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

[Peter Maydell <peter.maydell@linaro.org>]

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that when Make does its "is any makefile or include out of date and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
-- 
2.0.0