Makefile: Only build dtc if needed

At present U-Boot always builds dtc if CONFIG_OF_CONTROL is defined, even
when DTC is provided. The built dtc is not actually used, so this is a
waste of time.

Update the Makefile logic to build dtc only if one is not provided to the
build with the DTC variable. Add documentation to explain this.

This saves about 3.5 seconds of elapsed time on a clean build of
sandbox_spl for me.

Signed-off-by: Simon Glass <sjg@chromium.org>

Makefile

@@ -415,7 +415,13 @@ PERL		= perl
 PYTHON		?= python
 PYTHON2		= python2
 PYTHON3		?= python3
-DTC		?= $(objtree)/scripts/dtc/dtc
+
+# The devicetree compiler and pylibfdt are automatically built unless DTC is
+# provided. If DTC is provided, it is assumed the pylibfdt is available too.
+DTC_INTREE	:= $(objtree)/scripts/dtc/dtc
+DTC		?= $(DTC_INTREE)
+DTC_MIN_VERSION	:= 010406
+
 CHECK		= sparse
 
 CHECKFLAGS     := -D__linux__ -Dlinux -D__STDC__ -Dunix -D__unix__ \
@@ -1954,9 +1960,29 @@ endif
 
 endif
 
+# Check dtc and pylibfdt, if DTC is provided, else build them
 PHONY += scripts_dtc
 scripts_dtc: scripts_basic
-	$(Q)$(MAKE) $(build)=scripts/dtc
+	$(Q)if test "$(DTC)" = "$(DTC_INTREE)"; then \
+		$(MAKE) $(build)=scripts/dtc; \
+	else \
+		if ! $(DTC) -v >/dev/null; then \
+			echo '*** Failed to check dtc version: $(DTC)'; \
+			false; \
+		else \
+			if test "$(call dtc-version)" -lt $(DTC_MIN_VERSION); then \
+				echo '*** Your dtc is too old, please upgrade to dtc $(DTC_MIN_VERSION) or newer'; \
+				false; \
+			else \
+				if [ -n "$(CONFIG_PYLIBFDT)" ]; then \
+					if ! echo "import libfdt" | $(PYTHON3) 2>/dev/null; then \
+						echo '*** pylibfdt does not seem to be available with $(PYTHON3)'; \
+						false; \
+					fi; \
+				fi; \
+			fi; \
+		fi; \
+	fi
 
 # ---------------------------------------------------------------------------
 quiet_cmd_cpp_lds = LDS     $@

doc/build/gcc.rst

@@ -120,6 +120,27 @@ Further important build parameters are
 * O=<dir> - generate all output files in directory <dir>, including .config
 * V=1 - verbose build
 
+Devicetree compiler
+~~~~~~~~~~~~~~~~~~~
+
+Boards that use `CONFIG_OF_CONTROL` (i.e. almost all of them) need the
+devicetree compiler (dtc). Those with `CONFIG_PYLIBFDT` need pylibfdt, a Python
+library for accessing devicetree data. Suitable versions of these are included
+in the U-Boot tree in `scripts/dtc` and built automatically as needed.
+
+To use the system versions of these, use the DTC parameter, for example
+
+.. code-block:: bash
+
+    DTC=/usr/bin/dtc make
+
+In this case, dtc and pylibfdt are not built. The build checks that the version
+of dtc is new enough. It also makes sure that pylibfdt is present, if needed
+(see `scripts_dtc` in the Makefile).
+
+Note that the :doc:`tools` are always built with the included version of libfdt
+so it is not possible to build U-Boot tools with a system libfdt, at present.
+
 Other build targets
 ~~~~~~~~~~~~~~~~~~~
 

dts/Kconfig

@@ -5,9 +5,6 @@
 config SUPPORT_OF_CONTROL
 	bool
 
-config DTC
-	bool
-
 config PYLIBFDT
 	bool
 
@@ -42,7 +39,6 @@ menu "Device Tree Control"
 
 config OF_CONTROL
 	bool "Run-time configuration via Device Tree"
-	select DTC
 	select OF_LIBFDT if !OF_PLATDATA
 	select OF_REAL if !OF_PLATDATA
 	help

scripts/Makefile

@@ -10,4 +10,3 @@ always		:= $(hostprogs-y)
 
 # Let clean descend into subdirs
 subdir-	+= basic kconfig
-subdir-$(CONFIG_DTC)	+= dtc

scripts/dtc-version.sh

@@ -10,11 +10,16 @@
 dtc="$*"
 
 if [ ${#dtc} -eq 0 ]; then
-	echo "Error: No dtc command specified."
+	echo "Error: No dtc command specified"
 	printf "Usage:\n\t$0 <dtc-command>\n"
 	exit 1
 fi
 
+if ! which $dtc >/dev/null ; then
+	echo "Error: Cannot find dtc: $dtc"
+	exit 1
+fi
+
 MAJOR=$($dtc -v | head -1 | awk '{print $NF}' | cut -d . -f 1)
 MINOR=$($dtc -v | head -1 | awk '{print $NF}' | cut -d . -f 2)
 PATCH=$($dtc -v | head -1 | awk '{print $NF}' | cut -d . -f 3 | cut -d - -f 1)