Re: [Qemu-devel] [PATCH] Makefile: Don't build generated headers before Makefile is reread

[Paolo Bonzini <pbonzini@redhat.com>]

Il 28/06/2014 18:59, Peter Maydell ha scritto:
> Having a direct dependency
>    Makefile: $(GENERATED_HEADERS)
> can result in not-from-clean builds failing sometimes, because it means
> that when Make does its "is any makefile or include out of date and
> needing a rebuild?" check, as well as possibly running configure (to
> update config-host.mak) it will also rebuild GENERATED_HEADERS under
> the "old" config-host.mak regime. If the makefile rules for rebuilding
> the generated headers have changed in a way that means they're not
> compatible with the old config-host.mak variable names, the build will
> fail. Even if it does work, it's wasted effort since we'll need to
> rebuild them with the right config-host.mak settings immediately.
>
> Instead, make the generated headers depend on config-host.mak
> and config-target.mak. This means we'll definitely rebuild them
> if the configuration changes, but it will be done after Make
> reloads its makefiles and includes so will happen with the
> correct set of config-host.mak settings.
>
> We rely on the various .o files having correct autogenerated
> dependency rules to cause the generated headers to be generated
> as and when they are needed.
>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> The specific example of this is the change from CONFIG_TRACE_BACKEND
> to CONFIG_TRACE_BACKENDS, which I am still hitting on a pretty
> regular basis. This patch fixes that problem and I don't think it
> has any unpleasant side effects; Paolo, have I missed anything?
>
>  Makefile        | 8 +++-----
>  Makefile.target | 2 +-
>  2 files changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 145adb6..dca33b4 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -546,11 +546,9 @@ ifdef SIGNCODE
>  endif # SIGNCODE
>  endif # CONFIG_WIN
>
> -# Add a dependency on the generated files, so that they are always
> -# rebuilt before other object files
> -ifneq ($(filter-out %clean,$(MAKECMDGOALS)),$(if $(MAKECMDGOALS),,fail))
> -Makefile: $(GENERATED_HEADERS)
> -endif
> +# Make the generated files depend on config-host.mak so that if
> +# the configuration settings change we will rebuild them
> +$(GENERATED_HEADERS): config-host.mak
>
>  # Include automatically generated dependency files
>  # Dependencies in Makefile.objs files come from our recursive subdir rules
> diff --git a/Makefile.target b/Makefile.target
> index 6089d29..ea8614a 100644
> --- a/Makefile.target
> +++ b/Makefile.target
> @@ -198,4 +198,4 @@ ifdef CONFIG_TRACE_SYSTEMTAP
>  endif
>
>  GENERATED_HEADERS += config-target.h
> -Makefile: $(GENERATED_HEADERS)
> +$(GENERATED_HEADERS): config-target.mak config-devices.mak

config-devices.mak is not reflected in any C header file.  Apart from this,

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

Paolo