Re: [Intel-gfx] [PATCH 10/10] drm/i915/gt: Declare when we enabled timeslicing

From: Chris Wilson <chris@chris-wilson.co.uk>
To: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>,
	intel-gfx@lists.freedesktop.org
Cc: Kenneth Graunke <kenneth@whitecape.org>
Subject: Re: [Intel-gfx] [PATCH 10/10] drm/i915/gt: Declare when we enabled timeslicing
Date: Tue, 07 Apr 2020 11:55:05 +0100	[thread overview]
Message-ID: <158625690563.8918.10030260069115914165@build.alporthouse.com> (raw)
In-Reply-To: <ccf7f6e5-0295-a179-bf1f-eb4de8570b58@linux.intel.com>

Quoting Tvrtko Ursulin (2020-04-07 11:50:31)
> 
> On 03/04/2020 10:13, Chris Wilson wrote:
> > Let userspace know if they can trust timeslicing by including it as part
> > of the I915_PARAM_HAS_SCHEDULER::I915_SCHEDULER_CAP_TIMESLICING
> > 
> > v2: Only declare timeslicing if we can safely preempt userspace.
> > 
> > Fixes: 8ee36e048c98 ("drm/i915/execlists: Minimalistic timeslicing")
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Kenneth Graunke <kenneth@whitecape.org>
> > ---
> >   drivers/gpu/drm/i915/gt/intel_engine.h      | 3 ++-
> >   drivers/gpu/drm/i915/gt/intel_engine_user.c | 5 +++++
> >   include/uapi/drm/i915_drm.h                 | 1 +
> >   3 files changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gt/intel_engine.h b/drivers/gpu/drm/i915/gt/intel_engine.h
> > index b469de0dd9b6..424672ee7874 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_engine.h
> > +++ b/drivers/gpu/drm/i915/gt/intel_engine.h
> > @@ -339,7 +339,8 @@ intel_engine_has_timeslices(const struct intel_engine_cs *engine)
> >       if (!IS_ACTIVE(CONFIG_DRM_I915_TIMESLICE_DURATION))
> >               return false;
> >   
> > -     return intel_engine_has_semaphores(engine);
> > +     return (intel_engine_has_semaphores(engine) &&
> > +             intel_engine_has_preemption(engine));
> 
> This is turning off timeslicing on Gen8? Well it wouldn't have worked 
> anyway, outside the batch boundaries.. so it does sound technically correct.

...

> Split uapi from Gen8 fix?

I don't regard gen8 as broken per se, for the kernel could preempt
between batches -- but under the spotlight of "can userspace use this",
it clearly cannot. Hence why I put them both together, it is not until
userspace needs to control itself, that it becomes a problem.

The igt that test this purposefully do not run on gen8 because I was
aware of the limitations. That should have been a big clue, but it
wasn't until I looked at it from an actual user's perspective did I
realise how important that little detail was. :(
-Chris
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