The way to use Kconfig in U-Boot

Michal Simek michal.simek at
Thu May 28 07:40:04 CEST 2020

Hi Simon,

On 28. 05. 20 5:08, Simon Glass wrote:
> Hi Michal,
> On Tue, 26 May 2020 at 00:35, Michal Simek <michal.simek at> wrote:
>> On 25. 05. 20 17:29, Tom Rini wrote:
>>> On Mon, May 25, 2020 at 08:57:27AM -0600, Simon Glass wrote:
>>>> Hi Michal,
>>>> On Mon, 25 May 2020 at 03:16, Michal Simek <monstr at> wrote:
>>>>> 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:
>>>>>>>>>>>>> config SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR
>>>>>>>>>>>>> hex "Address on the MMC to load U-Boot from"
>>>>>>>>>>>>> depends on SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR
>>>>>>>>>>>>> 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
>>>>> example).
>>>> The chosen node is for passing things to linux, not (generally) for
>>>> use within U-Boot.
>>> Well, it would be worth double checking what DT-the-spec says for
>>> /chosen and also what other projects are using.  Maybe a patch to the DT
>>> spec would be in order, maybe not.
>> "The /chosen node does not represent a real device in the system but
>> describes parameters chosen or specified by the
>> system firmware at run time. It shall be a child of the root node."
>> We can extend it to also mentioned that it can be parameters for
>> firmware itself.
>> Also there is a table where we can list u-boot, are optional parameters
>> used for U-Boot bootloader.
> I think it would be better to just have a new node, perhaps something
> U-Boot-specific, or perhaps something intended for use by firmware and
> not passed to the kernel, where these properties would just cause
> confusion.

Xen is adding stuff to chosen node for quite a long time. Not sure if
this was reviewed but I can't see the reason why we can't do it too.

It means style like
chosen {
	xxx {
		compatible = "u-boot,XXX";

we can spec that xxx,XXX and as the part of DT fixups which u-boot does
before passing control(for example align memory node, mac address) to
Linux we can remove the entire node from DT.


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