[PATCH 1/3] dts/upstream: write fdtfile path to /chosen/fdtfile

[Casey Connolly]

On 6/11/25 14:13, Mark Kettenis wrote:
>> From: Casey Connolly <casey.connolly at linaro.org>
>> Date: Tue, 10 Jun 2025 18:23:33 +0200
> 
> Hi Casey,
> 
>> Since we're using the upstream file structure, we know at build time
>> the path to each DTB that we build. These same paths are used relative
>> to some (maybe version specific) dtb directory on the boot partition
>> or ESP.
>>
>> By recording this path at build time and embedding it into the
>> devicetree, we can set the fdtfile environment variable with 100%
>> certainty that it is correct.
>>
>> Devicetree file names are already treated as stable kernel API upstream,
>> since they are used by OS loaders extensively.
> 
> But when something similar was suggested some time ago, the Linux
> kernel folks explicitly stated that the file names are ~*not*
> considered to be ABI.

They don't /want/ it to be ABI, but that ship has long since sailed as 
the filename is how the dtb is loaded. If this wasn't meant to be the 
case then "make dtbs_install" should mangle filenames somehow.

> 
> So I think this proposal should be discussed in a broader context,
> probably on the devicetree-spec at vger.kernel.org mailing list.

One could argue that since this property is added by the U-Boot build 
system during compilation, and only expected to be consumed by U-Boot 
itself, is could fall outside the purview here. But yeah... probably

.

> 
>> Embedding the path into the devicetree itself just simplifies the
>> process and allows U-Boot or the OS loader to select a devicetree of a
>> different version for the specific board that U-Boot is running on.
>>
>> Signed-off-by: Casey Connolly <casey.connolly at linaro.org>
>> ---
>>   scripts/Makefile.lib | 15 +++++++++++++++
>>   1 file changed, 15 insertions(+)
>>
>> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
>> index e89a4a51b74ddccfd9f0a6463cdeded12eaa5235..1f95bda0597c14d2ee785eb8058d607a6c819a56 100644
>> --- a/scripts/Makefile.lib
>> +++ b/scripts/Makefile.lib
>> @@ -360,16 +360,31 @@ $(obj)/%.dtbo.S: $(obj)/%.dtbo
>>   ifeq ($(CONFIG_OF_LIBFDT_OVERLAY),y)
>>   DTC_FLAGS += -@
>>   endif
>>   
>> +ifeq ($(CONFIG_ARM64),y)
>> +dt_dir := dts/upstream/src/arm64
>> +else
>> +dt_dir := dts/upstream/src/$(ARCH)
>> +endif
>> +
>> +ifeq ($(CONFIG_OF_UPSTREAM),y)
>> +# Embed the upstream path to the DTB into it as /chosen/fdtfile. This property is used
>> +# at runtime to set the fdtfile variable
>> +dt_extra_snippet = / { chosen { fdtfile = "$(subst $(dt_dir)/,,$@)"; }; };
>> +else
>> +dt_extra_snippet =
>> +endif
>> +
>>   quiet_cmd_dtc = DTC     $@
>>   # Modified for U-Boot
>>   # Bring in any U-Boot-specific include at the end of the file
>>   # And finally any custom .dtsi fragments specified with CONFIG_DEVICE_TREE_INCLUDES
>>   cmd_dtc = mkdir -p $(dir ${dtc-tmp}) ; \
>>   	(cat $< > $(pre-tmp)); \
>>   	$(foreach f,$(subst $(quote),,$(dtsi_include_list) $(CONFIG_DEVICE_TREE_INCLUDES)), \
>>   	  echo '$(pound)include "$(f)"' >> $(pre-tmp);) \
>> +	[[ -z "$(dt_extra_snippet)" ]] || echo '$(dt_extra_snippet)' >> $(pre-tmp); \
>>   	$(HOSTCC) -E $(dtc_cpp_flags) -I$(obj) -x assembler-with-cpp -o $(dtc-tmp) $(pre-tmp) ; \
>>   	$(DTC) -O dtb -o $@ -b 0 \
>>   		-i $(dir $<) -i $(u_boot_dtsi_loc) $(DTC_FLAGS) \
>>   		-d $(depfile).dtc.tmp $(dtc-tmp) || \
>>
>> -- 
>> 2.49.0
>>
>>

-- 
Casey (she/they)