[17/30] Documentation: kconfig: document a new Kconfig macro language

Message ID 1523595999-27433-18-git-send-email-yamada.masahiro@socionext.com
State New
Headers show
Series
  • kconfig: move compiler capability tests to Kconfig
Related show

Commit Message

Masahiro Yamada April 13, 2018, 5:06 a.m.
Add a document for the macro language introduced to Kconfig.

The motivation of this work is to move the compiler option tests to
Kconfig from Makefile.  A number of kernel features require the
compiler support.  Enabling such features blindly in Kconfig ends up
with a lot of nasty build-time testing in Makefiles.  If a chosen
feature turns out unsupported by the compiler, what the build system
can do is either to disable it (silently!) or to forcibly break the
build, despite Kconfig has let the user to enable it.

This change was strongly prompted by Linus Torvalds.  You can find
his suggestions [1] [2] in ML.  The original idea was to add a new
'option', but I found generalized text expansion would make Kconfig
more powerful and lovely.  While polishing up the implementation, I
noticed sort of similarity between Make and Kconfig.  This might be
too immature to be called 'language', but anyway here it is.  All
ideas are from Make (you can even say it is addicted), so people
will easily understand how it works.

[1]: https://lkml.org/lkml/2016/12/9/577
[2]: https://lkml.org/lkml/2018/2/7/527

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

---

Changes in v3: None
Changes in v2: None

 Documentation/kbuild/kconfig-macro-language.txt | 179 ++++++++++++++++++++++++
 MAINTAINERS                                     |   2 +-
 2 files changed, 180 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/kbuild/kconfig-macro-language.txt

-- 
2.7.4

Comments

Randy Dunlap April 14, 2018, 11:33 p.m. | #1
On 04/12/18 22:06, Masahiro Yamada wrote:
> Add a document for the macro language introduced to Kconfig.

> 

> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

> ---

> 

> Changes in v3: None

> Changes in v2: None

> 

>  Documentation/kbuild/kconfig-macro-language.txt | 179 ++++++++++++++++++++++++

>  MAINTAINERS                                     |   2 +-

>  2 files changed, 180 insertions(+), 1 deletion(-)

>  create mode 100644 Documentation/kbuild/kconfig-macro-language.txt

> 

> diff --git a/Documentation/kbuild/kconfig-macro-language.txt b/Documentation/kbuild/kconfig-macro-language.txt

> new file mode 100644

> index 0000000..1f6281b

> --- /dev/null

> +++ b/Documentation/kbuild/kconfig-macro-language.txt

> @@ -0,0 +1,179 @@

> +Concept

> +-------

> +

> +The basic idea was inspired by Make. When we look at Make, we notice sort of

> +two languages in one. One language describes dependency graphs consisting of

> +targets and prerequisites. The other is a macro language for performing textual

> +substitution.

> +

> +There is clear distinction between the two language stages. For example, you

> +can write a makefile like follows:

> +

> +    APP := foo

> +    SRC := foo.c

> +    CC := gcc

> +

> +    $(APP): $(SRC)

> +            $(CC) -o $(APP) $(SRC)

> +

> +The macro language replaces the variable references with their expanded form,

> +and handles as if the source file were input like follows:

> +

> +    foo: foo.c

> +            gcc -o foo foo.c

> +

> +Then, Make analyzes the dependency graph and determines the targets to be

> +updated.

> +

> +The idea is quite similar in Kconfig - it is possible to describe a Kconfig

> +file like this:

> +

> +    CC := gcc

> +

> +    config CC_HAS_FOO

> +            def_bool $(shell $(srctree)/scripts/gcc-check-foo.sh $(CC))

> +

> +The macro language in Kconfig processes the source file into the following

> +intermediate:

> +

> +    config CC_HAS_FOO

> +            def_bool y

> +

> +Then, Kconfig moves onto the evaluation stage to resolve inter-symbol

> +dependency, which is explained in kconfig-language.txt.

> +

> +

> +Variables

> +---------

> +

> +Like in Make, a variable in Kconfig works as a macro variable.  A macro

> +variable is expanded "in place" to yield a text string that may then expanded


                                                               may then be expanded

> +further. To get the value of a variable, enclose the variable name in $( ).

> +As a special case, single-letter variable names can omit the parentheses and is


                                                                            and are

> +simply referenced like $X. Unlike Make, Kconfig does not support curly braces

> +as in ${CC}.

> +

> +There are two types of variables: simply expanded variables and recursively

> +expanded variables.

> +

> +A simply expanded variable is defined using the := assignment operator. Its

> +righthand side is expanded immediately upon reading the line from the Kconfig

> +file.

> +

> +A recursively expanded variable is defined using the = assignment operator.

> +Its righthand side is simply stored as the value of the variable without

> +expanding it in any way. Instead, the expansion is performed when the variable

> +is used.

> +

> +There is another type of assignment operator; += is used to append text to a

> +variable. The righthand side of += is expanded immediately if the lefthand

> +side was originally defined as a simple variable. Otherwise, its evaluation is

> +deferred.

