Re: [PATCH 00/23] kconfig: move compiler capability tests to Kconfig

[Sam Ravnborg <sam@ravnborg.org>]

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.

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