Re: [PATCH 1/8] drm/i915: expose helper mapping exec flag engine to intel_engine_cs

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

On 03/05/2018 18:31, Lionel Landwerlin wrote:
> On 03/05/18 18:12, Tvrtko Ursulin wrote:
>>
>> On 30/04/2018 15:37, Lionel Landwerlin wrote:
>>> On 25/04/18 12:50, Chris Wilson wrote:
>>>> Quoting Lionel Landwerlin (2018-04-25 12:45:14)
>>>>> This function will be used later by the per (context,engine) power
>>>>> programming interface.
>>>> No. This is not the appropriate uABI, please see
>>>> intel_engine_lookup_user().
>>>> -Chris
>>>>
>>> uAPI wise, does this sound okay? :
>>>
>>> #define I915_CONTEXT_PARAM_SSEU         0x7
>>>          __u64 value;
>>>
>>> struct drm_i915_gem_context_param_sseu {
>>> /*
>>>           * Engine to be configured or queried.
>>>           */
>>>          __u32 class;
>>>          __u32 instance;
>>>
>>> /*
>>>           * Setting slice_mask or subslice_mask to 0 will make the 
>>> context use
>>>           * masks reported respectively by I915_PARAM_SLICE_MASK or
>>>           * I915_PARAM_SUBSLICE_MASK.
>>>           */
>>>          union {
>>>                  struct {
>>>                          __u8 slice_mask;
>>>                          __u8 subslice_mask;
>>>                          __u8 min_eus_per_subslice;
>>>                          __u8 max_eus_per_subslice;
>>>                  } packed;
>>>                  __u64 value;
>>>          };
>>> };
>>
>> Is it possible (now or in the future) to have different configs per 
>> slice or subslice? And if so, is the way this uAPI handles that 
>> passing these packets multiple times every time with different slices 
>> and subslice mask?
>>
>> For instance:
>>
>> class=0, instance=0, slice_mask=0x1, subslice=0x10
>> class=0, instance=0, slice_mask=0x10, subslice=0x1
>>
>> Is this something we should support?
> 
> It's not supported in any configuration I'm aware of. It's always a 
> uniform subslice programming across slices. > It's actually a number of slice/subslice to enable (through the register
> interface), not a mask (i.e. we can't choose which ones get used) which 
> is what this interface exposes.

I thought for Gen11 it cannot be just number of slices but must be a 
mask? since media will want to make sure some slices are not enabled, 
and some are, for its workloads.

But in general I guess I don't understand the semantics of slice and 
subslice masks (and eu counts). Say if you pass in slice_mask=0x1 
subslice_mask=0x1 - this means you are only enabling subslice 0x1 on 
slice 0x1 - but what happens on other slices? They are all disabled?

> It's a fair comment though. We didn't plan for asymmetric slice fusing 
> initially and it just happened.

Okay so you think uAPI should support it to be more future proof?

> One other thing I've been wondering is whether we should strictly 
> validate the input from userspace.
> Right now we min/max the numbers and never raise any error.

It does feel low level enough that we should reject userspace trying to 
feed in nonsense for its own good.

Regards,

Tvrtko
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