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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Wed, 25 Sep 2019 23:19:22 +0200
Message-ID: <CAKv+Gu9299nphaf88-SdF843w0ZbY0WD8MisAx+JMw5=GVVN9g@mail.gmail.com>
Subject: Re: [RFC PATCH 11/18] int128: move __uint128_t compiler test to Kconfig
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        David Miller <davem@davemloft.net>,
        Greg KH <gregkh@linuxfoundation.org>,
        "Jason A . Donenfeld" <Jason@zx2c4.com>,
        Samuel Neves <sneves@dei.uc.pt>,
        Dan Carpenter <dan.carpenter@oracle.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Eric Biggers <ebiggers@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>
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On Wed, 25 Sep 2019 at 23:01, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Wed, Sep 25, 2019 at 9:14 AM Ard Biesheuvel
> <ard.biesheuvel@linaro.org> wrote:
> >
> >  config ARCH_SUPPORTS_INT128
> >         bool
> > +       depends on !$(cc-option,-D__SIZEOF_INT128__=0)
>
> Hmm. Does this actually work?
>
> If that "depends on" now ends up being 'n', afaik the people who
> _enable_ it just do a
>
>        select ARCH_SUPPORTS_INT128
>
> and now you'll end up with the Kconfig erroring out with
>
>    WARNING: unmet direct dependencies detected for ARCH_SUPPORTS_INT128
>
> and then you end up with CONFIG_ARCH_SUPPORTS_INT128 anyway, instead
> of the behavior you _want_ to get, which is to not get that CONFIG
> defined at all.
>
> So I heartily agree with your intent, but I don't think that model
> works. I think you need to change the cases that currently do
>
>        select ARCH_SUPPORTS_INT128
>
> to instead have that cc-option test.
>
> And take all the above with a pinch of salt. Maybe what you are doing
> works, and I am just missing some piece of the puzzle. But I _think_
> it's broken, and you didn't test with a compiler that doesn't support
> that thing properly.
>

I think you may be right.

Instead, I'll add a separate CC_HAS_INT128 symbol with the
$(cc-option) test, and replace occurrences of

select ARCH_SUPPORTS_INT128

with

select ARCH_SUPPORTS_INT128 if CC_HAS_INT128

which is a slightly cleaner approach in any case.