> +

> +

> +Functions

> +---------

> +

> +Like Make, Kconfig supports both built-in and user-defined functions. A

> +function invocation looks much like a variable reference, but includes one or

> +more parameters separated by commas:

> +

> +  $(function-name arg1, arg2, arg3)

> +

> +Some functions are implemented as a built-in function. Currently, Kconfig

> +supports the following:

> +

> + - $(shell command)

> +

> +  The 'shell' function accepts a single argument that is expanded and passed

> +  to a subshell for execution. The standard output of the command is then read

> +  and returned as the value of the function. Every newline in the output is

> +  replaced with a space. Any trailing newlines are deleted. The standard error

> +  is not returned, nor is any program exit status.

> +

> + - $(warning text)

> +

> +  The 'warning' function prints its arguments to stderr. The output is prefixed

> +  with the name of the current Kconfig file, the current line number. It


                                          file and the current line number. It

> +  evaluates to an empty string.

> +

> + - $(info text)

> +

> +  The 'info' function is similar to 'warning' except that it sends its argument

> +  to stdout without any Kconfig name or line number.


Are current Kconfig file name and line number available so that someone can
construct their own $(info message) messages?

> +

> +A user-defined function is defined by using the = operator. The parameters are

> +referenced within the body definition with $1, $2, etc. (or $(1), $(2), etc.)

> +In fact, a user-defined function is internally treated as a recursive variable.


so the difference is just whether there are arguments?
A recursive variable does not have arguments and a function always has at least
one argument?


> +

> +A user-defined function is referenced in the same way as a built-in function:

> +

> +    $(my_func, arg0, arg1, arg2)


Syntax given above is:
+  $(function-name arg1, arg2, arg3)

with no comma after the function name.  Which is it?

> +

> +Note 1:

> +There is a slight difference in the whitespace handling of the function call

> +between Make and Kconfig. In Make, leading whitespaces are trimmed from the

> +first argument. So, $(info    FOO) is equivalent to $(info FOO). Kconfig keeps

> +any leading whitespaces except the one right after the function name, which

> +works as a separator. So, $(info    FOO) prints "   FOO" to the stdout.

> +

> +Note 2:

> +In Make, a user-defined function is referenced by using a built-in function,

> +'call', like this:

> +

> +    $(call my_func, arg0, arg1, arg2)

> +

> +However, Kconfig did not adopt this form just for the purpose of shortening the

> +syntax.

> +

> +

> +Caveats

> +-------

> +

> +A variable (or function) can not be expanded across tokens. So, you can not use


                            cannot                                     cannot

> +a variable as a shorthand for an expression that consists of multiple tokens.

> +The following works:

> +

> +    RANGE_MIN := 1

> +    RANGE_MAX := 3

> +

> +    config FOO

> +            int "foo"

> +            range $(RANGE_MIN) $(RANGE_MAX)

> +

> +But, the following does not work:

> +

> +    RANGES := 1 3

> +

> +    config FOO

> +            int "foo"

> +            range $(RANGES)

> +

> +A variable can not be expanded to any keyword in Kconfig.  The following does


              cannot

> +not work:

> +

> +    MY_TYPE := tristate

> +

> +    config FOO

> +            $(MY_TYPE) "foo"

> +            default y

> +

> +Obviously from the design, $(shell command) is expanded in the textual

> +substitution phase.  You can not pass symbols to the 'shell' function.


                            cannot

> +The following does not work as expected.

> +

> +    config ENDIAN_OPTION

> +            string

> +            default "-mbig-endian" if CPU_BIG_ENDIAN

> +            default "-mlittle-endian" if CPU_LITTLE_ENDIAN

> +

> +    config CC_HAS_ENDIAN_OPTION

> +            def_bool $(shell $(srctree)/scripts/gcc-check-option ENDIAN_OPTION)

> +

> +Instead, you can do like follows so that any function call is statically

> +expanded.

> +

> +    config CC_HAS_ENDIAN_OPTION

> +            bool


fix indentation?

> +	    default $(shell $(srctree)/scripts/gcc-check-option -mbig-endian) if CPU_BIG_ENDIAN

> +	    default $(shell $(srctree)/scripts/gcc-check-option -mlittle-endian) if CPU_LITTLE_ENDIAN



-- 
~Randy
Ulf Magnusson April 15, 2018, 8:08 a.m. | #2
On Fri, Apr 13, 2018 at 7:06 AM, Masahiro Yamada
<yamada.masahiro@socionext.com> wrote:
> Add a document for the macro language introduced to Kconfig.

>

> The motivation of this work is to move the compiler option tests to

> Kconfig from Makefile.  A number of kernel features require the

> compiler support.  Enabling such features blindly in Kconfig ends up

> with a lot of nasty build-time testing in Makefiles.  If a chosen

> feature turns out unsupported by the compiler, what the build system

> can do is either to disable it (silently!) or to forcibly break the

> build, despite Kconfig has let the user to enable it.

