The source repository (whether BK or any other system) is not the
problem. You can get the timestamps right in the source but the moment
you generate and ship a diff then you lose control of timestamps. See
the long screed below about the problems with shipping generated files,
from kbuild-2.5.txt.
SHIPPING GENERATED FILES
In general it is bad practice to include generated files in the kernel
source tree. The only exceptions are when the generating code is not in
the kernel or when the generating code is in the kernel but it requires
additional tools which not all users have installed. An example of a
generated file without the code to regenerate it is firmware for a network
driver. An example of a generation process that requires extra tools is
drivers/char/defkeymap.c which requires the loadkeys program.
There is no justification for shipping any other generated file as part of
the kernel. This is especially true if the user is always expected to
regenerate the file, even more so if the generated data depends upon the
user's .config. Under these circumstances it is dangerous to ship a
generated file because it can be used when it does not reflect the user's
kernel.
When a generated file has no kernel support to regenerate it, the file can
be shipped as is. The question of whether including generated files in the
kernel without the underlying code is a violation of the kernel license is
outside the scope of this document.
For generated files where the code is included but not all users can run
it, you must be careful about when the generated file is rebuilt. make
looks at the time stamp of the generating and generated files to decide if
the output needs to be regenerated. This works for the person making the
changes because they control when the files are updated.
Time stamps do not work when the change is issued to the rest of the world
as a diff. After patch has run you cannot guarantee which of the updated
files has the more recent time. The patch program does not preserve time
stamps (it must not) so the time stamps on the changed files depend on the
order of the entries in the patch set. There is no defined order for
entries in a patch set, it depends on the program that generated the
difference listing. GNU diff generates patches in alphabetical order but
only when it is given an entire directory. When diff is invoked from a
source repository tool it is typically called once for each file and the
file order is controlled by the repository tool. Even if your repository
generates the patch in the correct order, that order can change when the
patch is merged into the kernel.
Since you cannot rely on time stamps to decide if a generated and shipped
file needs to be regenerated, you must use another mechanism for this case.
There are two basic possibilities, manual or automatic (otherwise known as
lazy or correct).
In the manual case you ignore changes to the generated or generating files
until the user performs some manual operation, such as selecting
CONFIG_REGENERATE_foo. This is easy to implement but it is also the lazy
approach, making the end user do extra work to save the maintainer doing
anything. This goes against the whole idea of *automatically* deciding
what needs to be rebuilt and relies on human intervention.
In the automatic case the maintainer has to do more work but the make rules
can automatically determine if the end user has changed any of the related
files and regenerate automatically. This is the correct method. In the
absence of reliable time stamps you need another method to check if the
generating and generated files are in sync or not. The obvious method is a
checksum of files.
The 2.5 implementation for correctly handling generated files has been
skipped, read Documentation/kbuild/kbuild-2.5.txt for the gory details.
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