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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 2 May 2021 14:08:31 -0700
Message-ID: <CAHk-=wjGJskk5EwnDCccs6DcLytE2yx76+P_W-n1-B5zq0M3KA@mail.gmail.com>
Subject: Re: [PATCH] Raise the minimum GCC version to 5.2
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     Joe Perches <joe@perches.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Arnd Bergmann <arnd@arndb.de>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Paul Mackerras <paulus@samba.org>,
        linux-riscv@lists.infradead.org,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Will Deacon <will@kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
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On Sun, May 2, 2021 at 1:38 PM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> The point is, you inconvenience users if you require a compiler version
> they do not already have.  Five years might be fine, but three years is
> not.

So this should be our main issue - not how old a compiler is, but how
our compiler version limitations end up possibly making it harder for
users to upgrade.

Of course, one issue there is whether said users would have upgraded
regardless - if you have a very old distribution, how likely are you
to upgrade the kernel at all?

But we do very much want to encourage people to upgrade their kernels,
even if they might be running otherwise fairly old user space. If for
no other reason than that it's good for our kernel coverage testing -
the more different distributions people test a new kernel with, the
better. And some of the less common architectures have their own
issues, with distros possibly not even supporting them any more (if
they ever did - considering all the odd ad-hoc cross-compiler builds
people have had..)

This is why "our clang support requires a very recent version of
clang" is not relevant - distributions won't have old versions of
clang anyway, and even if they do, such distributions will be
gcc-based, so "build the kernel with clang" for that situation is
perhaps an exercise for some intrepid person who is willing to do odd
and unusual things, and might as well build their own clang version
too.

So I really wish people didn't get hung about some "three years ago"
or similar. It's not relevant.

What is relevant is what version of gcc various distributions actually
have reasonably easily available, and how old and relevant the
distributions are. We did decide that (just as an example) RHEL 7 was
too old to worry about when we updated the gcc version requirement
last time.

Last year, Arnd and Kirill (maybe others were involved too) made a
list of distros and older gcc versions. But I don't think anybody
actually _maintains_ such a list. It would be perhaps interesting to
have some way to check what compiler versions are being offered by
different distros.

           Linus