hasty opened a new pull request #414:
URL: https://github.com/apache/mynewt-newt/pull/414
This resolves a limitation of the split image build process, allowing for
larger builds.
Prior to this change, the split build process worked like so:
1. Build the app image, linking against the regular "bsp.linkerscript" ld
script.
2. Build the loader image, also linking against the regular
"bsp.linkerscript" ld script.
3. Compare the resulting elfs from app and loader, look for common
functions, and substitute their addresses from the loader's elf into the app's
elf.
4. Re-link the app against the special "bsp.part2linkerscript" ld script,
which maps everything into slot 2's address space, except for those addresses
already mapped into the loader's elf in step 3 (and also do some
hardware-specific monkeying, if needed).
That doesn't use the split image setup to its full potential, though. The
regular linker script needs to enforce that the resulting image fits inside the
provided flash area, but in the case of the build in step 1, this flash area is
less than what is actually available, as it counts the various symbols that
will be re-linked and eliminated from the image in step 4. That artificially
limits the size of the app to its own code, plus the shared code, minus any
non-overlapping code in the loader image. In reality, it has (almost) the
entirely of both flash areas to play with, as long as the portion shared with
the loader doesn't overflow in step 2, and it doesn't overstep the various
needs of the boot loader (though existing scripts don't account for that as it
stands).
This PR changes step 1 to use an optional third linker script specified by
"bsp.wholeapplinkerscript". This allows a bsp to specify an initial app linker
script that allows the full use of both image slots; in the absence of such a
script, it falls back on existing behavior. The resulting app image may not be
workable if too much of it consists of dependencies on loader code, but that
will be revealed later by a failure in step 3.
It also changes the name of the setting in step 4 to
"bsp.splitapplinkerscript", which is more accurate and implies the connection
between the two settings ("bsp.part2linkerscript" was misleading anyway, as
technically that was stage 3 of the linking process). It maintains backwards
compatibility with existing bsp.yml files that still use
"bsp.part2linkerscript", but in the presence of both, prefers
"bsp.splitapplinkerscript".
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