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

Masahiro Yamada
2024-09-06T01:57:20Z

On Thu, Sep 5, 2024 at 11:17 PM Rob Herring <robh@kernel.org> wrote:
>
> 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.

I learned CONFIG_ARM_APPENDED_DTB today.

> 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?

I am fine with hard-coded "depends on"
if this feature is discouraged.

> > +
> > 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.

I think so.

include/asm-generic/vmlinux.lds.h defines it as 32.

We can loosen the alignment to 8, but for safety,
I just copied this from scripts/Makefile.lib
because 32-byte alignment is what we do now.

--
Best Regards
Masahiro Yamada