Re: [PATCH] drm/i915/gt: fix unsigned integer to signed assignment

[Tvrtko Ursulin]

On 16/02/2022 09:19, Ville Syrjälä wrote:
> On Wed, Feb 16, 2022 at 11:02:06AM +0200, Jani Nikula wrote:
> > On Wed, 16 Feb 2022, Jiapeng Chong <jiapeng.chong@xxxxxxxxxxxxxxxxx> wrote:
> > > Eliminate the follow smatch warning:
> > >
> > > drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c:4640
> > > guc_create_virtual() warn: assigning (-2) to unsigned variable
> > > 've->base.instance'.
> > >
> > > drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c:4641
> > > guc_create_virtual() warn: assigning (-2) to unsigned variable
> > > 've->base.uabi_instance'.
> > >
> > > Reported-by: Abaci Robot <abaci@xxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Jiapeng Chong <jiapeng.chong@xxxxxxxxxxxxxxxxx>
> >
> > The report seems to be valid, but I don't think this is the fix.
> >
> > Where do we even check for invalid instance/uabi_instance in code?
>
> The whole thing seems rather poorly documented as there's a matching
> uabi struct with __u16's and the negative values are defined right
> there in the uapi header as well.

Negative ones are exception values to be used in conjunction with the virtual engine uapi (see "DOC: Virtual Engine uAPI" and also comment next to I915_CONTEXT_PARAM_ENGINES).

AFAIK assigning negative int to unsigned int is defined and fine.

Compiler does warn on comparisons which is why we have:

./gem/i915_gem_busy.c:  if (id == (u16)I915_ENGINE_CLASS_INVALID)
./gem/i915_gem_busy.c:  if (id == (u16)I915_ENGINE_CLASS_INVALID)
./gem/i915_gem_context.c:               if (ci.engine_class == (u16)I915_ENGINE_CLASS_INVALID &&
./gem/i915_gem_context.c:                   ci.engine_instance == (u16)I915_ENGINE_CLASS_INVALID_NONE)

So I think no action needed here.

Regards,

Tvrtko
 
> > BR,
> > Jani.
> >
> >
> > > ---
> > >   drivers/gpu/drm/i915/gt/intel_engine_types.h | 4 ++--
> > >   1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/i915/gt/intel_engine_types.h b/drivers/gpu/drm/i915/gt/intel_engine_types.h
> > > index 36365bdbe1ee..dc7cc06c68e7 100644
> > > --- a/drivers/gpu/drm/i915/gt/intel_engine_types.h
> > > +++ b/drivers/gpu/drm/i915/gt/intel_engine_types.h
> > > @@ -328,10 +328,10 @@ struct intel_engine_cs {
> > >   	intel_engine_mask_t logical_mask;
> > >   
> > >   	u8 class;
> > > -	u8 instance;
> > > +	s8 instance;
> > >   
> > >   	u16 uabi_class;
> > > -	u16 uabi_instance;
> > > +	s16 uabi_instance;
> > >   
> > >   	u32 uabi_capabilities;
> > >   	u32 context_size;
> >
> > --
> > Jani Nikula, Intel Open Source Graphics Center