Re: [RFC] [PATCH V2] Adding DTB to architecture independent vmlinux

[Dirk Brandewie <dirk.brandewie-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Hi David,

Thanks for looking at the patch.

On 10/27/2010 05:57 PM, David VomLehn wrote:
> On Wed, Oct 27, 2010 at 05:30:27PM -0700, Dirk Brandewie wrote:
>> Here is V2 of the patch.
> ...
>> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
>> index 3068e1e..0f5eb1d 100644
>> --- a/arch/x86/kernel/Makefile
>> +++ b/arch/x86/kernel/Makefile
>> @@ -120,6 +120,21 @@ obj-$(CONFIG_X86_CHECK_BIOS_CORRUPTION) += check.o
>>   obj-$(CONFIG_SWIOTLB)			+= pci-swiotlb.o
>>   obj-$(CONFIG_X86_OF)			+= prom.o
>>
>> +ifeq ($(CONFIG_KERNEL_DTB),y)
>> +ifneq ($(PLATFORM_DTB),)
>> +obj-y += $(PLATFORM_DTB).dtb.o
>> +endif
>> +endif
>> +
>> +dtstree	:= $(srctree)/arch/x86/boot/dts
>> +
>> +$(obj)/%.dtb: $(dtstree)/%.dts
>> +	$(call if_changed,dtc)
>> +
>> +$(obj)/%.dtb.S: $(obj)/%.dtb
>> +	@echo '.section .dtb,"a"'>  $@
>> +	@echo '.incbin "$<" '>>  $@
>> +
>
> I've been playing a bit with the patch, and would suggest something
> like the following for the second target:
>
> 	$(obj)/%.dtb.S: $(obj)/%.dtb
> 		@echo '#include<asm/page.h>'>$@
> 		@echo '.balign PAGE_SIZE'>>  $@
> 		@echo '.section .kernel:dtb,"a"'>>  $@
> 		@echo '.global __$(*F)_dtb'>>  $@
> 		@echo '__$(*F)_dtb:'>>  $@
> 		@echo '.incbin "$<" '>>  $@
> 		@echo '.balign PAGE_SIZE'>>  $@
>
> Advantages:
> 1.	Each blob gets a name that can be used to refer to it. This
> 	allows multiple blobs to be built into a kernel, each with
> 	its own name.  The name of each blob is taken from the file
> 	name, so a source
> 	file abc.dts would produce a blob referred to as __abc_dtb.
> 2.	Blobs are aligned on a page boundary and extend to the nearest
> 	page boundary. Any blobs you don't care about can then easily
> 	be completely freed.
>

I like this idea in general but I have to admit I don't completely understand
all the implications of adding multiple blobs.  I can see a use case where a
vendor would want to make a single static image that supported multiple
platforms.  I am not sure how common this would be.  One issue I see with
linking multiple DTB's into the image is now the platform init code is back to
trying to detect which hardware it is running on to select the correct blob to
get its configuration from.

As part of the complete DTB story on x86 I am adding the ability to pass the DTB
in from the boot loader.  I think this mechanism would be the more common case
for supporting multiple platforms with a single vmlinux image.


> You might then use:
>
> 	obj-y += $(addprefix .dtb.o,$(PLATFORM_DTB))
>
> to add blob names, but I'm not completely confident this is the way to go.
I am still trying to settle on the right way to specify the DTB to be linked in
that won't generate a lot of hate from the build/distribution people where they
are supporting multiple platforms with/without the need of the DTB.

--Dirk