>

> This change was strongly prompted by Linus Torvalds.  You can find

> his suggestions [1] [2] in ML.  The original idea was to add a new

> 'option', but I found generalized text expansion would make Kconfig

> more powerful and lovely.  While polishing up the implementation, I

> noticed sort of similarity between Make and Kconfig.  This might be

> too immature to be called 'language', but anyway here it is.  All

> ideas are from Make (you can even say it is addicted), so people

> will easily understand how it works.

>

> [1]: https://lkml.org/lkml/2016/12/9/577

> [2]: https://lkml.org/lkml/2018/2/7/527

>

> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

> ---

>

> Changes in v3: None

> Changes in v2: None

>

>  Documentation/kbuild/kconfig-macro-language.txt | 179 ++++++++++++++++++++++++

>  MAINTAINERS                                     |   2 +-

>  2 files changed, 180 insertions(+), 1 deletion(-)

>  create mode 100644 Documentation/kbuild/kconfig-macro-language.txt

>

> diff --git a/Documentation/kbuild/kconfig-macro-language.txt b/Documentation/kbuild/kconfig-macro-language.txt

> new file mode 100644

> index 0000000..1f6281b

> --- /dev/null

> +++ b/Documentation/kbuild/kconfig-macro-language.txt

> @@ -0,0 +1,179 @@

> +Concept

> +-------

> +

> +The basic idea was inspired by Make. When we look at Make, we notice sort of

> +two languages in one. One language describes dependency graphs consisting of

> +targets and prerequisites. The other is a macro language for performing textual

> +substitution.

> +

> +There is clear distinction between the two language stages. For example, you

> +can write a makefile like follows:

> +

> +    APP := foo

> +    SRC := foo.c

> +    CC := gcc

> +

> +    $(APP): $(SRC)

> +            $(CC) -o $(APP) $(SRC)

> +

> +The macro language replaces the variable references with their expanded form,

> +and handles as if the source file were input like follows:

> +

> +    foo: foo.c

> +            gcc -o foo foo.c

> +

> +Then, Make analyzes the dependency graph and determines the targets to be

> +updated.

> +

> +The idea is quite similar in Kconfig - it is possible to describe a Kconfig

> +file like this:

> +

> +    CC := gcc

> +

> +    config CC_HAS_FOO

> +            def_bool $(shell $(srctree)/scripts/gcc-check-foo.sh $(CC))

> +

> +The macro language in Kconfig processes the source file into the following

> +intermediate:

> +

> +    config CC_HAS_FOO

> +            def_bool y

> +

> +Then, Kconfig moves onto the evaluation stage to resolve inter-symbol

> +dependency, which is explained in kconfig-language.txt.

> +

> +

> +Variables

> +---------

> +

> +Like in Make, a variable in Kconfig works as a macro variable.  A macro

> +variable is expanded "in place" to yield a text string that may then expanded

> +further. To get the value of a variable, enclose the variable name in $( ).

> +As a special case, single-letter variable names can omit the parentheses and is

> +simply referenced like $X. Unlike Make, Kconfig does not support curly braces

> +as in ${CC}.


Do we need single-letter variable names for anything? It looks like
we're deviating
a bit from Make behavior already.

I suspect they're just a side effect of Make having automatic variables like $@.
The Make manual discourages them otherwise:

"A dollar sign followed by a character other than a dollar sign,
open-parenthesis or
open-brace treats that single character as the variable name. Thus, you could
reference the variable x with `$x'. However, this practice is strongly
discouraged,
except in the case of the automatic variables (see section Automatic
Variables)."

Cheers,
Ulf
Masahiro Yamada April 17, 2018, 3:05 p.m. | #3
2018-04-15 8:33 GMT+09:00 Randy Dunlap <rdunlap@infradead.org>:
> On 04/12/18 22:06, Masahiro Yamada wrote:

>> Add a document for the macro language introduced to Kconfig.

>>

>> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

>> ---

>>

>> Changes in v3: None

>> Changes in v2: None

>>

>>  Documentation/kbuild/kconfig-macro-language.txt | 179 ++++++++++++++++++++++++

>>  MAINTAINERS                                     |   2 +-

>>  2 files changed, 180 insertions(+), 1 deletion(-)

>>  create mode 100644 Documentation/kbuild/kconfig-macro-language.txt

>>

>> diff --git a/Documentation/kbuild/kconfig-macro-language.txt b/Documentation/kbuild/kconfig-macro-language.txt

>> new file mode 100644

>> index 0000000..1f6281b

>> --- /dev/null

>> +++ b/Documentation/kbuild/kconfig-macro-language.txt

>> @@ -0,0 +1,179 @@

>> +Concept

>> +-------

>> +

>> +The basic idea was inspired by Make. When we look at Make, we notice sort of

>> +two languages in one. One language describes dependency graphs consisting of

>> +targets and prerequisites. The other is a macro language for performing textual

>> +substitution.

>> +

>> +There is clear distinction between the two language stages. For example, you

>> +can write a makefile like follows:

>> +

>> +    APP := foo

>> +    SRC := foo.c

>> +    CC := gcc

>> +

>> +    $(APP): $(SRC)

>> +            $(CC) -o $(APP) $(SRC)

>> +

>> +The macro language replaces the variable references with their expanded form,

>> +and handles as if the source file were input like follows:

>> +

>> +    foo: foo.c

>> +            gcc -o foo foo.c

>> +

>> +Then, Make analyzes the dependency graph and determines the targets to be

>> +updated.

