kbuild: use $(abspath ...) instead of $(shell cd ... && /bin/pwd)

Kbuild conventionally uses $(shell cd ... && /bin/pwd) idiom to get
the absolute path of the directory because GNU Make 3.80, the minimal
supported version at that time, did not support $(abspath ...) or
$(realpath ...).

Commit 37d69ee30808 ("docs: bump minimal GNU Make version to 3.81")
dropped the GNU Make 3.80 support, so we are now allowed to use those
make-builtin helpers.

This conversion will provide better portability without relying on
the pwd command or its location /bin/pwd.

I am intentionally using $(realpath ...) instead $(abspath ...) in
some places. The difference between the two is $(realpath ...)
returns an empty string if the given path does not exist. It is
convenient in places where we need to error-out if the makefile fails
to create an output directory.

Signed-off-by: Masahiro Yamada <[e-mail]>
Acked-by: Thierry Reding <[e-mail]>

Makefile

···
 ifneq ($(KBUILD_OUTPUT),)
 # check that the output directory actually exists
 saved-output := $(KBUILD_OUTPUT)
-KBUILD_OUTPUT := $(shell mkdir -p $(KBUILD_OUTPUT) && cd $(KBUILD_OUTPUT) \
-								&& /bin/pwd)
+$(shell [ -d $(KBUILD_OUTPUT) ] || mkdir -p $(KBUILD_OUTPUT))
+KBUILD_OUTPUT := $(realpath $(KBUILD_OUTPUT))
 $(if $(KBUILD_OUTPUT),, \
      $(error failed to create output directory "$(saved-output)"))
 
···
 	$(Q)$(MAKE) -f $(srctree)/Makefile headers_check
 endif
 ifdef CONFIG_GDB_SCRIPTS
-	$(Q)ln -fsn `cd $(srctree) && /bin/pwd`/scripts/gdb/vmlinux-gdb.py
+	$(Q)ln -fsn $(abspath $(srctree)/scripts/gdb/vmlinux-gdb.py)
 endif
 ifdef CONFIG_TRIM_UNUSED_KSYMS
 	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/adjust_autoksyms.sh \
···
 	@rm -rf $(MODLIB)/kernel
 	@rm -f $(MODLIB)/source
 	@mkdir -p $(MODLIB)/kernel
-	@ln -s `cd $(srctree) && /bin/pwd` $(MODLIB)/source
+	@ln -s $(abspath $(srctree)) $(MODLIB)/source
 	@if [ ! $(objtree) -ef  $(MODLIB)/build ]; then \
 		rm -f $(MODLIB)/build ; \
 		ln -s $(CURDIR) $(MODLIB)/build ; \
···
 # Clear a bunch of variables before executing the submake
 tools/: FORCE
 	$(Q)mkdir -p $(objtree)/tools
-	$(Q)$(MAKE) LDFLAGS= MAKEFLAGS="$(tools_silent) $(filter --j% -j,$(MAKEFLAGS))" O=$(shell cd $(objtree) && /bin/pwd) subdir=tools -C $(src)/tools/
+	$(Q)$(MAKE) LDFLAGS= MAKEFLAGS="$(tools_silent) $(filter --j% -j,$(MAKEFLAGS))" O=$(abspath $(objtree)) subdir=tools -C $(src)/tools/
 
 tools/%: FORCE
 	$(Q)mkdir -p $(objtree)/tools
-	$(Q)$(MAKE) LDFLAGS= MAKEFLAGS="$(tools_silent) $(filter --j% -j,$(MAKEFLAGS))" O=$(shell cd $(objtree) && /bin/pwd) subdir=tools -C $(src)/tools/ $*
+	$(Q)$(MAKE) LDFLAGS= MAKEFLAGS="$(tools_silent) $(filter --j% -j,$(MAKEFLAGS))" O=$(abspath $(objtree)) subdir=tools -C $(src)/tools/ $*
 
 # Single targets
 # ---------------------------------------------------------------------------

scripts/gdb/linux/Makefile

···
 always := gdb-scripts
 
-SRCTREE := $(shell cd $(srctree) && /bin/pwd)
+SRCTREE := $(abspath $(srctree))
 
 $(obj)/gdb-scripts:
 ifneq ($(KBUILD_SRC),)

tools/power/cpupower/Makefile

···
 
 ifneq ($(OUTPUT),)
 # check that the output directory actually exists
-OUTDIR := $(shell cd $(OUTPUT) && /bin/pwd)
+OUTDIR := $(realpath $(OUTPUT))
 $(if $(OUTDIR),, $(error output directory "$(OUTPUT)" does not exist))
 endif
 

tools/scripts/Makefile.include

···
 ifneq ($(O),)
 ifeq ($(origin O), command line)
-	dummy := $(if $(shell test -d $(O) || echo $(O)),$(error O=$(O) does not exist),)
-	ABSOLUTE_O := $(shell cd $(O) ; pwd)
+	ABSOLUTE_O := $(realpath $(O))
+	dummy := $(if $(ABSOLUTE_O),,$(error O=$(O) does not exist))
 	OUTPUT := $(ABSOLUTE_O)/$(if $(subdir),$(subdir)/)
 	COMMAND_O := O=$(ABSOLUTE_O)
 ifeq ($(objtree),)
···
 
 # check that the output directory actually exists
 ifneq ($(OUTPUT),)
-OUTDIR := $(shell cd $(OUTPUT) && /bin/pwd)
+OUTDIR := $(realpath $(OUTPUT))
 $(if $(OUTDIR),, $(error output directory "$(OUTPUT)" does not exist))
 endif