linux-kernel - Re: [PATCH] rpmb: use IS_REACHABLE instead of IS

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Message-Id: <cc808e76-4c2d-4a52-996c-2ff6d955e0a1@app.fastmail.com>
Date: Mon, 02 Sep 2024 10:32:39 +0000
From: "Arnd Bergmann" <arnd@...db.de>
To: "Jens Wiklander" <jens.wiklander@...aro.org>
Cc: linux-kernel@...r.kernel.org, "Ulf Hansson" <ulf.hansson@...aro.org>,
 "Linus Walleij" <linus.walleij@...aro.org>,
 "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
 "kernel test robot" <lkp@...el.com>
Subject: Re: [PATCH] rpmb: use IS_REACHABLE instead of IS_ENABLED

On Mon, Sep 2, 2024, at 09:54, Jens Wiklander wrote:
>
> Thanks for the suggestion.
>
> I tried adding the dependency above for MMC_BLOCK and OPTEE.
>
> Setting CONFIG_RPMB to "y" works as expected CONFIG_MMC_BLOCK and
> CONFIG_OPTEE can be configured as both "y" or "m".
>
> Setting CONFIG_RPMB to "m" will make CONFIG_MMC_BLOCK and CONFIG_OPTEE
> "m" too, even if they are set as "y" in the defconfig. So the module
> status seems to spread to the options with the dependency. For
> tristate options, it guarantees that the expected features are
> available even if it changes from built-in to a loadable module.

Correct.

> It will do nothing for bool options, but that's a hypothetical case
> for now.  Wouldn't spreading the built-in status to CONFIG_RPMB be
> better? So CONFIG_RPMB turns into "y" if an option configured as
> built-in depends on it. I don't know how to do that though, so that
> could be saved for when it's needed.

That would require using 'select' instead of 'depends on', but that has
other problems.

If a symbol that uses RPMB is 'bool', you need to do one of two things:

- if this is a driver that is always built-in, it would instead need
  'depends on RPMB!=m' or 'depends on RPMB=y || !RPMB', both have
  the same effect.

- If there is a bool symbol that turns on an optional feature in
  a modular driver, it would have to be something like this:

config MMC_USE_RPMB
     bool "Build MMC support for RPMB"
     depends on MMC && RPMB
     depends on RPMB=y || RPMB=MMC

> I'll send out patches for MMC_BLOCK and OPTEE options shortly if we're
> happy with the "depends on RPMB || !RPMB" approach.

Sounds good, thanks.

      Arnd