>> +

>> +The idea is quite similar in Kconfig - it is possible to describe a Kconfig

>> +file like this:

>> +

>> +    CC := gcc

>> +

>> +    config CC_HAS_FOO

>> +            def_bool $(shell $(srctree)/scripts/gcc-check-foo.sh $(CC))

>> +

>> +The macro language in Kconfig processes the source file into the following

>> +intermediate:

>> +

>> +    config CC_HAS_FOO

>> +            def_bool y

>> +

>> +Then, Kconfig moves onto the evaluation stage to resolve inter-symbol

>> +dependency, which is explained in kconfig-language.txt.

>> +

>> +

>> +Variables

>> +---------

>> +

>> +Like in Make, a variable in Kconfig works as a macro variable.  A macro

>> +variable is expanded "in place" to yield a text string that may then expanded

>

>                                                                may then be expanded

>

>> +further. To get the value of a variable, enclose the variable name in $( ).

>> +As a special case, single-letter variable names can omit the parentheses and is

>

>                                                                             and are

>

>> +simply referenced like $X. Unlike Make, Kconfig does not support curly braces

>> +as in ${CC}.

>> +

>> +There are two types of variables: simply expanded variables and recursively

>> +expanded variables.

>> +

>> +A simply expanded variable is defined using the := assignment operator. Its

>> +righthand side is expanded immediately upon reading the line from the Kconfig

>> +file.

>> +

>> +A recursively expanded variable is defined using the = assignment operator.

>> +Its righthand side is simply stored as the value of the variable without

>> +expanding it in any way. Instead, the expansion is performed when the variable

>> +is used.

>> +

>> +There is another type of assignment operator; += is used to append text to a

>> +variable. The righthand side of += is expanded immediately if the lefthand

>> +side was originally defined as a simple variable. Otherwise, its evaluation is

>> +deferred.

>> +

>> +

>> +Functions

>> +---------

>> +

>> +Like Make, Kconfig supports both built-in and user-defined functions. A

>> +function invocation looks much like a variable reference, but includes one or

>> +more parameters separated by commas:

>> +

>> +  $(function-name arg1, arg2, arg3)

>> +

>> +Some functions are implemented as a built-in function. Currently, Kconfig

>> +supports the following:

>> +

>> + - $(shell command)

>> +

>> +  The 'shell' function accepts a single argument that is expanded and passed

>> +  to a subshell for execution. The standard output of the command is then read

>> +  and returned as the value of the function. Every newline in the output is

>> +  replaced with a space. Any trailing newlines are deleted. The standard error

>> +  is not returned, nor is any program exit status.

>> +

>> + - $(warning text)

>> +

>> +  The 'warning' function prints its arguments to stderr. The output is prefixed

>> +  with the name of the current Kconfig file, the current line number. It

>

>                                           file and the current line number. It

>

>> +  evaluates to an empty string.

>> +

>> + - $(info text)

>> +

>> +  The 'info' function is similar to 'warning' except that it sends its argument

>> +  to stdout without any Kconfig name or line number.

>

> Are current Kconfig file name and line number available so that someone can

> construct their own $(info message) messages?


Not available for now, but it would be easy to support such special variables
that expand to the current file name, and line number.
For example,  '$(file)' and '$(lineno)' ?


>> +

>> +A user-defined function is defined by using the = operator. The parameters are

>> +referenced within the body definition with $1, $2, etc. (or $(1), $(2), etc.)

>> +In fact, a user-defined function is internally treated as a recursive variable.

>

> so the difference is just whether there are arguments?

> A recursive variable does not have arguments and a function always has at least

> one argument?


At least in my implementation, yes, they are implemented in the same way.
The difference is "what we call it".

I just called it in an intuitive name.

If there is no argument, people generally assume it is a variable.
If it takes arguments, a function.
But, in fact, they are the same.



Probably, similar situation in GNU Make.

For example, you can use $(call ...) to expand a variable

Test Makefile:


A = 1

all:
       @echo $(A)
       @echo $(call A)


Both print the value of A.








>

>> +

>> +A user-defined function is referenced in the same way as a built-in function:

>> +

>> +    $(my_func, arg0, arg1, arg2)

>

> Syntax given above is:

> +  $(function-name arg1, arg2, arg3)

>

> with no comma after the function name.  Which is it?



Sorry, no comma after the function name.

But, I personally think a comma after the function name is more consistent.




>> +

>> +Note 1:

>> +There is a slight difference in the whitespace handling of the function call

>> +between Make and Kconfig. In Make, leading whitespaces are trimmed from the

>> +first argument. So, $(info    FOO) is equivalent to $(info FOO). Kconfig keeps

>> +any leading whitespaces except the one right after the function name, which

>> +works as a separator. So, $(info    FOO) prints "   FOO" to the stdout.

