Re: [PATCH] drm/i915: Avoid BIT(max) - 1 and use GENMASK(max, 0)

[Joonas Lahtinen <joonas.lahtinen@linux.intel.com>]

On ti, 2017-02-07 at 14:03 +0000, Chris Wilson wrote:
> On Tue, Feb 07, 2017 at 03:51:41PM +0200, Joonas Lahtinen wrote:
> > 
> > "BIT(max) - 1" will overflow when max = 32, and GCC will complain.
> > We already have GENMASK for generating the mask, use it!
> > 
> > Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > Cc: Chris Wilson <chris@chris-wilson.co.uk>
> > ---
> >  drivers/gpu/drm/i915/intel_device_info.c | 2 +-
> >  drivers/gpu/drm/i915/intel_fbdev.c       | 2 +-
> >  drivers/gpu/drm/i915/intel_runtime_pm.c  | 2 +-
> >  3 files changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_device_info.c b/drivers/gpu/drm/i915/intel_device_info.c
> > index fcf8181..1c06fbb 100644
> > --- a/drivers/gpu/drm/i915/intel_device_info.c
> > +++ b/drivers/gpu/drm/i915/intel_device_info.c
> > @@ -234,7 +234,7 @@ static void broadwell_sseu_info_init(struct drm_i915_private *dev_priv)
> > > >  	 * The subslice disable field is global, i.e. it applies
> > > >  	 * to each of the enabled slices.
> > > >  	 */
> > > > -	sseu->subslice_mask = BIT(ss_max) - 1;
> > > > +	sseu->subslice_mask = GENMASK(ss_max, 0);
> 
> Too simple!
> 
> 	BUILD_BUG_ON(BIT(ss_max)-1 != GENMASK(ss_max, 0));
> 
> Needs to be GENMASK(ss_max - 1, 0); the high bit is inclusive.
> 
> Otherwise, the list matches up with a quick grep for BIT*(x)-1.

Whoop, seem to have nucleared my kernel config. I think better check if
the same warning is triggered too, with GENMASK :)

Regards, Joonas
-- 
Joonas Lahtinen
Open Source Technology Center
Intel Corporation
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