linux-kernel - Re: [RFC 1/3] kbuild: add support for ensuring headers are self-contained

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Message-ID: <20190524174011.GA23737@ravnborg.org>
Date:   Fri, 24 May 2019 19:40:11 +0200
From:   Sam Ravnborg <sam@...nborg.org>
To:     Jani Nikula <jani.nikula@...el.com>
Cc:     linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>
Subject: Re: [RFC 1/3] kbuild: add support for ensuring headers are
 self-contained

Hi Jani

> Sometimes it's useful to be able to explicitly ensure certain headers
> remain self-contained, i.e. that they are compilable as standalone
> units, by including and/or forward declaring everything they depend on.
> 
> Add special target header-test-y where individual Makefiles can add
> headers to be tested if CONFIG_HEADER_TEST is enabled. This will
> generate a dummy C file per header that gets built as part of extra-y.

Very useful, thanks.
I have cooked up something ad-hoc a couple of times but having it as a
standard feature in the build system is much better.
The we can let some of our infrastructure pick up an issues
automatically.

> 
> Cc: Chris Wilson <chris@...is-wilson.co.uk>
> Cc: Masahiro Yamada <yamada.masahiro@...ionext.com>
> Cc: Michal Marek <michal.lkml@...kovi.net>
> Signed-off-by: Jani Nikula <jani.nikula@...el.com>
> ---
>  Documentation/kbuild/makefiles.txt |  7 +++++++
>  init/Kconfig                       |  9 +++++++++
>  scripts/Makefile.build             | 10 ++++++++++
>  scripts/Makefile.lib               |  3 +++
>  4 files changed, 29 insertions(+)
> 
> diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt
> index 03c065855eaf..73df58e5ea0c 100644
> --- a/Documentation/kbuild/makefiles.txt
> +++ b/Documentation/kbuild/makefiles.txt
> @@ -1036,6 +1036,13 @@ When kbuild executes, the following steps are followed (roughly):
>  	In this example, extra-y is used to list object files that
>  	shall be built, but shall not be linked as part of built-in.a.
>  
> +    header-test-y
> +
> +	header-test-y specifies headers (*.h) in the current directory that
> +	should be compile tested to ensure they are self-contained,
> +	i.e. compilable as standalone units. If CONFIG_HEADER_TEST is enabled,
> +	this autogenerates dummy sources to include the headers, and builds them
> +	as part of extra-y.
Do we want to restrict this to current directory only?
Sometimes we could use this for headers in include/ but let it
trigger for the relevant subsystem.
So for example drivers/gpu/drm/Makefile will include the rules
for all headers in include/drm/*

The alternative would be Makefiles (of Kbuild files)
scattered in the directories with headers and then some
infrastructure to visit those.

Follow patch extend the header-test feature to work with
headers in include/

Example:
# Header files from this directory
header-test-y += drm_crtc_helper_internal.h
header-test-y += drm_crtc_internal.h
..
.
# Header files from include/drm
header-test-y += drm/amd_asic_type.h
header-test-y += drm/ati_pcigart.h
...


In the patch $* is used to get the "stem" from the pattern.
This is the filname of the header file without extension.


	Sam


diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 4d4bf698467a..ca132ab3a551 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -295,11 +295,10 @@ $(obj)/%.lst: $(src)/%.c FORCE
 # ---------------------------------------------------------------------------
 
 quiet_cmd_header_test = HDRTEST $@
-      cmd_header_test = echo "\#include \"$(<F)\"" > $@
+      cmd_header_test = echo "\#include <$(2).h>" > $@
 
-# FIXME: would be nice to be able to limit this implicit rule to header-test-y
-$(obj)/%.header_test.c: $(src)/%.h FORCE
-	$(call if_changed,header_test)
+$(obj)/%.header_test.c:
+	$(call cmd,header_test,$*)
 
 # Compile assembler sources (.S)
 # ---------------------------------------------------------------------------