Re: [PATCH] [v2] Kbuild: move to -std=gnu11

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On 5/18/22 00:46, Arnd Bergmann wrote:
On Mon, May 16, 2022 at 3:19 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
On 5/16/22 06:31, Greg KH wrote:
On Mon, May 16, 2022 at 06:10:23AM -0700, Guenter Roeck wrote:
On Mon, Feb 28, 2022 at 11:27:43AM +0100, Arnd Bergmann wrote:
From: Arnd Bergmann <arnd@xxxxxxxx>

During a patch discussion, Linus brought up the option of changing
the C standard version from gnu89 to gnu99, which allows using variable
declaration inside of a for() loop. While the C99, C11 and later standards
introduce many other features, most of these are already available in
gnu89 as GNU extensions as well.

The downside is that backporting affected patches to older kernel branches
now fails with error messages such as

mm/kfence/core.c: In function ‘kfence_init_pool’:
mm/kfence/core.c:595:2: error: ‘for’ loop initial declarations are only allowed in C99 or C11 mode

Just something to keep in mind when writing patches.

I just ran across this very issue on this commit.  It's an easy fixup
for 5.17.y to make this work, so I did that in my tree.  If this gets to
be too much, we might need to reconsider adding c11 to older stable

I think I'll do just that for ChromeOS; I don't want to have to deal
with the backports, and we are using recent compilers anyway.

I think it would be better not to have the --std=gnu11 change in the older
stable kernels by default, as this has introduced build warnings and other
smaller issues, as well as raising the minimum compiler version.

The users that are stuck on older kernels for some reason tend to
overlap with those on older compilers. One example here is Android,
which used to ship with a gcc-4.9 build as the only non-clang toolchain,
and was using this for building their kernels. If someone wants to
pull in stable updates into an older Android, this would fail with
-std=gnu11. Others may be in the same situation.

Changing some of the 5.x stable branches to -std=gnu11 is probably
less of a problem, but I would not know where to draw the line exactly.
Maybe check with the Android team to see what the newest kernel is
that they expect to be built with the old gcc-4.9.

I don't think they still build anything with gcc. We (ChromeOS) only
need it for test builds of chromeos-4.4 (sigh), and that will hopefully
be gone in a couple of months.

We already enabled -std=gnu11 in chromeos-5.10 and chromeos-5.15.
We'll see if that is possible with chromeos-5.4 as well.
We won't bother with older kernel branches, but those should not
get many patches from upstream outside stable release merges,
so it is less of a problem.

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