From 93b196532254366f653b4d763f69e49ff193f06c Mon Sep 17 00:00:00 2001
From: Simon Glass <sjg@chromium.org>
Date: Wed, 22 Sep 2021 11:34:44 -0600
Subject: 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 | 30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

(limited to 'Makefile')

diff --git a/Makefile b/Makefile
index f911f70344..32d29b87db 100644
--- a/Makefile
+++ b/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     $@
-- 
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