kbuild: dtc: Allow adding device tree fragments via config

This introduces a config variable that allows adding additional fragments
to the Barebox device tree(s).

Example uses are adjusting the flash partition layout, adding barebox
state variables, or adding an I2C device.  These can be now be done with
build configuration only, without needing to patch the existing dts
files in the Barebox source.

The advantage is greater when an external build system, such as Yocto or
Buildroot, is being used to build Barebox.  The build system can drop in
a dts fragment to partition flash and build from unaltered Barebox
source.  This avoids the need for cumbersome maintenance of patch files
to modify Barebox's source for each flash partition layout.

Preprocessing the dts file gains another layer, where a generated dts
source consisting of an include directive for the original dts source is
followed by more includes for each fragment.  This is piped to the
existing preprocessor call on stdin to avoid another temporary file.
cpp/dtc will correctly identify errors in the source files they occur
in.  The -MT option is used so the cpp auto-dependencies reference the
original dts source and not the generated code passed on stdin.

A preprocessor macro named after the base dts file, e.g. foo-bar.dts
will define foo_bar_dts, will be defined so that the fragments can
possibly operate differently based on which image's dts is being built.

Signed-off-by: Trent Piepho <trent.piepho@igorinstitute.com>
Link: https://lore.barebox.org/20210922181336.1350460-1-trent.piepho@igorinstitute.com
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

1 files changed, 24 insertions, 0 deletions

diff --git a/common/Kconfig b/common/Kconfig
index a9feae2ae8..506b1dd39a 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -1091,6 +1091,30 @@ config SYSTEMD_OF_WATCHDOG
 	  in the kernel device tree. If the kernel is booted without a device
 	  tree or with one that lacks aliases, nothing is added.
 
+config EXTERNAL_DTS_FRAGMENTS
+	string "external dts file fragments"
+	depends on OFTREE
+	help
+	  List of dts fragment files that will be appended to Barebox's device
+	  tree(s) source when building the dtb file(s).  If multiple files are
+	  listed, they will be appended in order.  Relative filenames will use
+	  the dtc include search path.
+
+	  A preprocessor macro based on the name of the main dts will be
+	  defined, which allows the dts fragments to based on which image of a
+	  multi image build they are being used in. Given the dts filename
+	  used for a board is "foo-board.dts" the external dts usage can be
+	  limited to that board with
+
+	  #ifdef foo_board_dts
+	  ...
+	  #endif
+
+	  It not intended that this be put into into Barebox defconfig files.
+	  Instead, it's an external build system, like Yocto or buildroot, to
+	  add dts fragments from outside the Barebox source tree into the
+	  Barebox build.
+
 menu "OP-TEE loading"
 
 config OPTEE_SIZE