Date: Tue, 18 Jan 2005 20:05:13 +0100
From: Sam Ravnborg <sam@ravnborg.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Sam Ravnborg <sam@ravnborg.org>, linux-kernel@vger.kernel.org
Subject: Re: kbuild: Implicit dependence on the C compiler
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	Sam Ravnborg <sam@ravnborg.org>, linux-kernel@vger.kernel.org
References: <cshbd7$nff$1@terminus.zytor.com> <20050117220052.GB18293@mars.ravnborg.org> <41EC363D.1090106@zytor.com>
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Hi hpa
> >It better be difficult. You want to recompile when changing gcc.
> >Try this untested patch.
> Sorry, but that's baloney.  Saying "it better be difficult" is 
> equivalent to saying "kbuild is smarter than you."

To give some background info about why kbuild does what it does.
A kernel being compiled partly with and partly without say -regparm=3
will result in a non-workable kernel.

The same goes for a kernel that is partly built using gcc 2.96, partly
using 3.3.4 for example.

So kbuild pr. default will force a recompile for any .o file where
opions to gcc differ, or name of gcc has changed. Today no check has
been implemented to check the actual gcc executable timestamp - and
neither is this planned.

Default behaviour today is to recompile if anything change.

But as hpa points outs this hits us with nfs mounted kernel tree when
performing a make install - because install has vmlinux as prerequsite.
So this leaves us with at least two possibilitites:
1) Unconditionally execute make install assuming vmlinux is up-to-date.
   make modules_install run unconditionally, so this is already know
   practice
2) Detect that aother user is running the build - and therefore skip
   the kernel and the module build.
   This is a rather intrusive change since with current kbuild structure
   it is rather difficult to stop this in all relevant cases.

	Sam
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