[Intel-gfx] [PATCH] drm/i915: Align engine->uabi_class/instance with i915_drm.h

[Tvrtko Ursulin]

On 15/01/2020 15:35, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2020-01-15 15:28:19)
>>
>> On 15/01/2020 15:24, Tvrtko Ursulin wrote:
>>> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>>
>>> In our ABI we have defined I915_ENGINE_CLASS_INVALID_NONE and
>>> I915_ENGINE_CLASS_INVALID_VIRTUAL as negative values which creates
>>> implicit coupling with type widths used in, also ABI, struct
>>> i915_engine_class_instance.
>>>
>>> When for instance we export engine->uabi_class
>>> I915_ENGINE_CLASS_INVALID_VIRTUAL from our our tracepoints, because the
>>> type of the former is u8 in contrast to u16 defined in the ABI, 254 will
>>> be returned instead of 65534 which userspace would legitimately expect.
>>>
>>> Therefore we need to align the type used to store engine ABI class and
>>> instance.
>>>
>>> I did not find any other user visible inconsistency apart from the
>>> tracepoints so I think importance of the fix is low.
>>
>> Alternatives:
>>
>> 1.
>> Embed struct i915_engine_class_instance in struct intel_engine_cs, but
>> downside is more churn.
> 
> Could do. It would seem to make sense.
>   
>> 2.
>> Only tweak the tracepoints to cast back and forth, but is it possible to
>> cast from unsigned to signed and get a negative number?
>>
>> 3.
>> Do nothing, does anyone cares?
> 
> It actually changes the value reported by GET_ENGINES for a virtual
> engine, right?
> 
> engine->uabi_instance is u8, so u16 ci.engine_instance =
> engine->uabi_instance is zero extended, not sign extended.
> 
> And we did say that is expected to be (u16)-2 already.
> 
> So cc:stable

You are correct, yep, was thinking about engine query where it doesn't 
apply but forgot about get_engines where it does.

>>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
> 
>>> ---
>>>    drivers/gpu/drm/i915/gem/i915_gem_busy.c     | 12 ++++++------
>>>    drivers/gpu/drm/i915/gt/intel_engine_types.h |  4 ++--
>>>    2 files changed, 8 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_busy.c b/drivers/gpu/drm/i915/gem/i915_gem_busy.c
>>> index 3d4f5775a4ba..25235ef630c1 100644
>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_busy.c
>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_busy.c
>>> @@ -9,16 +9,16 @@
>>>    #include "i915_gem_ioctls.h"
>>>    #include "i915_gem_object.h"
>>>    
>>> -static __always_inline u32 __busy_read_flag(u8 id)
>>> +static __always_inline u32 __busy_read_flag(u16 id)
>>>    {
>>> -     if (id == (u8)I915_ENGINE_CLASS_INVALID)
>>> +     if (id == (u16)I915_ENGINE_CLASS_INVALID)
>>>                return 0xffff0000u;
>>>    
>>>        GEM_BUG_ON(id >= 16);
>>>        return 0x10000u << id;
>>>    }
>>>    
>>> -static __always_inline u32 __busy_write_id(u8 id)
>>> +static __always_inline u32 __busy_write_id(u16 id)
>>>    {
>>>        /*
>>>         * The uABI guarantees an active writer is also amongst the read
>>> @@ -29,14 +29,14 @@ static __always_inline u32 __busy_write_id(u8 id)
>>>         * last_read - hence we always set both read and write busy for
>>>         * last_write.
>>>         */
>>> -     if (id == (u8)I915_ENGINE_CLASS_INVALID)
>>> +     if (id == (u16)I915_ENGINE_CLASS_INVALID)
>>>                return 0xffffffffu;
>>>    
>>>        return (id + 1) | __busy_read_flag(id);
>>>    }
>>>    
>>>    static __always_inline unsigned int
>>> -__busy_set_if_active(const struct dma_fence *fence, u32 (*flag)(u8 id))
>>> +__busy_set_if_active(const struct dma_fence *fence, u32 (*flag)(u16 id))
>>>    {
>>>        const struct i915_request *rq;
>>>    
>>> @@ -57,7 +57,7 @@ __busy_set_if_active(const struct dma_fence *fence, u32 (*flag)(u8 id))
>>>                return 0;
>>>    
>>>        /* Beware type-expansion follies! */
>>> -     BUILD_BUG_ON(!typecheck(u8, rq->engine->uabi_class));
>>> +     BUILD_BUG_ON(!typecheck(u16, rq->engine->uabi_class));
>>>        return flag(rq->engine->uabi_class);
>>>    }
>>>    
>>> diff --git a/drivers/gpu/drm/i915/gt/intel_engine_types.h b/drivers/gpu/drm/i915/gt/intel_engine_types.h
>>> index 00287515e7af..350da59e605b 100644
>>> --- a/drivers/gpu/drm/i915/gt/intel_engine_types.h
>>> +++ b/drivers/gpu/drm/i915/gt/intel_engine_types.h
>>> @@ -278,8 +278,8 @@ struct intel_engine_cs {
>>>        u8 class;
>>>        u8 instance;
>>>    
>>> -     u8 uabi_class;
>>> -     u8 uabi_instance;
>>> +     u16 uabi_class;
>>> +     u16 uabi_instance;
> 
> Bah, doesn't this leave us with a u16 hole!

I don't see anything to fill it with. Could expand class and instance to 
u16 as well and see if it has any negative effect on code size. But in a 
separate patch.

Regards,

Tvrtko