>> +

>> +Note 2:

>> +In Make, a user-defined function is referenced by using a built-in function,

>> +'call', like this:

>> +

>> +    $(call my_func, arg0, arg1, arg2)

>> +

>> +However, Kconfig did not adopt this form just for the purpose of shortening the

>> +syntax.

>> +

>> +

>> +Caveats

>> +-------

>> +

>> +A variable (or function) can not be expanded across tokens. So, you can not use

>

>                             cannot                                     cannot

>

>> +a variable as a shorthand for an expression that consists of multiple tokens.

>> +The following works:

>> +

>> +    RANGE_MIN := 1

>> +    RANGE_MAX := 3

>> +

>> +    config FOO

>> +            int "foo"

>> +            range $(RANGE_MIN) $(RANGE_MAX)

>> +

>> +But, the following does not work:

>> +

>> +    RANGES := 1 3

>> +

>> +    config FOO

>> +            int "foo"

>> +            range $(RANGES)

>> +

>> +A variable can not be expanded to any keyword in Kconfig.  The following does

>

>               cannot

>

>> +not work:

>> +

>> +    MY_TYPE := tristate

>> +

>> +    config FOO

>> +            $(MY_TYPE) "foo"

>> +            default y

>> +

>> +Obviously from the design, $(shell command) is expanded in the textual

>> +substitution phase.  You can not pass symbols to the 'shell' function.

>

>                             cannot

>

>> +The following does not work as expected.

>> +

>> +    config ENDIAN_OPTION

>> +            string

>> +            default "-mbig-endian" if CPU_BIG_ENDIAN

>> +            default "-mlittle-endian" if CPU_LITTLE_ENDIAN

>> +

>> +    config CC_HAS_ENDIAN_OPTION

>> +            def_bool $(shell $(srctree)/scripts/gcc-check-option ENDIAN_OPTION)

>> +

>> +Instead, you can do like follows so that any function call is statically

>> +expanded.

>> +

>> +    config CC_HAS_ENDIAN_OPTION

>> +            bool

>

> fix indentation?


Good catch.  I will fix them all.



>> +         default $(shell $(srctree)/scripts/gcc-check-option -mbig-endian) if CPU_BIG_ENDIAN

>> +         default $(shell $(srctree)/scripts/gcc-check-option -mlittle-endian) if CPU_LITTLE_ENDIAN

>

>

> --

> ~Randy

> --

> To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in

> the body of a message to majordomo@vger.kernel.org

> More majordomo info at  http://vger.kernel.org/majordomo-info.html




-- 
Best Regards
Masahiro Yamada
Masahiro Yamada April 17, 2018, 3:07 p.m. | #4
2018-04-15 17:08 GMT+09:00 Ulf Magnusson <ulfalizer@gmail.com>:
> On Fri, Apr 13, 2018 at 7:06 AM, Masahiro Yamada

> <yamada.masahiro@socionext.com> wrote:

>> Add a document for the macro language introduced to Kconfig.

>>

>> The motivation of this work is to move the compiler option tests to

>> Kconfig from Makefile.  A number of kernel features require the

>> compiler support.  Enabling such features blindly in Kconfig ends up

>> with a lot of nasty build-time testing in Makefiles.  If a chosen

>> feature turns out unsupported by the compiler, what the build system

>> can do is either to disable it (silently!) or to forcibly break the

>> build, despite Kconfig has let the user to enable it.

>>

>> This change was strongly prompted by Linus Torvalds.  You can find

>> his suggestions [1] [2] in ML.  The original idea was to add a new

>> 'option', but I found generalized text expansion would make Kconfig

>> more powerful and lovely.  While polishing up the implementation, I

>> noticed sort of similarity between Make and Kconfig.  This might be

>> too immature to be called 'language', but anyway here it is.  All

>> ideas are from Make (you can even say it is addicted), so people

>> will easily understand how it works.

>>

>> [1]: https://lkml.org/lkml/2016/12/9/577

>> [2]: https://lkml.org/lkml/2018/2/7/527

>>

>> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

>> ---

>>

>> Changes in v3: None

>> Changes in v2: None

>>

>>  Documentation/kbuild/kconfig-macro-language.txt | 179 ++++++++++++++++++++++++

>>  MAINTAINERS                                     |   2 +-

>>  2 files changed, 180 insertions(+), 1 deletion(-)

>>  create mode 100644 Documentation/kbuild/kconfig-macro-language.txt

>>

>> diff --git a/Documentation/kbuild/kconfig-macro-language.txt b/Documentation/kbuild/kconfig-macro-language.txt

>> new file mode 100644

>> index 0000000..1f6281b

>> --- /dev/null

>> +++ b/Documentation/kbuild/kconfig-macro-language.txt

>> @@ -0,0 +1,179 @@

>> +Concept

>> +-------

>> +

>> +The basic idea was inspired by Make. When we look at Make, we notice sort of

>> +two languages in one. One language describes dependency graphs consisting of

>> +targets and prerequisites. The other is a macro language for performing textual

>> +substitution.

