linux-kernel - Re: Kconfig rules (depends, imply)

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Message-ID: <d136df0c-34fc-d21c-1353-fa82deb62975@infradead.org>
Date:   Tue, 31 May 2022 13:25:52 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Muni Sekhar <munisekharrms@...il.com>,
        kernelnewbies <kernelnewbies@...nelnewbies.org>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: Kconfig rules (depends, imply)

Hi--

On 5/31/22 05:14, Muni Sekhar wrote:
> Hi all,
> 
> menuconfig SOUNDWIRE
>         tristate "SoundWire support"
>         depends on ACPI || OF
> 
> "depends on ACPI || OF" mean, SOUNDWIRE depends on both ACPI and OF modules?
> 

"||" is logical OR, so SOUNDWIRE depends on either ACPI or OF (or both
of them can be enabled).

> 
> config SOUNDWIRE_QCOM
>         tristate "Qualcomm SoundWire Master driver"
>         imply SLIMBUS
> 
> What does "imply SLIMBUS" mean?

See Documentation/kbuild/kconfig-language.rst:

- weak reverse dependencies: "imply" <symbol> ["if" <expr>]

  This is similar to "select" as it enforces a lower limit on another
  symbol except that the "implied" symbol's value may still be set to n
  from a direct dependency or with a visible prompt.

So (in your example) SLIMBUS is desired but not required.
That means that the code that is built for SOUNDWIRE_QCOM
probably needs to use IS_REACHABLE(CONFIG_SLIMBUS) to
see if that SLIMBUS code is available at build time.

HTH.
-- 
~Randy