The way to use Kconfig in U-Boot

Michal Simek monstr at
Mon May 25 11:16:17 CEST 2020

On 22. 05. 20 19:58, Tom Rini wrote:
> On Fri, May 22, 2020 at 11:38:58PM +0800, Bin Meng wrote:
>> Hi Simon,
>> On Fri, May 22, 2020 at 10:10 PM Simon Glass <sjg at> wrote:
>>> Hi Bin,
>>> On Fri, 22 May 2020 at 08:00, Tom Rini <trini at> wrote:
>>>> On Fri, May 22, 2020 at 09:53:07PM +0800, Bin Meng wrote:
>>>>> Hi Simon,
>>>>> On Fri, May 22, 2020 at 9:38 PM Simon Glass <sjg at> wrote:
>>>>>> Hi,
>>>>>> On Fri, 22 May 2020 at 06:32, Tom Rini <trini at> wrote:
>>>>>>> On Fri, May 22, 2020 at 06:20:39PM +0800, Bin Meng wrote:
>>>>>>>> Hi,
>>>>>>>> Kconfig is a flexible language and there are different ways to set a
>>>>>>>> value for a specific platform.
>>>>>>>> We can either:
>>>>>>>> - Use Kconfig overriding functionality
>>>>>>>> - Use Kconfig conditional set syntax like "default xxx if FOO"
>>>>>>>> Based on current Kconfig files hierarchy, in the root directory we
>>>>>>>> have the following come at the very beginning:
>>>>>>>> # Allow defaults in arch-specific code to override any given here
>>>>>>>> source "arch/Kconfig"
>>>>>>>> Based on this I thought our original design was to use the overriding.
>>>>>>>> But it seems not everyone is consistent on doing such. For example, we
>>>>>>>> have a bunch of unmaintainable (IMO) Kconfig options like this:
>>>>>>>> hex "Address on the MMC to load U-Boot from"
>>>>>>>> default 0x50 if ARCH_SUNXI
>>>>>>>> default 0x75 if ARCH_DAVINCI
>>>>>>>> default 0x8a if ARCH_MX6 || ARCH_MX7
>>>>>>>> default 0x100 if ARCH_UNIPHIER
>>>>>>>> default 0x140 if ARCH_MVEBU
>>>>>>>> default 0x200 if ARCH_SOCFPGA || ARCH_AT91
>>>>>>>> default 0x300 if ARCH_ZYNQ || ARCH_KEYSTONE || OMAP34XX || OMAP44XX || \
>>>>>>>>          OMAP54XX || AM33XX || AM43XX || ARCH_K3
>>>>>>>> default 0x4000 if ARCH_ROCKCHIP
>>>>>>>> help
>>>>>>>>   Address on the MMC to load U-Boot from, when the MMC is being used
>>>>>>>>   in raw mode. Units: MMC sectors (1 sector = 512 bytes).
>>>>>>>> The "default xxx if FOO" list is crazy!
>>>>>>>> I think we need to discuss and come up with a unified way of doing this.
>>>>>>>> I personally am in favor of the overriding mechanism, which is how
>>>>>>>> current x86 architecture Kconfig is organized. In the x86 arch
>>>>>>>> Kconfig, we have:
>>>>>>>> # board-specific options below
>>>>>>>> source "board/advantech/Kconfig"
>>>>>>>> ...
>>>>>>>> # platform-specific options below
>>>>>>>> source "arch/x86/cpu/apollolake/Kconfig"
>>>>>>>> ...
>>>>>>>> # architecture-specific options below
>>>>>>>> So that board behavior overrides platform/SoC behavior over
>>>>>>>> architecture behavior, and over the U-Boot common Kconfig options.
>>>>>>>> This to me is very clear.
>>>>>>> The problem I believe with overrides is that causes such churn to the
>>>>>>> defconfigs on re-sync as they're used.
>>>>>>> Personally I think this shows one of the problems with Kconfig as a
>>>>>>> language and the need for some tool to take these values from something
>>>>>>> else and spit out defines.  Perhaps now that u-boot, is a defined DT
>>>>>>> prefix we could something-something our way through storing these in
>>>>>>> -u-boot.dtsi files and use dtoc to get a header we use everywhere ala
>>>>>>> kconfig.h?
>>>>>> I think for the case Bin mentions yes we could do that. Certainly not
>>>>>> nice to put this sort of thing in Kconfig. Perhaps a DT 'config/' node
>>>>>> a bit like the existing 'chosen', that dtoc emits?
>>>>> Yes, for the example I gave, we can certainly put such information
>>>>> into DT config/ node. But this example may not be a good one because
>>>>> we cannot just put every Kconfig option into DT.
>>> Bin can you suggest an example with lots of variability, that is not
>>> suitable for DT?
>> How about this one?
>> config BUILD_TARGET
>>         string "Build target special images"
>>         default "u-boot-with-spl.sfp" if TARGET_SOCFPGA_ARRIA10
>>         default "u-boot-with-spl.sfp" if TARGET_SOCFPGA_GEN5
>>         default "u-boot-spl.kwb" if ARCH_MVEBU && SPL
>>         default "u-boot-elf.srec" if RCAR_GEN3
>>         default "u-boot.itb" if SPL_LOAD_FIT && (ARCH_ROCKCHIP || \
>>                                 ARCH_SUNXI || RISCV || ARCH_ZYNQMP)
>>         default "u-boot.kwb" if ARCH_KIRKWOOD
>>         default "u-boot-with-spl.bin" if ARCH_AT91 && SPL_NAND_SUPPORT
>>         default "u-boot-with-spl.imx" if ARCH_MX6 && SPL
>>         help
>>           Some SoCs need special image types (e.g. U-Boot binary
>>           with a special header) as build targets. By defining
>>           CONFIG_BUILD_TARGET in the SoC / board header, this
>>           special image will be automatically built upon calling
>>           make / buildman.
>> It might not be a good example neither :)
>> But what I really wanted to get some agreement among custodians about
>> the style. Which style do we want to go?
> I've been encouraging the "put defaults together" style as that I hope
> makes the problem scope for "now make handling this cleaner" easier to
> see.  When something is scattered in N files it's easier to miss what
> everyone looks like.  As you note, that too might fit well enough into a
> /config node or similar type thing instead.  It didn't really belong in
> include/config/${board}.h (or more often one of the common include files
> there) and doesn't quite fit in with Kconfig well either.

Just keep in your mind that DT config {} node is something what u-boot
is using but never been reviewed by DT guys. We should normally use
chosen node instead (for example u-boot,spl-boot-order is IMHO one good

I am trying to keep u-boot and linux dts in sync. I know we have
-uboot.dtsi where a lot of them are just adding u-boot,dm-XX; or binman
but if we want to use configuration via DT more then we should do it
properly in a compatible way.
(Just a side node we should move u-boot,dm-pre-reloc = <&.. &.. &..> to
chosen node and iterate over instead of putting property to every node.

Take a look at
and follow up

I am definitely fine with moving stuff to DT but we should do it in
similar way how Linux is doing it today. Which means use the whole
infrastructure for checking these DTs.
I would also suggest to clean up current u-boot specific DT properties,
reviewed them, document them, fix them if necessary and then start to
add new one on the top.


Michal Simek, Ing. (M.Eng), OpenPGP -> KeyID: FE3D1F91
w: p: +42-0-721842854
Maintainer of Linux kernel - Xilinx Microblaze
Maintainer of Linux kernel - Xilinx Zynq ARM and ZynqMP ARM64 SoCs
U-Boot custodian - Xilinx Microblaze/Zynq/ZynqMP/Versal SoCs

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