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From:   Ulf Magnusson <ulfalizer@gmail.com>
Date:   Mon, 19 Feb 2018 16:18:16 +0100
Message-ID: <CAFkk2KT1krKuDkWt0eHcasfC61fTVQNf-1kqw7icLP9OYGYkXQ@mail.gmail.com>
Subject: Re: [PATCH 00/23] kconfig: move compiler capability tests to Kconfig
To:     Sam Ravnborg <sam@ravnborg.org>
Cc:     Masahiro Yamada <yamada.masahiro@socionext.com>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
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Hello,

On Sun, Feb 18, 2018 at 11:13 PM, Sam Ravnborg <sam@ravnborg.org> wrote:
> Hi Masahiro.
>
> On Sat, Feb 17, 2018 at 03:38:28AM +0900, Masahiro Yamada wrote:
>> I brushed up the implementation in this version.
>>
>> In the previous RFC, CC_HAS_ was described by using 'option shell=',
>> like this:
>>
>> config CC_HAS_STACKPROTECTOR
>>         bool
>>         option shell="$CC -Werror -fstack-protector -c -x c /dev/null"
>>
>> After I thought a bit more, the following syntax is more grammatical,
>> and flexible.
>>
>> config CC_HAS_STACKPROTECTOR
>>         bool
>>         default $(shell $CC -Werror -fstack-protector -c -x c /dev/null)
>
> Looks good - but maybe we should go one step further.
>
> So we in the syntax explicit handles:
> - shell commands
> - other commands, defined as strings
> - environment variables
> - config variables
>
> Each case is explicit - so the reader is not confused what is used when.
>
> $(shell foo)    - output of the shell command foo. Uses $SHELL as the shell.
>                   May include optional paramters.
>                   foo may be a config variable referenced using ${} or a config variable prefixed with $
>                 Example:
>
>                 config BUILD_DIR
>                         string
>                         default $(shell cd ${objtree}; pwd)
>
> $(call bar)     - output of the bar command that may take optional parameters.
>                   bar may be a text string, a config variable or an environment variable
>                    The definition of bar may reference the parameters using $(1), $(2)
>                    In this context a config variable needs to be prefixed with $
>
>                 Example:
>
>                 config reverse
>                         string
>                         default $(2) $(1)
>
>                 config NEW_ORDER
>                         string
>                         $(call $reverse, A, B)  # Will assign REVERSE the value "B A"
>
>
>                 Example2:
>
>                 config CC_OPTION
>                         string
>                         default $(shell ${srctree}/scripts/cc-option ${CC} $(1) $(2))
>
>                 config CC_OPTIMIZE
>                         string
>                         $(call $CC_OPTION, -Oz, -Os)
>
>
> ${FOO}          - environment variable
>
> The above is inspired by how make implement similar functionality.
>
> 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.

>
> The above proposal include the functionality of the macro stuff proposed in this patch-set.
> But with a simpler syntax and we keep all the other kconfig logic (depends on etc) - so
> users will not be limited in their creativity.
>
>> Current limitations:
>>
>> Dependency on outside scripts.
>> Inter-option dependency:
>> Functions are evaluated statically:
>
> Same limitations exists with the syntax suggested above.
>
>         Sam

Cheers,
Ulf