Re: [PATCH v2 02/15] drm/i915/dsb: DSB context creation.

[Animesh Manna <animesh.manna@intel.com>]

On 10/17/2019 8:08 PM, Tvrtko Ursulin wrote:
>
> On 17/10/2019 14:53, Animesh Manna wrote:
>> On 10/17/2019 6:39 PM, Tvrtko Ursulin wrote:
>>> On 17/10/2019 13:52, Animesh Manna wrote:
>>>> On 10/17/2019 2:05 PM, Tvrtko Ursulin wrote:
>>>>> On 22/08/2019 13:09, Chris Wilson wrote:
>>>>>> Quoting Animesh Manna (2019-08-22 13:05:06)
>>>>>>> Hi,
>>>>>>>
>>>>>>>
>>>>>>> On 8/21/2019 11:41 PM, Chris Wilson wrote:
>>>>>>>> Quoting Animesh Manna (2019-08-21 07:32:22)
>>>>>>>>> +       vma = i915_gem_object_ggtt_pin(obj, NULL, 0, 0, 
>>>>>>>>> PIN_MAPPABLE);
>>>>>>>> Only this (currently) still requires struct_mutex
>>>>>>>
>>>>>>> Sure will add.
>>>>>>>>
>>>>>>>> Does it have to mappable? Is that the HW constraint?
>>>>>>>
>>>>>>> Yes, as per HW design need a cpu mapped buffer to write 
>>>>>>> opcode+data from
>>>>>>> driver.
>>>>>>
>>>>>> PIN_MAPPABLE refers to the iomem aperture portion of the Global 
>>>>>> GTT (i.e.
>>>>>> the low 64-512MiB). You never use a GGTT mmap for your CPU 
>>>>>> access, so the
>>>>>> placement should be entirely dictated by the DSB requirements. If 
>>>>>> you
>>>>>> don't need to be in the low region, don't force it to be, so we have
>>>>>> less congestion for the objects that have to be placed in that 
>>>>>> region.
>>>>>
>>>>> I was doing a mini audit of what uses the aperture these days and 
>>>>> noticed this code has been merged in the meantime, but AFAICS this 
>>>>> question from Chris hasn't been answered? At least not on the 
>>>>> mailing list. So does it need to be in the aperture region or not?
>>>>
>>>> Hi,
>>>>
>>>> Based on recommendation from H/w team used PIN_MAPPABLE, not very 
>>>> sure about internal details.
>>>
>>> What did the recommendation exactly say? That it has to be in GGTT 
>>> or aperture?
>>
>> It said:
>> "GMM to allocate buffer from global GTT, get CPU mapped address as 
>> well (not stolen memory) ... ".
>
> So it's possible you don't need PIN_MAPPABLE.

As per I remember from initial discussion from h/w team DSB is not cache 
coherent. Due to that we have used I915_MAP_WC during mapping the buffer 
and want to keep the buffer in aperture region.
Could not find the discussion thread as it was quiet old like around 
initial enablement of DSB feature.

>
>
> Do we have some test coverage for this? In other words if I send a 
> patch which removes it, will we know if the feature is healthy?

Gamma lut programming is enabled through DSB. kms_color igt is having 
the coverage for this.

Not sure cache coherence issue is easily reproducible or not, will go 
with expert's opinion.

Regards,
Animesh

>
> Regards,
>
> Tvrtko

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