[Intel-gfx] [PATCH] drm/i915: Allow null render state batchbuffers bigger than one page

[Oscar Mateo]

On 05/03/2017 08:52 AM, Mika Kuoppala wrote:
> Oscar Mateo <oscar.mateo at intel.com> writes:
>
>> On 05/02/2017 09:17 AM, Mika Kuoppala wrote:
>>> Chris Wilson <chris at chris-wilson.co.uk> writes:
>>>
>>>> On Fri, Apr 28, 2017 at 09:11:06AM +0000, Oscar Mateo wrote:
>>>>> The new batchbuffer for CNL surpasses the 4096 byte mark.
>>>>>
>>>>> Cc: Mika Kuoppala <mika.kuoppala at intel.com>
>>>>> Cc: Ben Widawsky <ben at bwidawsk.net>
>>>>> Signed-off-by: Oscar Mateo <oscar.mateo at intel.com>
>>>> Evil, 4k+ of nothing-ness that userspace then has to configure for itself
>>>> for correctness anyway.
>>>>
>>>> Patch looks ok, but still question the sanity.
>>> Is there a requirement for CNL to init the renderstate?
>>>
>>> I would like to drop the render state init from CNL if
>>> we can't find evidence that it needs it. Bspec indicates
>>> that it doesnt.
>>>
>>> -Mika
>> Hi Mika,
>>
>> I can double-check with the hardware architects, but word around here is
>> that render state init has never stopped being a requirement. Where did
>> you see in the BSpec that it is not required for CNL?
>>
> It would be great if you could refresh the answer and perhaps
> even get some answers to the 'why' parts.
>
> In the "Context Descriptor Format" section, it says:
> "Render CS Only: Render state need not be initialized; the Render
> Context Restore Inhibit bit in the Context/Save image in memory should
> be set to prevent restoring garbage render context."
>
> -Mika

:_(

The same section also says:

“See the Logical Ring Context Format section for details.”

And then “Logical Ring Context Format” section goes on to say:

“It is tedious for software to populate the engine context as per the 
requirements, it is recommended to implicitly use engine to populate 
this portion of the context. […] Software must program all the state 
required to initialize the engine in the ring buffer which would 
initialize the hardware state.”

I’ll try to clarify it...
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