The way to use Kconfig in U-Boot

Tom Rini trini at konsulko.com
Mon May 25 17:29:34 CEST 2020 

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 monstr.eu> 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 chromium.org> wrote:
> > >>>
> > >>> Hi Bin,
> > >>>
> > >>> On Fri, 22 May 2020 at 08:00, Tom Rini <trini at konsulko.com> 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 chromium.org> wrote:
> > >>>>>>
> > >>>>>> Hi,
> > >>>>>>
> > >>>>>> On Fri, 22 May 2020 at 06:32, Tom Rini <trini at konsulko.com> 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.

> > 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.
> 
> I think this is a worthy effort. I hope you can have some success. If
> you do I am very happy to help. But if the answer is that U-Boot has
> to only use Linux bindings, then it isn't going to work so well. I'd
> like to see more acceptance of the constraints of a boot loader with
> the bindings. The discussion over the 'clock' property in a debug UART
> is an example of that.

Yes, the "clocks" thing is a good example of why we need to be a bit
cautious.  We've had a few cases of having to rework our code to deal
with DT changes / corrections.

> > (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.
> 
> (side node...it has a u-boot, prefix, so surely it is fine from a
> binding perspective...this is the sort of Linux-centric approach that
> drove me nuts when I tried to upstream things)
> 
> This has been discussed before and it was the other option when I
> originally implemented it. I was worried about confusion efficiency at
> the time. I think it is possible to implement it reasonably
> efficiently by having a list of node offsets to take notice of, for
> the prior-to=relocation. For SPL/TPL it has already been dealt with by
> fdtgrep.
> 
> But for confusion I am not so sure. The way it works at the moment we
> can put the tags in a .dtsi file that is shared among multiple boards.
> We really don't have a way to merge multiple properties into one,
> which is what I think would be needed here.

I also remain unconvinced that it would be acceptable for the primary
authoritative DTS files for a platform to contain u-boot,dm-pre-reloc,
etc properties given other pushback over the years about changes that
result in growth of the DTB binary and in turn memory consumption in
Linux, etc.  I think it's good if we can inject what we need via
-u-boot.dtsi files and keep everything else in-sync as-is.

> > Take a look at
> > https://lists.denx.de/pipermail/u-boot/2020-April/405155.html
> > and follow up
> > https://lists.denx.de/pipermail/u-boot/2020-May/412102.html
> >
> 
> Looks good, particularly the last one. Did your patch get applied?

Yes, I believe that is the patch Rob applied.

> > 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.
> 
> Are we missing something? I certainly get plenty of warnings when I
> build stuff :-) Perhaps we should start by syncing over the DTs to get
> rid of all the warnings?

It would be good for various SoCs to re-sync with Linux and silence,
hopefully, a bunch of warnings.  I just worry about things breaking when
we resync so they need to be tested.

-- 
Tom
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