linux-kernel - Dependency issue through subdir's

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Date:	Mon, 25 Jan 2016 11:33:32 +0000
From:	Kieran Bingham <kieran@...ngham.co.uk>
To:	linux-kbuild@...r.kernel.org, mmarek@...e.com
Cc:	Jan Kiszka <jan.kiszka@...mens.com>,
	Lee Jones <lee.jones@...aro.org>,
	Peter Griffin <peter.griffin@...aro.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Dependency issue through subdir's

I am having difficulty with dependencies while trying to create an
automatically generated file in linux/scripts/gdb/

This autogenerated file, includes the kernel headers, so that we can
access kernel constants from GDB. As such, I need to parse C-headers
into a python-compatible file.

The commit-diff at :
https://git.linaro.org/people/kieran.bingham/linux.git/commitdiff/26194ca0579ec94b3630f1aac0e985242831aa1c
shows the current implementation.

This is functional on a make -j1 build, however at -j2 and above (from a
clean build), kbuild attempts to generate my file before
include/generated/timeconst.h resulting in the following error:

--- 8< ---
  GEN     scripts/gdb/linux/constants.py
In file included from sources/linux/include/linux/ktime.h:25:0,
                 from /linux/include/linux/rcupdate.h:47,
                 from /linux/include/linux/rbtree.h:34,
                 from /linux/include/linux/mm_types.h:9,
                 from /linux/arch/x86/include/asm/pgtable.h:448,
                 from /linux/scripts/gdb/linux/constants.py.in:16:
sources/linux/include/linux/jiffies.h:10:33: fatal error:
generated/timeconst.h: No such file or directory
--- >8 ---

I have tried to provide a dependency on include/generated/timeconst.h,
however, because scripts/gdb/ is a subdir-y target, it is run through a
submake, and the parent directory targets are not available.

I have also tried to look at how to delay the execution of the
scripts/gdb subdir - but subdir-ym is simply a sorted list, (and using
subdir-m += scripts/gdb didn't help)

I feel like I have the following possible approaches to solving this:

1) Create a 'subdir-late-' target group which builds specifically as
late in the build process as possible

2) Move the code that generates this file to /Kbuild. However I don't
like the idea of putting non-essential generator at the top level build

3) Add #ifndef __GENERATE_CONSTANTS_PY__ across any reference to
generated header files, such as in ktime.h, and define that with my
Preprocessor statement.

Are there any other solutions? I dislike all 3 of the options above ...

I'd appreciate any thoughts here.

--
Regards

Kieran Bingham