linux-kernel - Re: [PATCH v2] kbuild: fix if_change and friends to consider argument order

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Date:	Tue, 10 May 2016 21:22:07 +0200
From:	Michal Marek <mmarek@...e.com>
To:	Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:	linux-kbuild@...r.kernel.org, David Howells <dhowells@...hat.com>,
	David Woodhouse <David.Woodhouse@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] kbuild: fix if_change and friends to consider
	argument order

On Sat, May 07, 2016 at 03:48:26PM +0900, Masahiro Yamada wrote:
> Currently, arg-check is implemented as follows:
> 
>   arg-check = $(strip $(filter-out $(cmd_$(1)), $(cmd_$@)) \
>                       $(filter-out $(cmd_$@),   $(cmd_$(1))) )
> 
> This does not care about the order of arguments that appear in
> $(cmd_$(1)) and $(cmd_$@).  So, if_changed and friends never rebuild
> the target if only the argument order is changed.  This is a problem
> when the link order is changed.
> 
> Apparently,
> 
>   obj-y += foo.o
>   obj-y += bar.o
> 
> and
> 
>   obj-y += bar.o
>   obj-y += foo.o
> 
> should be distinguished because the link order determines the probe
> order of drivers.  So, built-in.o should be rebuilt when the order
> of objects is changed.
> 
> This commit fixes arg-check to compare the old/current commands
> including the argument order.
> 
> Of course, this change has a side effect; Kbuild will react to the
> change of compile option order.  For example, "-DFOO -DBAR" and
> "-DBAR -DFOO" should give no difference to the build result, but
> false positive should be better than false negative.

Agreed. Applied to kbuild.git#kbuild now.

Michal