kconfig: remove tristate choice support

I previously submitted a fix for a bug in the choice feature [1], where
I mentioned, "Another (much cleaner) approach would be to remove the
tristate choice support entirely".

There are more issues in the tristate choice feature. For example, you
can observe a couple of bugs in the following test code.

[Test Code]

    config MODULES
            def_bool y
            modules

    choice
            prompt "tristate choice"
            default A

    config A
            tristate "A"

    config B
            tristate "B"

    endchoice

Bug 1: the 'default' property is not correctly processed

'make alldefconfig' produces:

    CONFIG_MODULES=y
    # CONFIG_A is not set
    # CONFIG_B is not set

However, the correct output should be:

    CONFIG_MODULES=y
    CONFIG_A=y
    # CONFIG_B is not set

The unit test file, scripts/kconfig/tests/choice/alldef_expected_config,
is wrong as well.

Bug 2: choice members never get 'y' with randconfig

For the test code above, the following combinations are possible:

               A    B
        (1)    y    n
        (2)    n    y
        (3)    m    m
        (4)    m    n
        (5)    n    m
        (6)    n    n

'make randconfig' never produces (1) or (2).

These bugs are fixable, but a more critical problem is the lack of a
sensible syntax to specify the default for the tristate choice.
The default for the choice must be one of the choice members, which
cannot specify any of the patterns (3) through (6) above.

In addition, I have never seen it being used in a useful way.

The following commits removed unnecessary use of tristate choices:

 - df8df5e4bc37 ("usb: get rid of 'choice' for legacy gadget drivers")
 - bfb57ef0544a ("rapidio: remove choice for enumeration")

This commit removes the tristate choice support entirely, which allows
me to delete a lot of code, making further refactoring easier.

Note:
This includes the revert of commit fa64e5f6a35e ("kconfig/symbol.c:
handle choice_values that depend on 'm' symbols"). It was suspicious
because it did not address the root cause but introduced inconsistency
in visibility between choice members and other symbols.

[1]: https://lore.kernel.org/linux-kbuild/20240427104231.2728905-1-masahiroy@kernel.org/T/#m0a1bb6992581462ceca861b409bb33cb8fd7dbae

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

1 files changed, 3 insertions, 10 deletions

diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
index 1fb3f5e6193c..71b38a7670f3 100644
--- a/Documentation/kbuild/kconfig-language.rst
+++ b/Documentation/kbuild/kconfig-language.rst
@@ -409,16 +409,9 @@ choices::
 	"endchoice"
 
 This defines a choice group and accepts any of the above attributes as
-options. A choice can only be of type bool or tristate.  If no type is
-specified for a choice, its type will be determined by the type of
-the first choice element in the group or remain unknown if none of the
-choice elements have a type specified, as well.
-
-While a boolean choice only allows a single config entry to be
-selected, a tristate choice also allows any number of config entries
-to be set to 'm'. This can be used if multiple drivers for a single
-hardware exists and only a single driver can be compiled/loaded into
-the kernel, but all drivers can be compiled as modules.
+options.
+
+A choice only allows a single config entry to be selected.
 
 comment::