Messages in this thread | | | From | Masahiro Yamada <> | Date | Fri, 15 Sep 2023 02:23:25 +0900 | Subject | Re: [PATCH] Documentation: kbuild: explain handling optional dependencies |
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On Thu, Sep 14, 2023 at 11:57 PM Arnd Bergmann <arnd@arndb.de> wrote: > > On Thu, Sep 14, 2023, at 15:42, Jani Nikula wrote: > > On Wed, 13 Sep 2023, Arnd Bergmann <arnd@kernel.org> wrote: > >> From: Arnd Bergmann <arnd@arndb.de> > >> > >> +Optional dependencies > >> +~~~~~~~~~~~~~~~~~~~~~ > >> + > >> +Some drivers are able to optionally use a feature from another module > >> +or build cleanly with that module disabled, but cause a link failure > >> +when trying to use that loadable module from a built-in driver. > >> + > >> +The most common way to express this optional dependency in Kconfig logic > >> +uses the slighly counterintuitive > >> + > >> + config FOO > >> + bool "Support for foo hardware" > >> + depends on BAR || !BAR > > > > depends on BAR || BAR=n > > > > seems to be an alternative that's about as common: > > > > $ git grep "depends on \([A-Z0-9_]\+\) || \!\1" | wc -l > > 109 > > $ git grep "depends on \([A-Z0-9_]\+\) || \1=n" | wc -l > > 107 > > > > Maybe worth mentioning both? > > I fear that would add more confusion than it avoids: > "!BAR" is actually different from "BAR=n", but > "BAR || !BAR" is the same as "BAR || BAR=n" here, and > trying to explain this in the documentation would either > make it incorrect or unhelpfully complicated.
The rules are already explained in line 231-278 of Documentation/kbuild/kconfig-language.rst
y, m, n are internally 2, 1, 0.
!A returns (2 - A). A=B returns 2 if the equation is true, 0 otherwise. A||B returns max(A,B)
Given those in my mind, this is simple math.
For each case of BAR=y, =m, =n,
BAR 2 1 0 !BAR 0 1 2 BAR=n 0 0 2 BAR||!BAR 2 1 2 BAR||BAR=n 2 1 2 BAR!=m||m 2 1 2
So, the last three are equivalent. They are equally complicated and confusing, though.
After all, what we are doing is to create this matrix:
| WIREGUARD | y m n ---------------------------- y | O O O IPV6 m | X O O n | O O O
It is unclear why WIREGUARD must be entirely disabled just because of the optional feature being modular.
My preference is to use IS_REACHABLE(CONFIG_IPV6) instead of IS_ENABLED(CONFIG_IPV6) under drivers/net/wireguard, then get rid of "depends on IPV6 || !IPV6)
If you want to make it clearer on the Kconfig level, perhaps the following is also possible.
config WIREGUARD tristate "WireGuard"
config WIREGUARD_IPV6 def_bool y depends on WIREGUARD depends on IPV6 >= WIREGUARD
config IPV6 tristate "IPV6"
-- Best Regards Masahiro Yamada
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