Re: [PATCH v3] drm/i915/execlists: Use coherent writes into the context image

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

On 14/09/2018 09:21, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2018-09-14 09:14:54)
>>
>> On 13/09/2018 20:33, Chris Wilson wrote:
>>> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
>>> index d7fcbba8e982..7b1f322f232b 100644
>>> --- a/drivers/gpu/drm/i915/intel_lrc.c
>>> +++ b/drivers/gpu/drm/i915/intel_lrc.c
>>> @@ -1294,7 +1294,7 @@ static int __context_pin(struct i915_gem_context *ctx, struct i915_vma *vma)
>>>         * on an active context (which by nature is already on the GPU).
>>>         */
>>>        if (!(vma->flags & I915_VMA_GLOBAL_BIND)) {
>>> -             err = i915_gem_object_set_to_gtt_domain(vma->obj, true);
>>> +             err = i915_gem_object_set_to_wc_domain(vma->obj, true);
>>
>> I am still confused by this. Cache flushing effects of the old and new
>> call seem the same due object being in CPU write domain at this point.
>> What changes is that it will be marked differently from this point one.
>> Does that come into play later in the objects lifetime and where?
> 
> No, just taking the opportunity to use a more correct domain now that it
> exists and logically ties in with using WC.

Ok.

>>>                if (err)
>>>                        return err;
>>>        }
>>> @@ -1322,7 +1322,9 @@ __execlists_context_pin(struct intel_engine_cs *engine,
>>>        if (ret)
>>>                goto err;
>>>    
>>> -     vaddr = i915_gem_object_pin_map(ce->state->obj, I915_MAP_WB);
>>> +     vaddr = i915_gem_object_pin_map(ce->state->obj,
>>> +                                     i915_coherent_map_type(ctx->i915) |
>>> +                                     I915_MAP_OVERRIDE);
>>
>> Override MAP_WB from populate_lr_context - OK I think.
>>
>>>        if (IS_ERR(vaddr)) {
>>>                ret = PTR_ERR(vaddr);
>>>                goto unpin_vma;
>>> @@ -2753,7 +2755,7 @@ populate_lr_context(struct i915_gem_context *ctx,
>>>                void *defaults;
>>>    
>>>                defaults = i915_gem_object_pin_map(engine->default_state,
>>> -                                                I915_MAP_WB);
>>> +                                                I915_MAP_FORCE_WB);
>>>                if (IS_ERR(defaults)) {
>>>                        ret = PTR_ERR(defaults);
>>>                        goto err_unpin_ctx;
>>> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
>>> index d0ef50bf930a..1eb68d77b66c 100644
>>> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
>>> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
>>> @@ -1288,7 +1288,7 @@ alloc_context_vma(struct intel_engine_cs *engine)
>>>                }
>>>    
>>>                defaults = i915_gem_object_pin_map(engine->default_state,
>>> -                                                I915_MAP_WB);
>>> +                                                I915_MAP_FORCE_WB);
>>>                if (IS_ERR(defaults)) {
>>>                        err = PTR_ERR(defaults);
>>>                        goto err_map;
>>>
>>
>> These two do not need to be changed AFAICT.
> 
> I think we cannot rely on engine->default_state always being MAP_WB
> already at this point, due to not having an idle cycle between creation
> of engine->default_state on module_load and first use.
 >
 > Having thought of that last night, I did mean to add a call to
 > __i915_gem_park() during init so we forced ourselves to idle.

I don't follow - all places where we map it use MAP_WB. Isn't force flag 
just to override the existing different mapping?

Regards,

Tvrtko

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