[Intel-gfx] [PATCH 8/8] drm/i915: Allow user to set cache at BO creation

[Jordan Justen]

On 2023-04-24 02:08:43, Tvrtko Ursulin wrote:
> 
> Being able to "list" supported extensions sounds like a reasonable
> principle, albeit a departure from the design direction to date.
> Which means there are probably no quick solutions. Also, AFAIU, only
> PXP context create is the problematic one, right? Everything else is
> pretty much instant or delayed allocation so super cheap to probe by
> attempting to use.
> 
> If I got that right and given this series is about
> drm_i915_gem_create_ext I don't think this side discussion should be
> blocking it.

This still leaves the issue of no reasonable detection mechanism for
the extension. If the discussion gets too complicated, then can we add
a GET_PARAM for the SET_PAT extension? I'm hoping we could either come
up with something better reasonably quickly, or i915/Xe can add a new
param for each new extensions until a better approach is available.

> Furthermore the PXP context create story is even more complicated,
> in a way that it is not just about querying whether the extension is
> supported, but the expensive check is something more complicated.
> 
> Going back to implementation details for this proposed new feature,
> one alternative to query could be something like:
> 
>    drm_i915_gem_create_ext.flags |= I915_GEM_CREATE_EXT_FLAG_PROBE_EXTENSIONS;
> 
> That would be somewhat more light weight to implement that the
> i915_query route. And it appears it would work for all ioctls which
> support extensions apart for i915_context_param_engines.

This seems little better than the "try it, and if it works then it's
supported".

I'm not suggesting that userspace should be able to check that
scenario x+y+z will work, but more a list of extensions that
conceivably could work. Normally this should just a matter of the
kernel unconditionally adding the newly implemented extension to the
list returned in the query call.

If a GET_PARAM can be made for the PXP case, then it seems like a
query item returning CONTEXT_CREATE extensions could conditionally
omit that extension just as easily as implementing the proposed new
GET_PARAM.

-Jordan