Re: [PATCH] x86/config: Make the x86 defconfigs a bit more usable

[Masahiro Yamada <masahiroy@kernel.org>]

On Sun, Sep 4, 2022 at 6:48 PM Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> > > Unfortunately, even without the ARCH=i386 'make savedefconfig' doesn't
> > > seem to be doing the right thing & is dropping the '# CONFIG_64BIT is
> > > not set' line:
> >
> >
> > Oh, really?
> >
> > Without ARCH=i386, it works correctly for me.
> >
> >
> >
> > masahiro@zoe:~/ref/linux$ make i386_defconfig savedefconfig
> > #
> > # No change to .config
> > #
> > masahiro@zoe:~/ref/linux$ grep CONFIG_64BIT defconfig
> > # CONFIG_64BIT is not set
>
> Yeah, so why do these two seemingly identical commands produce two
> different .config's:
>
>   $ make ARCH=i386 defconfig
>   *** Default configuration is based on 'i386_defconfig'
>
>   $ make i386_defconfig
>
> ?





As Nathan explained in this thread,
the prompt of CONFIG_64BIT is hidden
when ARCH=i386.


config 64BIT
        bool "64-bit kernel" if "$(ARCH)" = "x86"
        default "$(ARCH)" != "i386"
        help
          Say yes to build a 64-bit kernel - formerly known as x86_64
          Say no to build a 32-bit kernel - formerly known as i386




[1] "make ARCH=i386 defconfig"

ARCH is i386, so the Kconfig snippet becomes as follows:


  config 64BIT
          bool
          default n




[2] "make i386_defconfig"


ARCH is x86, so the Kconfig snippet becomes as follows:


  config 64BIT
          bool "64-bit kernel"
          default y







"# ... is not set" is output when
the prompt is visible and the user did not enable it.












> Basically the canonical way to generate a defconfig is to provide an ARCH,
> then use the common 'defconfig' target:



I do not know what you mean by "canonical", but
there are only 3 architectures (x86, sparc, sh) that
allow two ways for configuration.

This is somewhat related to a historical reason
(commit 8c4d9b145ba3 mentioned this a little).

After arch/i386/ and arch/x86_64/ were merged into arch/x86,
ARCH=i386 and ARCH=x86_64 became a subset of ARCH=x86.






