[PATCH] i915: fix shift warning

[PATCH] i915: fix shift warning

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

Randconfig builds on 32-bit machines show lots of warnings for
the i915 driver for incorrect bit masks like:

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))

Since this is a 64-bit mask, use GENMASK_ULL instead of GENMASK.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/gpu/drm/i915/i915_drv.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 0a3ee4f9dc0a..ca32fa0d6a57 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1624,7 +1624,7 @@ tgl_revids_get(struct drm_i915_private *dev_priv)
 	unsigned int first__ = (first);					\
 	unsigned int count__ = (count);					\
 	((gt)->info.engine_mask &						\
-	 GENMASK(first__ + count__ - 1, first__)) >> first__;		\
+	 GENMASK_ULL(first__ + count__ - 1, first__)) >> first__;		\
 })
 #define VDBOX_MASK(gt) \
 	ENGINE_INSTANCES_MASK(gt, VCS0, I915_MAX_VCS)
-- 
2.29.2

Re: [PATCH] i915: fix shift warning

[Arnd Bergmann]

On Wed, Dec 30, 2020 at 4:56 PM Chris Wilson <chris@chris-wilson.co.uk> wrote:
>
> Quoting Arnd Bergmann (2020-12-30 15:39:14)
> > From: Arnd Bergmann <arnd@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.

        Arnd