kconfig: undocumented syntax

[erwango]

Describe the bug In the tree we can find the following undocumented Kconfig syntax:

config FOO
	default BAR

as a short for

config FOO
	default y if BAR

with FOO being a bool

For instance:

config USB_CDC_ACM
	default SERIAL

This syntax is unclear as when reading it can suggest that FOO is selecting BAR option. Also it is undocumented.

Expected behavior This syntax should be forbidden and existing occurrences cleaned up

Impact This makes kconfig.defconfig files reading quite confusing.

Logs and console output One example: https://github.com/zephyrproject-rtos/zephyr/blob/c211cb347e0af0a4931e0e7af3d93577bcc7af8f/boards/nordic/nrf52840dongle/Kconfig.defconfig#L30-L48

[jfischer-no]

Expected behavior This syntax should be forbidden and existing occurrences cleaned up

Certainly not, this is a common shorthand, see also Linux kernel tree. https://docs.zephyrproject.org/3.6.0/build/kconfig/tips.html#common-kconfig-shorthands

[tejlmand]

In the tree we can find the following undocumented Kconfig syntax: ... as a short for

no, actually that's not what this means, the construct:

config FOO
	default BAR

actually means default the value of FOO to the value of BAR.

Now with bools this really doesn't change anything in practice, because at the end of the day, whether you do: default y if BAR, meaning assign y if BAR is also y. or you say default BAR, assign the value of BAR to this setting, results in same value

but if starting to play with combinations of strings and bools, then the difference becomes clearer, for example:

config FOO_STRING
        string
        default "foo"

config BAR_BOOL
        bool
        default FOO_STRING # <-- illegal, because a string cannot be assigned to bool.

config BAR_BOOL
        bool
        default y if FOO_STRING = "foo"

[tejlmand]

Not a bug.

[erwango]

~~Thanks for the feedback. Then it would need to be documented in the, otherwise fairly complete, Kconfig tips and tricks doc page.~~

~~Does it sound reasonnable ?~~

Edit 1: ~~Ok, Thanks @jfischer-no for the pointer which I managed to miss somehow. Closing the point.~~

Edit 2: An example is used, but the shorthand is not explicitly documented in https://docs.zephyrproject.org/latest/build/kconfig/tips.html#common-kconfig-shorthands. Reopenning.

[tejlmand]

An example is used, but the shorthand is not explicitly documented in https://docs.zephyrproject.org/latest/build/kconfig/tips.html#common-kconfig-shorthands.

yep, seems that the docs could be improved, but that should be under setting of values and not a short-hand. A short-hand is where the short syntax actually means the same as the long syntax.

And that is not the case with default BAR, as that is assigning a value.

[tejlmand]

Addressed here: https://github.com/zephyrproject-rtos/zephyr/pull/73270