>> +

>> +There is clear distinction between the two language stages. For example, you

>> +can write a makefile like follows:

>> +

>> +    APP := foo

>> +    SRC := foo.c

>> +    CC := gcc

>> +

>> +    $(APP): $(SRC)

>> +            $(CC) -o $(APP) $(SRC)

>> +

>> +The macro language replaces the variable references with their expanded form,

>> +and handles as if the source file were input like follows:

>> +

>> +    foo: foo.c

>> +            gcc -o foo foo.c

>> +

>> +Then, Make analyzes the dependency graph and determines the targets to be

>> +updated.

>> +

>> +The idea is quite similar in Kconfig - it is possible to describe a Kconfig

>> +file like this:

>> +

>> +    CC := gcc

>> +

>> +    config CC_HAS_FOO

>> +            def_bool $(shell $(srctree)/scripts/gcc-check-foo.sh $(CC))

>> +

>> +The macro language in Kconfig processes the source file into the following

>> +intermediate:

>> +

>> +    config CC_HAS_FOO

>> +            def_bool y

>> +

>> +Then, Kconfig moves onto the evaluation stage to resolve inter-symbol

>> +dependency, which is explained in kconfig-language.txt.

>> +

>> +

>> +Variables

>> +---------

>> +

>> +Like in Make, a variable in Kconfig works as a macro variable.  A macro

>> +variable is expanded "in place" to yield a text string that may then expanded

>> +further. To get the value of a variable, enclose the variable name in $( ).

>> +As a special case, single-letter variable names can omit the parentheses and is

>> +simply referenced like $X. Unlike Make, Kconfig does not support curly braces

>> +as in ${CC}.

>

> Do we need single-letter variable names for anything? It looks like

> we're deviating

> a bit from Make behavior already.

>

> I suspect they're just a side effect of Make having automatic variables like $@.

> The Make manual discourages them otherwise:

>

> "A dollar sign followed by a character other than a dollar sign,

> open-parenthesis or

> open-brace treats that single character as the variable name. Thus, you could

> reference the variable x with `$x'. However, this practice is strongly

> discouraged,

> except in the case of the automatic variables (see section Automatic

> Variables)."

>


OK.  We do not need two ways to do the same thing.

I will consider it
although supporting single-letter variable is not costly.



-- 
Best Regards
Masahiro Yamada
Ulf Magnusson April 18, 2018, 8:33 a.m. | #5
On Tue, Apr 17, 2018 at 5:07 PM, Masahiro Yamada
<yamada.masahiro@socionext.com> wrote:
> 2018-04-15 17:08 GMT+09:00 Ulf Magnusson <ulfalizer@gmail.com>:

>> On Fri, Apr 13, 2018 at 7:06 AM, Masahiro Yamada

>> <yamada.masahiro@socionext.com> wrote:

>>> Add a document for the macro language introduced to Kconfig.

>>>

>>> The motivation of this work is to move the compiler option tests to

>>> Kconfig from Makefile.  A number of kernel features require the

>>> compiler support.  Enabling such features blindly in Kconfig ends up

>>> with a lot of nasty build-time testing in Makefiles.  If a chosen

>>> feature turns out unsupported by the compiler, what the build system

>>> can do is either to disable it (silently!) or to forcibly break the

>>> build, despite Kconfig has let the user to enable it.

>>>

>>> This change was strongly prompted by Linus Torvalds.  You can find

>>> his suggestions [1] [2] in ML.  The original idea was to add a new

>>> 'option', but I found generalized text expansion would make Kconfig

>>> more powerful and lovely.  While polishing up the implementation, I

>>> noticed sort of similarity between Make and Kconfig.  This might be

>>> too immature to be called 'language', but anyway here it is.  All

>>> ideas are from Make (you can even say it is addicted), so people

>>> will easily understand how it works.

>>>

>>> [1]: https://lkml.org/lkml/2016/12/9/577

>>> [2]: https://lkml.org/lkml/2018/2/7/527

>>>

>>> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

>>> ---

>>>

>>> Changes in v3: None

>>> Changes in v2: None

>>>

>>>  Documentation/kbuild/kconfig-macro-language.txt | 179 ++++++++++++++++++++++++

>>>  MAINTAINERS                                     |   2 +-

>>>  2 files changed, 180 insertions(+), 1 deletion(-)

>>>  create mode 100644 Documentation/kbuild/kconfig-macro-language.txt

>>>

>>> diff --git a/Documentation/kbuild/kconfig-macro-language.txt b/Documentation/kbuild/kconfig-macro-language.txt

>>> new file mode 100644

>>> index 0000000..1f6281b

>>> --- /dev/null

>>> +++ b/Documentation/kbuild/kconfig-macro-language.txt

>>> @@ -0,0 +1,179 @@

>>> +Concept

>>> +-------

>>> +

>>> +The basic idea was inspired by Make. When we look at Make, we notice sort of

>>> +two languages in one. One language describes dependency graphs consisting of

>>> +targets and prerequisites. The other is a macro language for performing textual

>>> +substitution.

