${KBUILD_BUILD_TIMESTAMP}
>>>  EOF
>>>  }
>>>
>>> diff --git a/meta/recipes-kernel/linux/files/debian/isar/common.tmpl
>>> b/meta/recipes-kernel/linux/files/debian/isar/common.tmpl
>>> index b7752fb6..ffe84745 100644
>>> --- a/meta/recipes-kernel/linux/files/debian/isar/common.tmpl
>>> +++ b/meta/recipes-kernel/linux/files/debian/isar/common.tmpl
>>> @@ -7,6 +7,7 @@ ARCH=${KERNEL_ARCH}
>>>  KERNEL_PKG_IMAGE=linux-image-${KERNEL_NAME_PROVIDED}
>>>  KERNEL_PKG_KERN_HEADERS=linux-headers-${KERNEL_NAME_PROVIDED}
>>>  KERNEL_PKG_LIBC_HEADERS=linux-libc-dev
>>> +KBUILD_BUILD_TIMESTAMP="${KBUILD_BUILD_TIMESTAMP}"
>>>
>>>  # Constants
>>>  KCONF=.config
>>> @@ -36,6 +37,7 @@ main() {
>>>
>>>      # variables to be exported
>>>      export ARCH
>>> +    export KBUILD_BUILD_TIMESTAMP
>>>
>>>      # are we cross-compiling?
>>>      BUILD_ARCH=$(dpkg-architecture -qDEB_BUILD_ARCH) diff --git
>>> a/meta/recipes-kernel/linux/linux-custom.inc
>>> b/meta/recipes-kernel/linux/linux-custom.inc
>>> index 8a5dbf82..f8e87453 100644
>>> --- a/meta/recipes-kernel/linux/linux-custom.inc
>>> +++ b/meta/recipes-kernel/linux/linux-custom.inc
>>> @@ -33,6 +33,8 @@ KERNEL_HEADERS_DEBIAN_DEPENDS ?= "libc6, \
>>>
>>>  KERNEL_LIBC_DEV_DEPLOY ?= "0"
>>>
>>> +KBUILD_BUILD_TIMESTAMP ??= "LANG=C $(date)"
>>
>> Can't we use SOURCE_DATE_EPOCH like the rest?
> 
> This variable is still not honored by the kernel (AFAIK).
> Also the kernel documentation only mentions KBUILD_BUILD_TIMESTAMP.
> There is a patch from 2015 which has not been applied [1].
> 
> We could do the mapping in ISAR, but I prefer to have it explicit.
> 
> [1] https://patchwork.kernel.org/project/linux-kbuild/patch/1443741332.2730.75.camel@decadent.org.uk/
> 

Then how does the regular Debian kernel set a stable date? We should
align to that and surely not force the user to set anything manually
(that is my interpretation of the behavior of the current patch).

Jan

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