[PATCH] i915: fix shift warning

[Chris Wilson]

Quoting Arnd Bergmann (2021-01-02 11:23:20)
> On Wed, Dec 30, 2020 at 4:56 PM Chris Wilson <chris at chris-wilson.co.uk> wrote:
> >
> > Quoting Arnd Bergmann (2020-12-30 15:39:14)
> > > From: Arnd Bergmann <arnd at arndb.de>
> > >
> > > Randconfig builds on 32-bit machines show lots of warnings for
> > > the i915 driver for incorrect bit masks like:
> >
> > mask is a u8.
> >
> > VCS0 is 2, I915_MAX_VCS 4
> >
> > (u8 & GENMASK(5, 2)) >> 2
> 
> Ah right, I misread the warning then.
> 
> > > drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:2584:9: error: shift count >= width of type [-Werror,-Wshift-count-overflow]
> > >         return hweight64(VDBOX_MASK(&i915->gt));
> > >                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > include/asm-generic/bitops/const_hweight.h:29:49: note: expanded from macro 'hweight64'
> > >  #define hweight64(w) (__builtin_constant_p(w) ? __const_hweight64(w) : __arch_hweight64(w))
> >
> > So it's upset by hweight64() on the unsigned long?
> 
> I suspect what is going on is that clang once again warns because it performs
> more code checks before dead-code elimination than gcc does. The warning is
> for the __const_hweight64() case, which is not actually used here because the
> input is not a compile-time constant.
> 
> > So hweight_long?
> 
> That seems to work, I'll send a new version with that.
> 
> > Or use a cast, hweight8((intel_engine_mask_t)VDMASK())?
> >
> > static __always_inline int engine_count(intel_engine_mask_t mask)
> > {
> >         return sizeof(mask) == 1 ? hweight8(mask) :
> >                 sizeof(mask) == 2 ? hweight16(mask) :
> >                 sizeof(mask) == 4 ? hweight32(mask) :
> >                 hweight64(mask);
> > }
> 
> Fine with me as well. If you prefer that way, I'll let you handle that.

While we wait for a generic hweight(), lets use hweight_long() here.
-Chris