>>> +

>>> +There is clear distinction between the two language stages. For example, you

>>> +can write a makefile like follows:

>>> +

>>> +    APP := foo

>>> +    SRC := foo.c

>>> +    CC := gcc

>>> +

>>> +    $(APP): $(SRC)

>>> +            $(CC) -o $(APP) $(SRC)

>>> +

>>> +The macro language replaces the variable references with their expanded form,

>>> +and handles as if the source file were input like follows:

>>> +

>>> +    foo: foo.c

>>> +            gcc -o foo foo.c

>>> +

>>> +Then, Make analyzes the dependency graph and determines the targets to be

>>> +updated.

>>> +

>>> +The idea is quite similar in Kconfig - it is possible to describe a Kconfig

>>> +file like this:

>>> +

>>> +    CC := gcc

>>> +

>>> +    config CC_HAS_FOO

>>> +            def_bool $(shell $(srctree)/scripts/gcc-check-foo.sh $(CC))

>>> +

>>> +The macro language in Kconfig processes the source file into the following

>>> +intermediate:

>>> +

>>> +    config CC_HAS_FOO

>>> +            def_bool y

>>> +

>>> +Then, Kconfig moves onto the evaluation stage to resolve inter-symbol

>>> +dependency, which is explained in kconfig-language.txt.

>>> +

>>> +

>>> +Variables

>>> +---------

>>> +

>>> +Like in Make, a variable in Kconfig works as a macro variable.  A macro

>>> +variable is expanded "in place" to yield a text string that may then expanded

>>> +further. To get the value of a variable, enclose the variable name in $( ).

>>> +As a special case, single-letter variable names can omit the parentheses and is

>>> +simply referenced like $X. Unlike Make, Kconfig does not support curly braces

>>> +as in ${CC}.

>>

>> Do we need single-letter variable names for anything? It looks like

>> we're deviating

>> a bit from Make behavior already.

>>

>> I suspect they're just a side effect of Make having automatic variables like $@.

>> The Make manual discourages them otherwise:

>>

>> "A dollar sign followed by a character other than a dollar sign,

>> open-parenthesis or

>> open-brace treats that single character as the variable name. Thus, you could

>> reference the variable x with `$x'. However, this practice is strongly

>> discouraged,

>> except in the case of the automatic variables (see section Automatic

>> Variables)."

>>

>

> OK.  We do not need two ways to do the same thing.

>

> I will consider it

> although supporting single-letter variable is not costly.

>

>

>

> --

> Best Regards

> Masahiro Yamada


Can you think of any cases where dynamic generation of Kconfig symbol
names would be a good solution by the way?

Personally, I'd make the semantics obvious by requiring quotes, so you
could do e.g.

    bar := "bar value"
    baz := "baz value"
    foo := " $(bar)  $(baz) "

and get " bar value  baz value " in 'foo'.

That'd get rid of those $(empty) hacks and the like, and would make
things super easy to understand.

If expansion was then also limited to within "" (so that you always
produce values), things would get absolutely ridiculously simple, both
behavior- and implementation-wise. All the unput() shenanigans would
disappear (shaving off code), and everyone would be able to tell at a
glance how stuff works.

I know that you're trying to create something that will be immediately
familiar to people used to Make, and that allows things to be easily
moved over, but I wonder if it's worthwhile to carry over the parts that
people find the most confusing and subtle about Make. Simply and
recursively expanded variables and functions are great and make sense,
but the rest I'm not so sure about, especially in Kconfig context.

When I see $(foo) outside of quotes, I assume a textual substitution,
like in Make, because it sticks out like a sore thumb compared to the
rest of the Kconfig syntax (I also think "Perl scalar"). It's actually a
token substitution here. "$(foo)" makes it obvious.

