Re: [PATCH v3 1/2] kbuild: move non-boot built-in DTBs to .rodata section

[Rob Herring <robh@kernel.org>]

On Mon, Sep 23, 2024 at 04:56:02PM +0900, Masahiro Yamada wrote:
> Commit aab94339cd85 ("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.
> 
> Fixes: 5c9dd72d8385 ("of: Add a KUnit test for overlays and test managed APIs")
> Fixes: 5776526beb95 ("clk: Add KUnit tests for clk fixed rate basic type")
> Fixes: 274aff8711b2 ("clk: Add KUnit tests for clks registered with struct clk_parent_data")
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> ---
> 
> Changes in v3:
>   - Move to .rodata section instead of .init.rodata
> 
>  scripts/Makefile.dtbs | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)

Acked-by: Rob Herring (Arm) <robh@kernel.org>