Re: [PATCH 04/15] kbuild: add generic support for built-in boot DTBs

Rob Herring
2024-09-05T14:17:25Z

On Thu, Sep 05, 2024 at 08:47:40AM +0900, Masahiro Yamada wrote:
> Some architectures embed boot DTBs in vmlinux. A potential issue for
> these architectures is a race condition during parallel builds because
> Kbuild descends into arch/*/boot/dts/ twice.
>
> One build thread is initiated by the 'dtbs' target, which is a
> prerequisite of the 'all' target in the top-level Makefile:
>
> ifdef CONFIG_OF_EARLY_FLATTREE
> all: dtbs
> endif
>
> For architectures that support the embedded boot dtb, arch/*/boot/dts/
> is visited also during the ordinary directory traversal in order to
> build obj-y objects that wrap DTBs.
>
> Since these build threads are unaware of each other, they can run
> simultaneously during parallel builds.
>
> This commit introduces a generic build rule to scripts/Makefile.vmlinux
> to support embedded boot DTBs in a race-free way. Architectures that
> want to use this rule need to select CONFIG_GENERIC_BUILTIN_DTB.
>
> After the migration, Makefiles under arch/*/boot/dts/ will be visited
> only once to build only *.dtb files.
>
> This change also aims to unify the CONFIG options used for embedded DTBs
> support. Currently, different architectures use different CONFIG options
> for the same purposes.
>
> The CONFIG options are unified as follows:
>
> - CONFIG_GENERIC_BUILTIN_DTB
>
> This enables the generic rule for embedded boot DTBs. This will be
> renamed to CONFIG_BUILTIN_DTB after all architectures migrate to the
> generic rule.
>
> - CONFIG_BUILTIN_DTB_NAME
>
> This specifies the path to the embedded DTB.
> (relative to arch/*/boot/dts/)
>
> - CONFIG_BUILTIN_DTB_ALL
>
> If this is enabled, all DTB files compiled under arch/*/boot/dts/ are
> embedded into vmlinux. Only used by MIPS.

I started to do this a long time ago, but then decided we didn't want to
encourage this feature. IMO it should only be for legacy bootloaders or
development/debug. And really, appended DTB is more flexible for the
legacy bootloader case.

In hindsight, a common config would have been easier to limit new
arches...

>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> ---
>
> Makefile | 7 ++++++-
> drivers/of/Kconfig | 6 ++++++
> scripts/Makefile.vmlinux | 44 ++++++++++++++++++++++++++++++++++++++++
> scripts/link-vmlinux.sh | 4 ++++
> 4 files changed, 60 insertions(+), 1 deletion(-)
>
> diff --git a/Makefile b/Makefile
> index 145112bf281a..1c765c12ab9e 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1417,6 +1417,10 @@ ifdef CONFIG_OF_EARLY_FLATTREE
> all: dtbs
> endif
>
> +ifdef CONFIG_GENERIC_BUILTIN_DTB
> +vmlinux: dtbs
> +endif
> +
> endif
>
> PHONY += scripts_dtc
> @@ -1483,7 +1487,8 @@ endif # CONFIG_MODULES
> CLEAN_FILES += vmlinux.symvers modules-only.symvers \
> modules.builtin modules.builtin.modinfo modules.nsdeps \
> compile_commands.json rust/test \
> - rust-project.json .vmlinux.objs .vmlinux.export.c
> + rust-project.json .vmlinux.objs .vmlinux.export.c \
> + .builtin-dtbs-list .builtin-dtb.S
>
> # Directories & files removed with 'make mrproper'
> MRPROPER_FILES += include/config include/generated \
> diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
> index dd726c7056bf..5142e7d7fef8 100644
> --- a/drivers/of/Kconfig
> +++ b/drivers/of/Kconfig
> @@ -2,6 +2,12 @@
> config DTC
> bool
>
> +config GENERIC_BUILTIN_DTB
> + bool

So that we don't add new architectures to this, I would like something
like:

# Do not add new architectures to this list
depends on MIPS || RISCV || MICROBLAZE ...

Yes, it's kind of odd since the arch selects the option...

For sure, we don't want this option on arm64. For that, I can rely on
Will and Catalin rejecting a select, but on some new arch I can't.

> +
> +config BUILTIN_DTB_ALL
> + bool

Can this be limited to MIPS?

> +
> menuconfig OF
> bool "Device Tree and Open Firmware support"
> help
> diff --git a/scripts/Makefile.vmlinux b/scripts/Makefile.vmlinux
> index 5ceecbed31eb..4626b472da49 100644
> --- a/scripts/Makefile.vmlinux
> +++ b/scripts/Makefile.vmlinux
> @@ -17,6 +17,50 @@ quiet_cmd_cc_o_c = CC $@
> %.o: %.c FORCE
> $(call if_changed_dep,cc_o_c)
>
> +quiet_cmd_as_o_S = AS $@
> + cmd_as_o_S = $(CC) $(a_flags) -c -o $@ $<
> +
> +%.o: %.S FORCE
> + $(call if_changed_dep,as_o_S)
> +
> +# Built-in dtb
> +# ---------------------------------------------------------------------------
> +
> +quiet_cmd_wrap_dtbs = WRAP $@
> + cmd_wrap_dtbs = { \
> + echo '\#include <asm-generic/vmlinux.lds.h>'; \
> + echo '.section .dtb.init.rodata,"a"'; \
> + while read dtb; do \
> + symbase=__dtb_$$(basename -s .dtb "$${dtb}" | tr - _); \
> + echo '.balign STRUCT_ALIGNMENT'; \

Is this always guaranteed to be at least 8 bytes? That's the required
alignment for dtbs and assumed by libfdt.

> + echo ".global $${symbase}_begin"; \
> + echo "$${symbase}_begin:"; \
> + echo '.incbin "'$$dtb'" '; \
> + echo ".global $${symbase}_end"; \
> + echo "$${symbase}_end:"; \
> + done < $<; \
> + } > $@