[RFC PATCH v2 1/5] kbuild: fit: Document FIT_COMPRESSION and FIT_DECOMPOSE_DTBS

[Douglas Anderson]

`FIT_COMPRESSION` and `FIT_DECOMPOSE_DTBS` can be passed on the `make`
command line when building `image.fit` (see `cmd_fit` in
`scripts/Makefile.lib`), but neither variable was documented in
`Documentation/kbuild/kbuild.rst` or mentioned in
`arch/arm64/Makefile`'s `make help` output.

Document both variables before adding new FIT Kbuild variables.

Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---

Changes in v2:
- New preparatory patch documenting preexisting FIT_COMPRESSION and
  FIT_DECOMPOSE_DTBS variables (Simon Glass)

 Documentation/kbuild/kbuild.rst | 12 ++++++++++++
 arch/arm64/Makefile             |  1 +
 2 files changed, 13 insertions(+)

diff --git a/Documentation/kbuild/kbuild.rst b/Documentation/kbuild/kbuild.rst
index e71a692a559a..744a32ccbd1c 100644
--- a/Documentation/kbuild/kbuild.rst
+++ b/Documentation/kbuild/kbuild.rst
@@ -364,3 +364,15 @@ LLVM
 ----
 If this variable is set to 1, Kbuild will use Clang and LLVM utilities instead
 of GCC and GNU binutils to build the kernel.
+
+FIT_COMPRESSION
+---------------
+Compression algorithm used when packaging the kernel and device tree blobs
+into ``image.fit`` (for example, ``gzip``, ``lz4``, ``xz``, ``zstd``, or
+``none``). Defaults to ``gzip``.
+
+FIT_DECOMPOSE_DTBS
+------------------
+If set to a non-empty value when building ``image.fit``, decompose composite
+DTBs (built from a base ``.dtb`` plus ``.dtbo`` overlays) into their component
+blobs and deduplicate them in the FIT.
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index ed28f94be428..8818d6a2015b 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -223,6 +223,7 @@ define archhelp
   echo  '* Image.gz      - Compressed kernel image (arch/$(ARCH)/boot/Image.gz)'
   echo  '  Image         - Uncompressed kernel image (arch/$(ARCH)/boot/Image)'
   echo  '  image.fit     - Flat Image Tree (arch/$(ARCH)/boot/image.fit)'
+  echo  '                  Configured by FIT_COMPRESSION and FIT_DECOMPOSE_DTBS'
   echo  '  install       - Install kernel (compressed if COMPRESSED_INSTALL set)'
   echo  '  zinstall      - Install compressed kernel'
   echo  '                  Install using (your) ~/bin/installkernel or'
-- 
2.56.0.611.g1030f0b940-goog