[Intel-gfx] [RFC v3] drm/i915: Select engines via class and instance in execbuffer2

[Tvrtko Ursulin]

On 18/05/2017 12:10, Chris Wilson wrote:
> On Thu, May 18, 2017 at 01:55:59PM +0300, Joonas Lahtinen wrote:
>> On ke, 2017-05-17 at 16:40 +0100, Tvrtko Ursulin wrote:
>>> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>>
>>> Building on top of the previous patch which exported the concept
>>> of engine classes and instances, we can also use this instead of
>>> the current awkward engine selection uAPI.
>>>
>>> This is primarily interesting for the VCS engine selection which
>>> is a) currently done via disjoint set of flags, and b) the
>>> current I915_EXEC_BSD flags has different semantics depending on
>>> the underlying hardware which is bad.
>>>
>>> Proposed idea here is to reserve 16-bits of flags, to pass in
>>> the engine class and instance (8 bits each), and a new flag
>>> named I915_EXEC_CLASS_INSTACE to tell the kernel this new engine
>>> selection API is in use.
>
> Note this text doesn't describe the interface in v3.
>
>> Would it make sense to use bitmask for future proofing? Then we could
>> allow passing multiple options in the future.
>
> No. The first use case has to be explicit control of engine. That's
> orthogonal to asking to select any of a particular class.

Was the suggestion to allow instance=any and instance=N? Or even all the 
way to instance=N-or-M?

If not the latter, then I can think of two other possible approaches:

1. Reserve 0xff as instance=any, aka 128 instances should be enough for 
everbody. :) Could simply enforce in the uAPI that instance == 
I915_EXEC_INSTANCE_MASK = -EINVAL for now or something.

2. Do nothing now and add say I915_EXEC_CLASS at a later point. This 
patch adds I915_EXEC_CLASS_INSTANCE so I915_EXEC_CLASS would not sound 
out of place.

Regards,

Tvrtko