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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Tue, 4 May 2021 14:07:58 +0200
Message-ID: <CAK8P3a0Ono1gMwt9tKUZtV_jX8iFxLp9rWiWUuE8jXQNhZ+yWA@mail.gmail.com>
Subject: Re: [PATCH] Raise the minimum GCC version to 5.2
To:     Arnd Bergmann <arnd@arndb.de>,
        Matthew Wilcox <willy@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Segher Boessenkool <segher@kernel.crashing.org>,
        Joe Perches <joe@perches.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Albert Ou <aou@eecs.berkeley.edu>,
        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>,
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        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Will Deacon <will@kernel.org>,
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On Tue, May 4, 2021 at 7:31 AM Alexander Dahl <ada@thorsis.com> wrote:
> Am Mon, May 03, 2021 at 11:25:21AM +0200 schrieb Arnd Bergmann:
> > On Mon, May 3, 2021 at 9:35 AM Alexander Dahl <ada@thorsis.com> wrote:
> > >
> > > Desktops and servers are all nice, however I just want to make you
> > > aware, there are embedded users forced to stick to older cross
> > > toolchains for different reasons as well, e.g. in industrial
> > > environment. :-)
> > >
> > > This is no show stopper for us, I just wanted to let you be aware.
> >
> > Can you be more specific about what scenarios you are thinking of,
> > what the motivations are for using an old compiler with a new kernel
> > on embedded systems, and what you think a realistic maximum
> > time would be between compiler updates?
>
> One reason might be certification. For certain industrial applications
> like support for complex field bus protocols, you need to get your
> devices tested by an external partner running extensive test suites.
> This is time consuming and expensive.
>
> Changing the toolchain of your system then, would be a massive change
> which would require recertification, while you could argue just
> updating a single component like the kernel and building everything
> again, does not require the whole testing process again.
>
> Thin ice, I know.

As Christophe said, I don't think this is a valid example. I agree that
if rebuilding everything with a new toolchain requires certification, you
shouldn't rebuild everything.

If replacing the kernel does not require recertification for your
specific system, that is fine, but that does not mean the new kernel
should be built with an outdated toolchain. If the certification
allows replacing linux-3.18 with linux-5.10 but doesn't allow building
the kernel with a different toolchain compared to the rest, then
the point of the certification is rather questionable.

Do you know specific certifications that would require you to
do this?

> One problem we actually ran into in BSPs like that (we build with
> ptxdist, however build system doesn't matter here, it could as well
> have been buildroot etc.) was things* failing to build with newer
> compilers, things we could not or did not want to fix, so staying with
> an older toolchain was the obvious choice.
>
> *Things as in bootloaders for an armv5 platform.
...
>
> What we actually did: building recent userspace and kernel with older
> toolchains, because bootloader.

It sounds like you are trying to make an argument in favour of
deprecating old toolchains *earlier* in new kernels ;-)

If we simply made it impossible to have users build kernels with
the same old toolchain that is needed for building the old bootloader
or the old user space, it sounds like more people would do the
right thing and build the updated kernels with a better tested
toolchain, or update their bootloader as well. The only downside
is that some users would choose to remain on the older kernels,
so it shouldn't be too aggressive either.

         Arnd