On Wed, Dec 10, 2008 at 12:10:48PM -0800, Andrew Morton wrote:
> On Wed, 10 Dec 2008 21:02:42 +0100
> Sam Ravnborg <sam@xxxxxxxxxxxx> wrote:
>
> > On Wed, Dec 10, 2008 at 02:48:35PM -0500, Steven Rostedt wrote:
> > >
> > > Hi,
> > >
> > > In include/linux/kernel.h I currently have the following lines at the
> > > bottom of the file:
> > >
> > > /* Rebuild everything on CONFIG_FTRACE_MCOUNT_RECORD */
> > > #ifdef CONFIG_FTRACE_MCOUNT_RECORD
> > > # define REBUILD_DUE_TO_FTRACE_MCOUNT_RECORD
> > > #endif
> > >
> > >
> > > This is only there to force a rebuild of all files when
> > > CONFIG_FTRACE_MCOUNT_RECORD is modified. Since the modification of that
> > > config causes the build to act different, we need to rebuild all C
> > > objects. I added the #ifdef in kernel.h as a hack to force the rebuild by
> > > the config dependencies used in kbuild.
> > >
> > > My question is, is there a better way to force a full rebuild on a
> > > modification of a config?
> >
> > Why is that hack required in the first place?
> > We rebuild if:
> > 1- target file is missing (not the case here)
> > 2- any prerequisite files are newer than target (not the case here)
> > 3- any CONFIG_* options used by any prerequisite has changed (not the case here)
> > [It is due to your hack]
> > 4- gcc changed (not the case here)
> > 5- options are added/deleted to gcc (this should be the case for relevant files)
> >
>
> But damn me it's hard to work out where and how this is implemented :(
The magic is to be found in Kbuild.include:
# Usage: $(call if_changed_rule,foo)
# Will check if $(cmd_foo) or any of the prerequisites changed,
# and if so will execute $(rule_foo).
if_changed_rule = $(if $(strip $(any-prereq) $(arg-check) ), \
@set -e; \
$(rule_$(1)))
This parts takes care of:
$(any-prereq) takes care of 1 and 2 in the above list
$(arg-check) takes care of 4 and 5 in the above list.
And record_mcount is part of the following definition:
define rule_cc_o_c
$(call echo-cmd,checksrc) $(cmd_checksrc) \
$(call echo-cmd,cc_o_c) $(cmd_cc_o_c); \
$(cmd_modversions) \
$(cmd_record_mcount) \
scripts/basic/fixdep $(depfile) $@ '$(call make-cmd,cc_o_c)' > \
$(dot-target).tmp; \
rm -f $(depfile); \
mv -f $(dot-target).tmp $(dot-target).cmd
endef
Here we use fixdep to create a set of prerequisites - one for each
CONFIG_ symbol that is found in the prerequisites. And fixdep reads the
list of prerequisites in the $(dep-file) that is created by gcc in the
cc_o_c command:
cmd_cc_o_c = $(CC) $(c_flags) -c -o $(@D)/.tmp_$(@F) $<
So after reading this you know almost as much about kbuild as I do.
I will fix the record_mcount stuff somehow.
But not tonight. Busy with work-work.
Sam
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