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

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 16 Feb 2022, Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> wrote:
> 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@linux.alibaba.com> 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@linux.alibaba.com>
>>>> Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
>>>
>>> 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.

We never check instance or uabi_instance members against
I915_ENGINE_CLASS_INVALID_VIRTUAL anywhere.

BR,
Jani.

>
> 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
>> 

-- 
Jani Nikula, Intel Open Source Graphics Center