Date:   Mon, 5 Oct 2020 10:31:57 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Rohit Maheshwari <rohitm@...lsio.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH net-next v2 1/2] Makefile.extrawarn: Add symbol for W=1
 warnings for today

On Thu, Oct 1, 2020 at 6:44 PM Andrew Lunn <andrew@...n.ch> wrote:
>
> On Thu, Oct 01, 2020 at 04:09:43PM -0700, Nick Desaulniers wrote:
> > On Wed, Sep 30, 2020 at 6:12 PM Andrew Lunn <andrew@...n.ch> wrote:
> > >
> > > There is a movement to try to make more and more of /drivers W=1
> > > clean. But it will only stay clean if new warnings are quickly
> > > detected and fixed, ideally by the developer adding the new code.
> > >
> > > To allow subdirectories to sign up to being W=1 clean for a given
> > > definition of W=1, export the current set of additional compile flags
> > > using the symbol KBUILD_CFLAGS_W1_20200930. Subdirectory Makefiles can
> > > then use:
> > >
> > > subdir-ccflags-y := $(KBUILD_CFLAGS_W1_20200930)
> > >
> > > To indicate they want to W=1 warnings as defined on 20200930.
> > >
> > > Additional warnings can be added to the W=1 definition. This will not
> > > affect KBUILD_CFLAGS_W1_20200930 and hence no additional warnings will
> > > start appearing unless W=1 is actually added to the command
> > > line. Developers can then take their time to fix any new W=1 warnings,
> > > and then update to the latest KBUILD_CFLAGS_W1_<DATESTAMP> symbol.
> >
> > I'm not a fan of this approach.  Are DATESTAMP configs just going to
> > keep being added? Is it going to complicate isolating the issue from a
> > randconfig build?  If we want more things to build warning-free at
> > W=1, then why don't we start moving warnings from W=1 into the
> > default, until this is no W=1 left?  That way we're cutting down on
> > the kernel's configuration combinatorial explosion, rather than adding
> > to it?
>
> Hi Nick
>
> I don't see randconfig being an issue. driver/net/ethernet would
> always be build W=1, by some stable definition of W=1. randconfig
> would not enable or disable additional warnings. It to make it clear,
> KBUILD_CFLAGS_W1_20200930 is not a Kconfig option you can select. It
> is a Makefile constant, a list of warnings which define what W=1 means
> on that specific day. See patch 1/2.
>
> I see a few issues with moving individual warnings from W=1 to the
> default:
>
> One of the comments for v1 of this patchset is that we cannot
> introduce new warnings in the build. The complete tree needs to clean
> of a particularly warning, before it can be added to the default list.
> But that is not how people are cleaning up code, nor how the
> infrastructure is designed. Those doing the cleanup are not take the
> first from the list, -Wextra and cleanup up the whole tree for that
> one warnings. They are rather enabling W=1 on a subdirectory, and
> cleanup up all warnings on that subdirectory. So using this approach,
> in order to move a warning from W=1 to the default, we are going to
> have to get the entire tree W=1 clean, and move them all the warnings
> are once.

Sorry, to be more specific about my concern; I like the idea of
exporting the W=* flags, then selectively applying them via
subdir-ccflags-y.  I don't like the idea of supporting W=1 as defined
at a precise point in time via multiple date specific symbols.  If
someone adds something to W=1, then they should need to ensure subdirs
build warning-free, so I don't think you need to "snapshot" W=1 based
on what it looked like on 20200930.

>
> People generally don't care about the tree as a whole. They care about
> their own corner. The idea of fixing one warning thought the whole
> tree is 'slicing and dicing' the kernel the wrong way. As we have seen
> with the recent work with W=1, the more natural way to slice/dice the
> kernel is by subdirectories.

I'm not sure I agree with this paragraph. ^  If a warning is not
enabled by default implicitly, then someone would need to clean the
tree to turn it on.  It's very messy to apply it on a child directory,
then try to work up.  We've done multiple tree wide warning cleanups
and it's not too bad.

>
> I do however understand the fear that we end up with lots of
> KBUILD_CFLAGS_W1_<DATESTAMP> constants. So i looked at the git history
> of script/Makefile.extrawarn. These are historically the symbols we
> would have, if we started this idea 1/1/2018:
>
> KBUILD_CFLAGS_W1_20200326    # CLANG only change
> KBUILD_CFLAGS_W1_20190907
> KBUILD_CFLAGS_W1_20190617    # CLANG only change
> KBUILD_CFLAGS_W1_20190614    # CLANG only change
> KBUILD_CFLAGS_W1_20190509
> KBUILD_CFLAGS_W1_20180919
> KBUILD_CFLAGS_W1_20180111
>
> So not too many.

It's a useful visualization.  I still would prefer W=1 to get enabled
by default if all of the CI systems have flushed out the existing
warnings.  That way we have one less combination of things to test;
not more.
--
Thanks,
~Nick Desaulniers

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