[PATCH v2 3/3] tools build: Correct bpf fixdep dependencies

[Brian Norris <briannorris@chromium.org>]

The dependencies in tools/lib/bpf/Makefile are incorrect. Before we
recurse to build $(BPF_IN_STATIC), we need to build its 'fixdep'
executable.

I can't use the usual shortcut from Makefile.include:

  <target>: <sources> fixdep

because its 'fixdep' target relies on $(OUTPUT), and $(OUTPUT) differs
in the parent 'make' versus the child 'make' -- so I imitate it via
open-coding.

I tweak a few $(MAKE) invocations while I'm at it, because
1. I'm adding a new recursive make; and
2. these recursive 'make's print spurious lines about files that are "up
   to date" (which isn't normally a feature in Kbuild subtargets) or
   "jobserver not available" (see [1])

I also need to tweak the assignment of the OUTPUT variable, so that
relative path builds work. For example, for 'make tools/lib/bpf', OUTPUT
is unset, and is usually treated as "cwd" -- but recursive make will
change cwd and so OUTPUT has a new meaning. For consistency, I ensure
OUTPUT is always an absolute path.

And $(Q) gets a backup definition in tools/build/Makefile.include,
because Makefile.include is sometimes included without
tools/build/Makefile, so the "quiet command" stuff doesn't actually work
consistently without it.

After this change, top-level builds result in an empty grep result from:

  $ grep 'cannot find fixdep' $(find tools/ -name '*.cmd')

[1] https://www.gnu.org/software/make/manual/html_node/MAKE-Variable.html
If we're not using $(MAKE) directly, then we need to use more '+'.

Signed-off-by: Brian Norris <briannorris@chromium.org>
---

Changes in v2:
 - also fix libbpf shared library rules
 - ensure OUTPUT is always set, and always an absolute path
 - add backup $(Q) definition in tools/build/Makefile.include

 tools/build/Makefile.include | 12 +++++++++++-
 tools/lib/bpf/Makefile       | 14 ++++++++++++--
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/tools/build/Makefile.include b/tools/build/Makefile.include
index 8dadaa0fbb43..0e4de83400ac 100644
--- a/tools/build/Makefile.include
+++ b/tools/build/Makefile.include
@@ -1,8 +1,18 @@
 # SPDX-License-Identifier: GPL-2.0-only
 build := -f $(srctree)/tools/build/Makefile.build dir=. obj
 
+# More than just $(Q), we sometimes want to suppress all command output from a
+# recursive make -- even the 'up to date' printout.
+ifeq ($(V),1)
+  Q ?=
+  SILENT_MAKE = +$(Q)$(MAKE)
+else
+  Q ?= @
+  SILENT_MAKE = +$(Q)$(MAKE) --silent
+endif
+
 fixdep:
-	$(Q)$(MAKE) -C $(srctree)/tools/build CFLAGS= LDFLAGS= $(OUTPUT)fixdep
+	$(SILENT_MAKE) -C $(srctree)/tools/build CFLAGS= LDFLAGS= $(OUTPUT)fixdep
 
 fixdep-clean:
 	$(Q)$(MAKE) -C $(srctree)/tools/build clean
diff --git a/tools/lib/bpf/Makefile b/tools/lib/bpf/Makefile
index 2cf892774346..630369c0091e 100644
--- a/tools/lib/bpf/Makefile
+++ b/tools/lib/bpf/Makefile
@@ -108,6 +108,8 @@ MAKEOVERRIDES=
 
 all:
 
+OUTPUT ?= ./
+OUTPUT := $(abspath $(OUTPUT))/
 export srctree OUTPUT CC LD CFLAGS V
 include $(srctree)/tools/build/Makefile.include
 
@@ -141,7 +143,13 @@ all: fixdep
 
 all_cmd: $(CMD_TARGETS) check
 
-$(BPF_IN_SHARED): force $(BPF_GENERATED)
+$(SHARED_OBJDIR):
+	$(Q)mkdir -p $@
+
+$(STATIC_OBJDIR):
+	$(Q)mkdir -p $@
+
+$(BPF_IN_SHARED): force $(BPF_GENERATED) | $(SHARED_OBJDIR)
 	@(test -f ../../include/uapi/linux/bpf.h -a -f ../../../include/uapi/linux/bpf.h && ( \
 	(diff -B ../../include/uapi/linux/bpf.h ../../../include/uapi/linux/bpf.h >/dev/null) || \
 	echo "Warning: Kernel ABI header at 'tools/include/uapi/linux/bpf.h' differs from latest version at 'include/uapi/linux/bpf.h'" >&2 )) || true
@@ -151,9 +159,11 @@ $(BPF_IN_SHARED): force $(BPF_GENERATED)
 	@(test -f ../../include/uapi/linux/if_xdp.h -a -f ../../../include/uapi/linux/if_xdp.h && ( \
 	(diff -B ../../include/uapi/linux/if_xdp.h ../../../include/uapi/linux/if_xdp.h >/dev/null) || \
 	echo "Warning: Kernel ABI header at 'tools/include/uapi/linux/if_xdp.h' differs from latest version at 'include/uapi/linux/if_xdp.h'" >&2 )) || true
+	$(SILENT_MAKE) -C $(srctree)/tools/build CFLAGS= LDFLAGS= OUTPUT=$(SHARED_OBJDIR) $(SHARED_OBJDIR)fixdep
 	$(Q)$(MAKE) $(build)=libbpf OUTPUT=$(SHARED_OBJDIR) CFLAGS="$(CFLAGS) $(SHLIB_FLAGS)"
 
-$(BPF_IN_STATIC): force $(BPF_GENERATED)
+$(BPF_IN_STATIC): force $(BPF_GENERATED) | $(STATIC_OBJDIR)
+	$(SILENT_MAKE) -C $(srctree)/tools/build CFLAGS= LDFLAGS= OUTPUT=$(STATIC_OBJDIR) $(STATIC_OBJDIR)fixdep
 	$(Q)$(MAKE) $(build)=libbpf OUTPUT=$(STATIC_OBJDIR)
 
 $(BPF_HELPER_DEFS): $(srctree)/tools/include/uapi/linux/bpf.h
-- 
2.45.2.803.g4e1b14247a-goog