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

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Date:   Wed, 16 Feb 2022 09:57:49 +0000
From:   Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
To:     Ville Syrjälä <ville.syrjala@...ux.intel.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     Jiapeng Chong <jiapeng.chong@...ux.alibaba.com>, airlied@...ux.ie,
        intel-gfx@...ts.freedesktop.org,
        Abaci Robot <abaci@...ux.alibaba.com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        christian.koenig@....com, linaro-mm-sig@...ts.linaro.org,
        rodrigo.vivi@...el.com, sumit.semwal@...aro.org,
        linux-media@...r.kernel.org
Subject: Re: [PATCH] drm/i915/gt: fix unsigned integer to signed assignment


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@...ux.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@...ux.alibaba.com>
>>> Signed-off-by: Jiapeng Chong <jiapeng.chong@...ux.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.

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
>