[Intel-gfx] [PATCH 2/2] drm/i915: Reduce context HW ID lifetime

[Lionel Landwerlin]

On 22/02/18 09:17, Chris Wilson wrote:
> Quoting Lionel Landwerlin (2018-02-21 01:12:46)
>> On 09/02/18 20:47, Chris Wilson wrote:
>>> Quoting Daniele Ceraolo Spurio (2018-02-09 18:56:36)
>>>> On 09/02/18 02:22, Chris Wilson wrote:
>>>>> Future gen reduce the number of bits we will have available to
>>>>> differentiate between contexts, so reduce the lifetime of the ID
>>>>> assignment from that of the context to its current active cycle (i.e.
>>>>> only while it is pinned for use by the HW, will it have a constant ID).
>>>>> This means that instead of a max of 2k allocated contexts (worst case
>>>>> before fun with bit twiddling), we instead have a limit of 2k in flight
>>>>> contexts (minus a few that have been pinned by the kernel or by perf).
>>>>>
>>>> Since we're moving to dynamic assignment would it be reasonable to move
>>>> the ID to intel_context with an ida per engine class instead of keeping
>>>> it at the i915_gem_context level? This would allow us to use more
>>>> contexts as long as they use different classes and it'd take us closer
>>>> to the way the HW thinks (lrcs on different classes are independent even
>>>> if they have the same ID). It could also make it easier to use the
>>>> sw_counter field to expand the maximum number of supported lrcs.
>>>> Just a thought, we can always do it as a follow up step.
>>> Certainly doable, leads to lots of refinement about focusing on engines
>>> rather than global requests (*shivers* every time I think about
>>> struct_mutex around requests). Afaict, even perf userspace will not care
>>> if the HW ID vary between engines for the same context; certainly that's
>>> the only thing that might notice.
>>> -Chris
>> Would it make sense to dedicate part of the HW ID bits to indicate the
>> engine and the rest a rolling integer that increments each time
>> something's submitted to the execlist ports?
> Hmm. At present, I guess only OA requires a known hw-id. I thought we
> would need it to service PASID faults (but I guess that isn't really our
> problem). Can we go completely hw-id less and just using a rolling id?

I think for some future changes it would be easier to have the engine id 
part of the hw-id.
That should be easy to change later on anyway.
Otherwise I don't have any problem with a rolling id.

Thanks!

-
Lionel