[Intel-gfx] [PATCH 16/21] drm/i915/gem: Delay context creation

[Tvrtko Ursulin]

On 30/04/2021 07:53, Daniel Vetter wrote:
> On Thu, Apr 29, 2021 at 11:35 PM Jason Ekstrand <jason at jlekstrand.net> wrote:
>>
>> On Thu, Apr 29, 2021 at 2:07 PM Daniel Vetter <daniel at ffwll.ch> wrote:
>>>
>>> On Thu, Apr 29, 2021 at 02:01:16PM -0500, Jason Ekstrand wrote:
>>>> On Thu, Apr 29, 2021 at 1:56 PM Daniel Vetter <daniel at ffwll.ch> wrote:
>>>>> On Thu, Apr 29, 2021 at 01:16:04PM -0500, Jason Ekstrand wrote:
>>>>>> On Thu, Apr 29, 2021 at 10:51 AM Daniel Vetter <daniel at ffwll.ch> wrote:
>>>>>>>> +     ret = set_proto_ctx_param(file_priv, pc, args);
>>>>>>>
>>>>>>> I think we should have a FIXME here of not allowing this on some future
>>>>>>> platforms because just use CTX_CREATE_EXT.
>>>>>>
>>>>>> Done.
>>>>>>
>>>>>>>> +     if (ret == -ENOTSUPP) {
>>>>>>>> +             /* Some params, specifically SSEU, can only be set on fully
>>>>>>>
>>>>>>> I think this needs a FIXME: that this only holds during the conversion?
>>>>>>> Otherwise we kinda have a bit a problem me thinks ...
>>>>>>
>>>>>> I'm not sure what you mean by that.
>>>>>
>>>>> Well I'm at least assuming that we wont have this case anymore, i.e.
>>>>> there's only two kinds of parameters:
>>>>> - those which are valid only on proto context
>>>>> - those which are valid on both (like priority)
>>>>>
>>>>> This SSEU thing looks like a 3rd parameter, which is only valid on
>>>>> finalized context. That feels all kinds of wrong. Will it stay? If yes
>>>>> *ugh* and why?
>>>>
>>>> Because I was being lazy.  The SSEU stuff is a fairly complex param to
>>>> parse and it's always set live.  I can factor out the SSEU parsing
>>>> code if you want and it shouldn't be too bad in the end.
>>>
>>> Yeah I think the special case here is a bit too jarring.
>>
>> I rolled a v5 that allows you to set SSEU as a create param.  I'm not
>> a huge fan of that much code duplication for the SSEU set but I guess
>> that's what we get for deciding to "unify" our context creation
>> parameter path with our on-the-fly parameter path....
>>
>> You can look at it here:
>>
>> https://gitlab.freedesktop.org/jekstrand/linux/-/commit/c805f424a3374b2de405b7fc651eab551df2cdaf#474deb1194892a272db022ff175872d42004dfda_283_588
> 
> Hm yeah the duplication of the render engine check is a bit annoying.
> What's worse, if you tthrow another set_engines on top it's probably
> all wrong then. The old thing solved that by just throwing that
> intel_context away.
> 
> You're also not keeping the engine id in the proto ctx for this, so
> there's probably some gaps there. We'd need to clear the SSEU if
> userspace puts another context there. But also no userspace does that.
> 
> Plus cursory review of userspace show
> - mesa doesn't set this
> - compute sets its right before running the batch
> - media sets it as the last thing of context creation

Noticed a long sub-thread so looked inside..

SSEU is a really an interesting one.

For current userspace limiting to context creation is fine, since it is 
only allowed for Icelake/VME use case. But if you notice the comment inside:

		/* ABI restriction - VME use case only. */

It is a hint there was, or could be, more to this uapi than that.

And from memory I think limiting to creation time will nip the hopes 
media had to use this dynamically on other platforms in the bud. So not 
that good really. They had convincing numbers what gets significantly 
better if we allowed dynamic control to this, just that as always, open 
source userspace was not there so we never allowed it. However if you 
come up with a new world order where it can only be done at context 
creation, as said already, the possibility for that improvement (aka 
further improving the competitive advantage) is most likely dashed.

Regards,

Tvrtko