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>

1 files changed, 21 insertions, 0 deletions

diff --git a/doc/build/gcc.rst b/doc/build/gcc.rst
index 0cdc307d57..6c4b4ad7a0 100644
--- a/doc/build/gcc.rst
+++ b/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
 ~~~~~~~~~~~~~~~~~~~