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

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Date:   Mon, 30 Apr 2018 13:51:45 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Chris Wilson <chris@...is-wilson.co.uk>
Cc:     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>,
        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

On Mon, Apr 30, 2018 at 09:01:49PM +0100, Chris Wilson wrote:
> 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?

If it affected a LTS build I'd say yes, but since that isn't the case
I think it's not necessary.

> > > 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.

drivers/gpu/drm/i915/intel_guc_submission.c:183:13: warning: function
  'has_doorbell' is not needed and will not be emitted
  [-Wunneeded-internal-declaration]
static bool has_doorbell(struct intel_guc_client *client)

The function is only called within a GEM_BUG_ON macro, which does not
evaluate the expression unless CONFIG_DRM_I915_DEBUG_GEM is set.

Instead of disabling the warning it would probably be better to mark
has_doorbell as __maybe_unused.