[Intel-gfx] [PATCH 1/2] drm/i915: Pre-calculate engine context size

[Zhi Wang]

Hi Daniele:
     Thanks for the reply! Joonas and I did some researches in irc after 
the email. We found B-spec did say the context image for render engine 
consist 20 pages in context layout section. It looks like a mistake in 
b-spec. Another interesting we found is the context image size for KBL 
halo is weird, not the same with other KBL SKUs.

Thanks,
Zhi.

于 04/27/17 00:20, Daniele Ceraolo Spurio 写道:
>
>
> On 26/04/17 02:57, Zhi Wang wrote:
>> Uh...sorry for not mentioning that before:), and stolen memory is not my
>> business. :(
>>
>> Actually we root-caused it.
>>
>> This is how we found this case:
>>
>> The story is W driver directly allocated the ring buffer after the
>> context image, and the context image size in W driver is 19 pages. GVT-g
>> will do shadow context during submission, we copy 20 pages from guest
>> context image, so you can see, an extra page is copied here as the
>> context image size is actual 19 pages. The extra page belows to ring
>> buffer. When guest updates that page with new commands during GVT-g
>> executing the workload, the extra page ( which is ring buffer page) will
>> be over-written with old content, since GVT-g will copy the shadow
>> context (20 pages) back to guest at this time.
>>
>> That's the full story. I send another email to Harsh. He should know if
>> the context image size of CHV is also 19 pages.
>>
>> Thanks,
>> Zhi.
>>
>
> I did a quick check and according to the specs both the BDW and the 
> CHV lrcs are formed by 18096 dwords plus the per-context HWSP, which 
> converts to 19 pages for both platforms.
>
> Regards,
> Daniele
>
>> 于 04/26/17 17:52, Joonas Lahtinen 写道:
>>> On ke, 2017-04-26 at 17:10 +0800, Zhi Wang wrote:
>>>> Hi Joonas:
>>>>       Can you change GEN8_LR_CONTEXT_RENDER_SIZE = (19 * PAGE_SIZE)?
>>>> Then we don't need the hack in GVT-g. :P Actually it's 19 pages not
>>>> 20 pages on BDW.
>>> The exception is only made for BDW, not Gen8 overall. Has the change
>>> been verified for CHV too?
>>>
>>> Why hasn't a patch to fix above been sent for i915 in the past? Just
>>> like in the stolen memory disabling case, bugs should be root caused
>>> and then fixed, not just worked around quickly.
>>>
>>> Regards, Joonas
>>
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