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:

quoted

Dne 2.11.2014 v 21:52 Jason Cooper napsal(a):

quoted

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.