The way to use Kconfig in U-Boot

Michal Simek michal.simek at
Fri May 29 13:03:24 CEST 2020

On 28. 05. 20 17:46, Simon Glass wrote:
> Hi Michal,
> On Wed, 27 May 2020 at 23:40, Michal Simek <michal.simek at> wrote:
>> 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.
> We could, but then 'chosen' becomes a misnomer. There is no need to
> try to reuse something else for this purpose, just because it is
> there. I think we should have our own 'config' node, or perhaps reuse
> the existing 'firmware' node with a 'u-boot' subnode. The smaller the
> overhead the better, for SPL.

I didn't find any information about firmware node in device tree spec
but coreboot is using this description and it went to kernel.

>From my point of view it can be whatever but we should really make sure
that it is the right way to go. It means if you think that config{} node
is the best solution let's just describe it and send it for review.
But at least today we have config node and there is no compatible
property there and none outside of u-boot has any idea what this is for.

I also found yesterday that misc/i2c-eeprom.c introduced
fixed-partitions which don't use reg property which is defined by Linux
dt spec but own offset/size properties (imx53-ppd-uboot.dtsi). That's
another example that we really need to start do things right.

Just a summary I am fine with any solution, node name, compatible string
but let's do it properly to be aligned with Linux DT. And the best do it
the same with Linux.

And I personally see different in connection to handling if we have
chosen {
	u-boot {

firmware {
	u-boot {


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