Cheers,
Ulf

Patch

diff --git a/Documentation/kbuild/kconfig-macro-language.txt b/Documentation/kbuild/kconfig-macro-language.txt
new file mode 100644
index 0000000..1f6281b
--- /dev/null
+++ b/Documentation/kbuild/kconfig-macro-language.txt
@@ -0,0 +1,179 @@ 
+Concept
+-------
+
+The basic idea was inspired by Make. When we look at Make, we notice sort of
+two languages in one. One language describes dependency graphs consisting of
+targets and prerequisites. The other is a macro language for performing textual
+substitution.
+
+There is clear distinction between the two language stages. For example, you
+can write a makefile like follows:
+
+    APP := foo
+    SRC := foo.c
+    CC := gcc
+
+    $(APP): $(SRC)
+            $(CC) -o $(APP) $(SRC)
+
+The macro language replaces the variable references with their expanded form,
+and handles as if the source file were input like follows:
+
+    foo: foo.c
+            gcc -o foo foo.c
+
+Then, Make analyzes the dependency graph and determines the targets to be
+updated.
+
+The idea is quite similar in Kconfig - it is possible to describe a Kconfig
+file like this:
+
+    CC := gcc
+
+    config CC_HAS_FOO
+            def_bool $(shell $(srctree)/scripts/gcc-check-foo.sh $(CC))
+
+The macro language in Kconfig processes the source file into the following
+intermediate:
+
+    config CC_HAS_FOO
+            def_bool y
+
+Then, Kconfig moves onto the evaluation stage to resolve inter-symbol
+dependency, which is explained in kconfig-language.txt.
+
+
+Variables
+---------
+
+Like in Make, a variable in Kconfig works as a macro variable.  A macro
+variable is expanded "in place" to yield a text string that may then expanded
+further. To get the value of a variable, enclose the variable name in $( ).
+As a special case, single-letter variable names can omit the parentheses and is
+simply referenced like $X. Unlike Make, Kconfig does not support curly braces
+as in ${CC}.
+
+There are two types of variables: simply expanded variables and recursively
+expanded variables.
+
+A simply expanded variable is defined using the := assignment operator. Its
+righthand side is expanded immediately upon reading the line from the Kconfig
+file.
+
+A recursively expanded variable is defined using the = assignment operator.
+Its righthand side is simply stored as the value of the variable without
+expanding it in any way. Instead, the expansion is performed when the variable
+is used.
+
+There is another type of assignment operator; += is used to append text to a
+variable. The righthand side of += is expanded immediately if the lefthand
+side was originally defined as a simple variable. Otherwise, its evaluation is
+deferred.
+
+
+Functions
+---------
+
+Like Make, Kconfig supports both built-in and user-defined functions. A
+function invocation looks much like a variable reference, but includes one or
+more parameters separated by commas:
+
+  $(function-name arg1, arg2, arg3)
+
+Some functions are implemented as a built-in function. Currently, Kconfig
+supports the following:
+
+ - $(shell command)
+
+  The 'shell' function accepts a single argument that is expanded and passed
+  to a subshell for execution. The standard output of the command is then read
+  and returned as the value of the function. Every newline in the output is
+  replaced with a space. Any trailing newlines are deleted. The standard error
+  is not returned, nor is any program exit status.
+
+ - $(warning text)
+
+  The 'warning' function prints its arguments to stderr. The output is prefixed
+  with the name of the current Kconfig file, the current line number. It
+  evaluates to an empty string.
+
+ - $(info text)
+
+  The 'info' function is similar to 'warning' except that it sends its argument
+  to stdout without any Kconfig name or line number.
+
+A user-defined function is defined by using the = operator. The parameters are
+referenced within the body definition with $1, $2, etc. (or $(1), $(2), etc.)
+In fact, a user-defined function is internally treated as a recursive variable.
+
+A user-defined function is referenced in the same way as a built-in function:
+
+    $(my_func, arg0, arg1, arg2)
+
+Note 1:
+There is a slight difference in the whitespace handling of the function call
+between Make and Kconfig. In Make, leading whitespaces are trimmed from the
+first argument. So, $(info    FOO) is equivalent to $(info FOO). Kconfig keeps
+any leading whitespaces except the one right after the function name, which
+works as a separator. So, $(info    FOO) prints "   FOO" to the stdout.
+
+Note 2:
+In Make, a user-defined function is referenced by using a built-in function,
+'call', like this:
+
+    $(call my_func, arg0, arg1, arg2)
+
+However, Kconfig did not adopt this form just for the purpose of shortening the
+syntax.
+
+
+Caveats
+-------
+
+A variable (or function) can not be expanded across tokens. So, you can not use
+a variable as a shorthand for an expression that consists of multiple tokens.
+The following works:
+
+    RANGE_MIN := 1
+    RANGE_MAX := 3
+
+    config FOO
+            int "foo"
+            range $(RANGE_MIN) $(RANGE_MAX)
+
+But, the following does not work:
+
+    RANGES := 1 3
+
+    config FOO
+            int "foo"
+            range $(RANGES)
+
+A variable can not be expanded to any keyword in Kconfig.  The following does
+not work:
+
+    MY_TYPE := tristate
+
+    config FOO
+            $(MY_TYPE) "foo"
+            default y
+
+Obviously from the design, $(shell command) is expanded in the textual
+substitution phase.  You can not pass symbols to the 'shell' function.
+The following does not work as expected.
+
+    config ENDIAN_OPTION
+            string
+            default "-mbig-endian" if CPU_BIG_ENDIAN
+            default "-mlittle-endian" if CPU_LITTLE_ENDIAN
+
+    config CC_HAS_ENDIAN_OPTION
+            def_bool $(shell $(srctree)/scripts/gcc-check-option ENDIAN_OPTION)
+
+Instead, you can do like follows so that any function call is statically
+expanded.
+
+    config CC_HAS_ENDIAN_OPTION
+            bool
+	    default $(shell $(srctree)/scripts/gcc-check-option -mbig-endian) if CPU_BIG_ENDIAN
+	    default $(shell $(srctree)/scripts/gcc-check-option -mlittle-endian) if CPU_LITTLE_ENDIAN
diff --git a/MAINTAINERS b/MAINTAINERS
index b60179d..b9dab38 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -7622,7 +7622,7 @@  M:	Masahiro Yamada <yamada.masahiro@socionext.com>
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git kconfig
 L:	linux-kbuild@vger.kernel.org
 S:	Maintained
-F:	Documentation/kbuild/kconfig-language.txt
+F:	Documentation/kbuild/kconfig*
 F:	scripts/kconfig/
 
 KDUMP