Hello,
I'd like comments on some changes to fdt_overlay_apply() that I have
prototyped to allow merging of two or more overlay blobs (overlay blob is
considered as a DT blob having unresolved external references listed in
__fixups__ section).
fdt_overlay_apply() API currently allows for an overlay DT blob to be merged
with a base blob. It assumes that all external symbols specified in overlay
blob's __fixups__ section are found in base blob's __symbols__ section and
aborts on the first instance where a symbol could not be found in base blob.
This is mostly fine as the primary use of overlay is on a target for its
bootloader to merge various overlay blobs based on h/w configuration detected?
We are exploring an extended use of fdt_overlay_apply() for offline use, i.e on
the host (build machine) side, which requires merging two overlay DT blobs i.e
the base (overlay) blob will not resolve all the external node references found
in overlay blob.
Currently all the device-tree (DT) code for a given soc is maintained in a
common kernel repository. For example, this common DT code will have code for
audio, video, fingerprint, bluetooth etc. Further this DT code is typically
split into a base (soc-common) code and board specific code, with the soc code
being compiled as soc.dtb and board specific code being compiled as respective
overlay blobs (board1.dtbo, board2.dtbo etc). soc.dtb and boardX.dtbo files are
flashed separately on target (besides improving the overall size of DT blobs
flashed on target, Android Treble also requires separation of soc and board DT
bits). Bootloader will pick one of the board overlay blobs and merge it with
soc.dtb, before booting kernel which is presented a unified DT blob (soc + board
overlay).
For ease of code maintenance and better control over release management, we are
exploring allowing some of the tech teams (audio/fingerprint sensor etc) to
maintain their kernel code (including their DT code) outside a common kernel
repository. In our experience, this simplifies number of branches maintained in
core kernel repo. New/experimental features in fingerprint sensor driver for
example that needs to be on a separate branch will not result in unnecessary
branching in core kenrel repo, affecting all other drivers.
In addition to compiling DT code outside core kernel tree, we also want to merge
the blobs back to respective blobs found in kernel build tree (soc.dtb or
boardX.dtbo), as otherwise relying on bootloader to do all the overlay impacts
boot-time.
This brings up the need to merge two overlay blobs (fingerprint-overlay.dtbo +
boardX.dtbo), which currently doesn't seem to be supported and which this patch
series aims to support.
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