[PATCH v2 1/1] kbuild: dtc: Allow adding device tree fragments via config

[Sascha Hauer]

Hi Trent,

On Wed, Sep 22, 2021 at 11:13:36AM -0700, Trent Piepho wrote:
> 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 at igorinstitute.com>
> ---
> Changes from v1:
>   - Remove 2nd config variable for in-tree fragments.
>   - Add macro to preproccessed dts

I tested this myself and it works as expected. Applied with a little
change

> +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.

Changed this to:

	  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

To make that point a little more clear.

Sascha

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