>
>    make ARCH=i386      defconfig
>    make ARCH=x86       defconfig
>    make ARCH=arm       defconfig
>    make ARCH=arm64     defconfig
>    make ARCH=powerpc   defconfig
>    make ARCH=s390      defconfig
>    make ARCH=sparc     defconfig
>    make ARCH=sparc64   defconfig
>    ...
>
> This is what my build test scripts are using to quick-test cross-arch
> builds. It's a straightforward way to generate defconfigs without knowing
> the details of the myriads of random defconfig targets some architectures
> have, such as:
>
>   kepler:~/tip> ls arch/arm/configs/
>   am200epdkit_defconfig     gemini_defconfig         multi_v5_defconfig    s5pv210_defconfig
>   aspeed_g4_defconfig       h3600_defconfig          multi_v7_defconfig    sama5_defconfig
>   aspeed_g5_defconfig       h5000_defconfig          mv78xx0_defconfig     sama7_defconfig
>   assabet_defconfig         hackkit_defconfig        mvebu_v5_defconfig    shannon_defconfig
>   at91_dt_defconfig         hisi_defconfig           mvebu_v7_defconfig    shmobile_defconfig
>   axm55xx_defconfig         imxrt_defconfig          mxs_defconfig         simpad_defconfig
>   badge4_defconfig          imx_v4_v5_defconfig      neponset_defconfig    socfpga_defconfig
>   bcm2835_defconfig         imx_v6_v7_defconfig      netwinder_defconfig   sp7021_defconfig
>   cerfcube_defconfig        integrator_defconfig     nhk8815_defconfig     spear13xx_defconfig
>   clps711x_defconfig        iop32x_defconfig         omap1_defconfig       spear3xx_defconfig
>   cm_x300_defconfig         ixp4xx_defconfig         omap2plus_defconfig   spear6xx_defconfig
>   cns3420vb_defconfig       jornada720_defconfig     orion5x_defconfig     spitz_defconfig
>   colibri_pxa270_defconfig  keystone_defconfig       oxnas_v6_defconfig    stm32_defconfig
>   colibri_pxa300_defconfig  lart_defconfig           palmz72_defconfig     sunxi_defconfig
>   collie_defconfig          lpc18xx_defconfig        pcm027_defconfig      tct_hammer_defconfig
>   corgi_defconfig           lpc32xx_defconfig        pleb_defconfig        tegra_defconfig
>   davinci_all_defconfig     lpd270_defconfig         pxa168_defconfig      trizeps4_defconfig
>   dove_defconfig            lubbock_defconfig        pxa255-idp_defconfig  u8500_defconfig
>   dram_0x00000000.config    magician_defconfig       pxa3xx_defconfig      versatile_defconfig
>   dram_0xc0000000.config    mainstone_defconfig      pxa910_defconfig      vexpress_defconfig
>   dram_0xd0000000.config    milbeaut_m10v_defconfig  pxa_defconfig         vf610m4_defconfig
>   ep93xx_defconfig          mini2440_defconfig       qcom_defconfig        viper_defconfig
>   eseries_pxa_defconfig     mmp2_defconfig           realview_defconfig    vt8500_v6_v7_defconfig
>   exynos_defconfig          moxart_defconfig         rpc_defconfig         xcep_defconfig
>   ezx_defconfig             mps2_defconfig           s3c2410_defconfig     zeus_defconfig
>   footbridge_defconfig      multi_v4t_defconfig      s3c6400_defconfig
>
> But this doesn't seem to be working reliably on i386:
>
>   kepler:~/tip> make i386_defconfig
>   #
>   # configuration written to .config
>   #
>   kepler:~/tip> /bin/cp .config .config.i386_defconfig.1
>
>   kepler:~/tip> make ARCH=i386 defconfig
>   *** Default configuration is based on 'i386_defconfig'
>   #
>   # configuration written to .config
>   #
>   kepler:~/tip> /bin/cp .config .config.i386_defconfig.2
>
>   kepler:~/tip> diff -up .config.i386_defconfig.1 .config.i386_defconfig.2
>   --- .config.i386_defconfig.1  2022-09-04 11:34:48.253202205 +0200
>   +++ .config.i386_defconfig.2  2022-09-04 11:35:04.268758331 +0200
>   @@ -1,6 +1,6 @@
>    #
>    # Automatically generated file; DO NOT EDIT.
>   -# Linux/x86 6.0.0-rc3 Kernel Configuration
>   +# Linux/i386 6.0.0-rc3 Kernel Configuration
>    #
>    CONFIG_CC_VERSION_TEXT="gcc (Ubuntu 12-20220319-1ubuntu1) 12.0.1 20220319 (experimental) [master r12-7719-g8ca61ad148f]"
>    CONFIG_CC_IS_GCC=y
>   @@ -261,7 +261,6 @@ CONFIG_PROFILING=y
>    CONFIG_TRACEPOINTS=y
>    # end of General setup
>
>   -# CONFIG_64BIT is not set
>    CONFIG_X86_32=y
>    CONFIG_X86=y
>    CONFIG_INSTRUCTION_DECODER=y
>
> Note how in the ARCH=i386 case the build system claims to use i386_defconfig:
>
>   *** Default configuration is based on 'i386_defconfig'
>
> But that doesn't seem to be identical with when we specify i386_defconfig
> directly as a target...
>
> What am I missing?
>
> Thanks,
>
>         Ingo





-- 
Best Regards
Masahiro Yamada