The way to use Kconfig in U-Boot
Simon Glass
sjg at chromium.org
Fri May 22 18:48:17 CEST 2020
Hi,
On Fri, 22 May 2020 at 10:07, Masahiro Yamada <masahiroy at kernel.org> wrote:
>
> On Sat, May 23, 2020 at 12:39 AM Bin Meng <bmeng.cn at gmail.com> 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 think Tom can make the decision here.
My 2p is that having the override mechanism is better than not (and
avoids horrible things like the above).
But for the above we really should be using DT in some way. That is
where our board/arch config (as opposed to feature selection) should
live.
>
>
>
>
>
> I do not see this kind of Kconfig problem in Linux.
> Not in Barebox, either.
>
> So, this is a problem of U-Boot, not of Kconfig.
>
> U-Boot conventionally prefixed all sort of parameters
> with CONFIG_, and put them in header files.
>
> Since Kconfig was introduced,
> we have converted them to Kconfig verbatim
> by using moveconfig tool.
>
>
> We always discuss "how to do it in Kconfig?"
> while they should not be CONFIG options in the first place.
>
>
>
> As for CONFIG_BUILD_TARGET, it is not a CONFIG option.
> It should be done in Makefile somehow.
>
>
> As for CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR,
> we can make it into a platform hook because
> we have a board file for each platform anyway.
>
>
> A board file for arch/arm/mach-sunxi/:
>
> unsigned int mmcsd_raw_mode_u_boot_sector(void)
> {
> return 0x50;
> }
>
>
> A board file for arch/arm/mach-davinci/:
>
> unsigned int mmcsd_raw_mode_u_boot_sector(void)
> {
> return 0x75;
> }
>
>
> Default:
>
> unsigned int __weak mmcsd_raw_mode_u_boot_sector(void)
> {
> return 0;
>
> }
>
Please no, this is a disaster IMO.
We have a device tree which is exactly set up to handle the hierarchy
of config in U-Boot. Weak functions and ad-hoc APIs are just going to
get out of hand. We should define these things in DT, add a binding
file and use that.
Regards,
Simon
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