kbuild: move non-boot built-in DTBs to .rodata section

Commit aab94339 ("of: Add support for linking device tree blobs
into vmlinux") introduced a mechanism to embed DTBs into vmlinux.

Initially, it was used for wrapping boot DTBs in arch/*/boot/dts/, but
it is now reused for more generic purposes, such as testing.

Built-in DTBs are discarded because KERNEL_DTB() is part of INIT_DATA,
as defined in include/asm-generic/vmlinux.lds.h.

This has not been an issue so far because OF unittests are triggered
during boot, as defined by late_initcall(of_unittest).

However, the recent clk KUnit test additions have caused problems
because KUnit can execute test suites after boot.

For example:

  # echo > /sys/kernel/debug/kunit/clk_register_clk_parent_data_device/run

This command triggers a stack trace because built-in DTBs have already
been freed.

While it is possible to move such test suites from kunit_test_suites to
kunit_test_init_section_suites, it would be preferable to avoid usage
limitations.

This commit moves non-boot built-in DTBs to the .rodata section. Since
these generic DTBs are looked up by name, they do not need to be placed
in the special .dtb.init.rodata section.

Boot DTBs should remain in .dtb.init.rodata because the arch boot code
generally does not know the DT name, thus it uses the __dtb_start symbol
to locate it.

This separation also ensures that the __dtb_start symbol references the
boot DTB. Currently, the .dtb.init.rodata is a mixture of both boot and
non-boot DTBs. The __dtb_start symbol must be followed by the boot DTB,
but we currently rely on the link order (i.e., the order in Makefiles),
which is very fragile.

The implementation is kind of cheesy; the section is .dtb.init.rodata
when $(obj) starts with arch/$(SRCARCH)/boot/dts, and .rodata section
otherwise. This will be refactored later.

Fixes: 5c9dd72d ("of: Add a KUnit test for overlays and test managed APIs")
Fixes: 5776526b ("clk: Add KUnit tests for clk fixed rate basic type")
Fixes: 274aff87 ("clk: Add KUnit tests for clks registered with struct clk_parent_data")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Rob Herring (Arm) <robh@kernel.org>

scripts/Makefile.dtbs

@@ -34,12 +34,14 @@ $(obj)/dtbs-list: $(dtb-y) FORCE
 # Assembly file to wrap dtb(o)
 # ---------------------------------------------------------------------------
 
+builtin-dtb-section = $(if $(filter arch/$(SRCARCH)/boot/dts%, $(obj)),.dtb.init.rodata,.rodata)
+
 # Generate an assembly file to wrap the output of the device tree compiler
 quiet_cmd_wrap_S_dtb = WRAP    $@
       cmd_wrap_S_dtb = {								\
 		symbase=__$(patsubst .%,%,$(suffix $<))_$(subst -,_,$(notdir $*));	\
 		echo '\#include <asm-generic/vmlinux.lds.h>';				\
-		echo '.section .dtb.init.rodata,"a"';					\
+		echo '.section $(builtin-dtb-section),"a"';				\
 		echo '.balign STRUCT_ALIGNMENT';					\
 		echo ".global $${symbase}_begin";					\
 		echo "$${symbase}_begin:";						\