linux-kernel - Re: [RFC PATCH] Don't reset timestamps in include/generated if not needed

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Date:	Sun, 8 Mar 2015 16:08:06 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	Michal Marek <mmarek@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>
Subject: Re: [RFC PATCH] Don't reset timestamps in include/generated if not needed

On Tue, Mar 3, 2015 at 5:07 PM, Valdis Kletnieks
<Valdis.Kletnieks@...edu> wrote:
>
> Kbuild regenerates bounds.h and asm-offsets.h, resetting the timestamps
> and forcing rebuilds even if the contents haven't changed.  Add a bit of
> shell magic to only replace the file if the contents have in fact changed,
> which should speed up git bisects and similar.
> ...
> RFC because I can't wrap my head around why this wasn't done ages ago.
> If I'm missing something something obvious, please apply a cluestick. :)
>
> Lightly tested - if I rm one of those two files, it gets rebuilt. If I
> insert some whitespace, it gets replaced. If I don't touch it, the datestamp
> doesn't change.

I don't think this is wrong, but I'd really prefer to do the whole
move-if-changed thing as a separate rule, and a separate command.

IOW, could we please have something that generates the "build.h" file
by separately creating a new temporary file, and then having the
traditional kind of move-if-changed definition

   define move-if-changed
        if ! cmp -s $(1) $(2); then mv -f $(1) $(2); else rm -f $(1); fi
   endef

kind of thing that just does

   $(call move-if-changed,build.h.tmp,build.h)

Hmm? That move-if-changed can be useful for other cases too, I've seen
something like it in a lot of makefiles. I'm surprised we don't have
one already.

That said, we already have that "filechk" define that does pretty much
all of this.

So I don't like the exact patch, but I do like the effect it gives. Michal?

                    Linus
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