linux-kernel - Re: [Intel-gfx] [PATCH v2] drm/i915: Disable some extra clang warnings

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Date:   Mon, 30 Apr 2018 21:01:49 +0100
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Matthias Kaehlcke <mka@...omium.org>,
        "Jani Nikula" <jani.nikula@...ux.intel.com>,
        "Joonas Lahtinen" <joonas.lahtinen@...ux.intel.com>,
        "Rodrigo Vivi" <rodrigo.vivi@...el.com>,
        "David Airlie" <airlied@...ux.ie>
Cc:     "Guenter Roeck" <groeck@...omium.org>,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH v2] drm/i915: Disable some extra clang warnings

Quoting Matthias Kaehlcke (2018-04-30 20:31:19)
> On Mon, Mar 19, 2018 at 12:14:51PM -0700, Matthias Kaehlcke wrote:
> > Commit 39bf4de89ff7 ("drm/i915: Add -Wall -Wextra to our build, set
> > warnings to full") enabled extra warnings for i915 to spot possible
> > bugs in new code, and then disabled a subset of these warnings to keep
> > the current code building without warnings (with gcc). Enabling the
> > extra warnings also enabled some additional clang-only warnings, as a
> > result building i915 with clang currently is extremely noisy. For now
> > also disable the clang warnings sign-compare, sometimes-uninitialized,
> > unneeded-internal-declaration and initializer-overrides. If desired
> > they can be re-enabled after the code has been fixed.
> > 
> > Fixes: 39bf4de89ff7 ("drm/i915: Add -Wall -Wextra to our build, set
> > warnings to full")

Do we need to backport this for a non-default build with a non-default
compiler?

> > Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> > ---
> > Changes in v2:
> > - rebased on drm-tip
> > - added comment indicating that disabled warnings are clang warnings
> > 
> >  drivers/gpu/drm/i915/Makefile | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
> > index 4eee91a3a236..9717c037b582 100644
> > --- a/drivers/gpu/drm/i915/Makefile
> > +++ b/drivers/gpu/drm/i915/Makefile
> > @@ -18,6 +18,11 @@ subdir-ccflags-y += $(call cc-disable-warning, type-limits)
> >  subdir-ccflags-y += $(call cc-disable-warning, missing-field-initializers)
> >  subdir-ccflags-y += $(call cc-disable-warning, implicit-fallthrough)
> >  subdir-ccflags-y += $(call cc-disable-warning, unused-but-set-variable)
> > +# clang warnings
> > +subdir-ccflags-y += $(call cc-disable-warning, sign-compare)

Too much mixup in the code to be fixed overnight indeed.

> > +subdir-ccflags-y += $(call cc-disable-warning, sometimes-uninitialized)

Annoyingly it appears that clang has more false positives.

> > +subdir-ccflags-y += $(call cc-disable-warning, unneeded-internal-declaration)

Example? I don't recall this one, so don't know if we should just not
fix it rather than suppress. I've used ignored-attributes, perhaps that
was for the same cause.

> > +subdir-ccflags-y += $(call cc-disable-warning, initializer-overrides)

While we used per-file to restrict this one, I don't think we care that
much for precision with clang as well.
-Chris