Re: [PATCH] drm/i915: Engine discovery query

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 01/10/2018 20:39, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2018-10-01 17:41:00)
>>
>> On 01/10/2018 17:24, Chris Wilson wrote:
>>> Quoting Tvrtko Ursulin (2018-10-01 17:15:24)
>>>> +       len = sizeof(struct drm_i915_query_engine_info) +
>>>> +             I915_NUM_ENGINES * sizeof(struct drm_i915_engine_info);
>>>
>>> Since I915_NUM_ENGINES is not ABI, and this will be using a 2-step
>>> query (1st to find length, 2nd to grab details), it should work with
>>> info->num_rings. If not, we have problems later ;)
>>
>> I don't follow, what do you plan to enumerate that would cause a problem?
> 
> All I'm suggesting is to avoid any notion that this length is fixed
> indefinitely :)

You mean extra paranoid step of making sure reported len exactly matches 
the number of reported engines * sizeof(engine_info)? uAPI is defined to 
query required memory block size and then read the reported num_engines 
so I think that's fine, but sure, I can make it match exactly.

Regards,

Tvrtko
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