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    SubjectRe: [PATCH 00/23] kconfig: move compiler capability tests to Kconfig
    On Wed, Feb 21, 2018 at 8:38 AM, Masahiro Yamada
    <yamada.masahiro@socionext.com> wrote:
    > 2018-02-20 0:18 GMT+09:00 Ulf Magnusson <ulfalizer@gmail.com>:
    >
    >>>
    >>> I'm not happy that we in one context can reference CONFIG variables
    >>> directly, but inside the $(call ...) and $(shell ...) needs the $ prefix.
    >>> But I could not come up with something un-ambigious where this could be avoided.
    >>
    >> I think we should be careful about allowing references to config
    >> symbols. It mixes up the parsing and evaluation phases, since $() is
    >> expanded during parsing (which I consider a feature and think is
    >> needed to retain sanity).
    >>
    >> Patch 06/23 removes the last existing instance of symbol references in
    >> strings by getting rid of 'option env'. That's an improvement to me.
    >> We shouldn't add it back.
    >
    >
    > This is really important design decision,
    > so I'd like to hear a little more from experts.
    >
    >
    > For example, x86 allows users to choose sub-arch, either 'i386' or 'x86_64'.
    >
    > https://github.com/torvalds/linux/blob/v4.16-rc2/arch/x86/Kconfig#L4
    >
    >
    >
    > If the user toggles CONFIG_64BIT,
    > the bi-arch compiler will work in a slightly different mode
    > (at least, back-end parts)
    >
    > So, my question is, is there a case,
    >
    > $(cc-option, -m32 -foo) is y, but
    > $(cc-option, -m64 -foo) is n ?
    > (or vice versa)
    >
    >
    > If the answer is yes, $(cc-option -foo) would have to be re-calculated
    > every time CONFIG_64BIT is toggled.
    >
    > This is what I'd like to avoid, though.

    The -m32/-m64 trick (and -mbig-endian/-mlittle-endian on other architectures
    as well as a couple of other flags) only works if the compiler is configured to
    support it. In other cases (e.g. big-endian xtensa), the kernel always
    detects what the compiler does and silently configures itself to match
    using Makefile logic.

    On x86, compilers are usually built as bi-arch, but you can build one that
    only allows one of them.

    I can see two reasonable ways out:

    - we don't use $(cc-option -foo) in a case like this, and instead require the
    user to have a matching toolchain.
    - we could make the 32/64 selection on x86 a 'choice' statement where
    each option depends on both the ARCH= variable and the
    $(cc-option, -m32)/ $(cc-option, -m64) output.

    Arnd

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