[PATCH] ARM: kbuild: Fix forced rebuild after 'make dtbs'

[Jason Cooper]

On Tue, Nov 25, 2014 at 11:51:50PM +0000, Russell King - ARM Linux wrote:
> On Tue, Nov 25, 2014 at 10:41:35PM +0100, Michal Marek wrote:
> > Dne 2.11.2014 v 21:52 Jason Cooper napsal(a):
> > > After this patch:
> > > 
> > >   f4d4ffc03efc kbuild: dtbs_install: new make target
> > > 
> > > was added the kernel tree, Linus Walleij noticed that 'make dtbs' forced
> > > a following 'make zImage' to rebuild the entire tree, even though
> > > nothing had changed.  His report:
> > > 
> > >   After this patch a while back I have observed the following behaviour
> > >   of the kernel build:
> > > 
> > >   make zImage
> > >   make zImage
> > >   -> incremental build, just relink
> > > 
> > >   make zImage
> > >   make dtbs
> > >   make zImage
> > >   -> The whole kernel gets rebuilt
> > > 
> > >   So now if I happen to recompile my device trees, I suddenly want
> > >   the entire zImage to be rebuilt to? It's by definition not changes
> > >   that affect the kernel build :-(
> > > 
> > >   I noticed this because my build scripts calls make dtbs && make
> > >   zImage, and started to rebuild absolutely everything all the time.
> > > 
> > > To fix this, we make only the dtbs_install target depend on the prepare
> > > target.  It's needed to make sure KERNELVERSION is calculated prior to
> > > installing.
> > 
> > If a mere 'make prepare' causes a rebuild of the whole kernel, then
> > there is something fishy in the ARM Makefiles. However, if you only need
> > the KERNELRELEASE variable, then you do not need to depend on prepare.
> > The main Makefile makes sure that silentoldconfig is ran and therefore
> > KERNELRELEASE set for all targets except make *config.
> 
> Running make prepare and then re-running a build doesn't rebuild anything
> for me.  However, I always build with O=  Also works if I do a make
> dtbs too.  So, everything seems to work as expected here.
> 
> I think this needs a bit more debugging to see why the whole kernel is
> being rebuilt - the kernel build system has methods to tell you why stuff
> is being built, which would be a good place to start.

Linus, since you were the one experiencing the regression, care to take
a whack at that?

thx,

Jason.