tools build: Fix Makefile(s) to properly invoke tools build

Several fixes were needed to allow following builds:
  $ make tools/tmon
  $ make -C <kernelsrc> tools/perf
  $ make -C <kernelsrc>/tools perf

 - some of the tools (perf) use same make variables as in
   kernel build, unsetting srctree and objtree
 - using original $(O) for O variable
 - perf build does not follow the descend function setup
   invoking it via it's own make rule

I tried the rest of the tools/Makefile targets and they
seem to work now.

Reported-by: Brian Norris <[email protected]>
Signed-off-by: Jiri Olsa <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Norris <[email protected]>
Cc: David Ahern <[email protected]>
Cc: David Howells <[email protected]>
Cc: Michal Marek <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Sam Ravnborg <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

1 files changed, 11 insertions, 1 deletions

diff --git a/tools/Makefile b/tools/Makefile
index 01995cc8ac44..b35102721cbb 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -1,3 +1,8 @@
+# Some of the tools (perf) use same make variables
+# as in kernel build.
+export srctree=
+export objtree=
+
 include scripts/Makefile.include
 
 help:
@@ -50,8 +55,13 @@ liblockdep: FORCE
 libapi: FORCE
 	$(call descend,lib/api)
 
+# The perf build does not follow the descend function setup,
+# invoking it via it's own make rule.
+PERF_O   = $(if $(O),$(O)/tools/perf,)
+
 perf: FORCE
-	$(call descend,$@)
+	$(Q)mkdir -p $(PERF_O) .
+	$(Q)$(MAKE) --no-print-directory -C perf O=$(PERF_O) subdir=
 
 selftests: FORCE
 	$(call descend,testing/$@)