Re: [PATCH v2] drm/i915/gt: Hold a wakeref for the active VM

[Tvrtko Ursulin]

On 04/04/2023 17:00, Andi Shyti wrote:
> Hi Tvrtko,
>
> > > > diff --git a/drivers/gpu/drm/i915/gt/intel_context.h b/drivers/gpu/drm/i915/gt/intel_context.h
> > > > index 0a8d553da3f439..48f888c3da083b 100644
> > > > --- a/drivers/gpu/drm/i915/gt/intel_context.h
> > > > +++ b/drivers/gpu/drm/i915/gt/intel_context.h
> > > > @@ -14,6 +14,7 @@
> > > >    #include "i915_drv.h"
> > > >    #include "intel_context_types.h"
> > > >    #include "intel_engine_types.h"
> > > > +#include "intel_gt_pm.h"
> > > >    #include "intel_ring_types.h"
> > > >    #include "intel_timeline_types.h"
> > > >    #include "i915_trace.h"
> > > > @@ -207,8 +208,11 @@ void intel_context_exit_engine(struct intel_context *ce);
> > > >    static inline void intel_context_enter(struct intel_context *ce)
> > > >    {
> > > >    	lockdep_assert_held(&ce->timeline->mutex);
> > > > -	if (!ce->active_count++)
> > > > -		ce->ops->enter(ce);
> > > > +	if (ce->active_count++)
> > > > +		return;
> > > > +
> > > > +	ce->ops->enter(ce);
> > > > +	intel_gt_pm_get(ce->vm->gt);
> > > >    }
> > > >    static inline void intel_context_mark_active(struct intel_context *ce)
> > > > @@ -222,8 +226,11 @@ static inline void intel_context_exit(struct intel_context *ce)
> > > >    {
> > > >    	lockdep_assert_held(&ce->timeline->mutex);
> > > >    	GEM_BUG_ON(!ce->active_count);
> > > > -	if (!--ce->active_count)
> > > > -		ce->ops->exit(ce);
> > > > +	if (--ce->active_count)
> > > > +		return;
> > > > +
> > > > +	intel_gt_pm_put_async(ce->vm->gt);
> > > > +	ce->ops->exit(ce);
> > >
> > > shouldn't these two be swapped?
>
> maybe I wasn't clear here... shouldn't it be

I missed this one.

> 	ce->ops->exit(ce);
> 	intel_gt_pm_put_async(ce->vm->gt);
>
> Don't we need to hold the pm until exiting?

I think it doesn't matter. The problematic edge case this is fixing is 
when ce->engine->gt is different from ce->vm->gt but at this point if it 
is safe to release one it must be safe to release the other too.

Regards,

Tvrtko


> > > >    }
> > > >    static inline struct intel_context *intel_context_get(struct intel_context *ce)
> > > > diff --git a/drivers/gpu/drm/i915/gt/intel_engine_pm.c b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> > > > index e971b153fda976..ee531a5c142c77 100644
> > > > --- a/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> > > > +++ b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> > > > @@ -114,6 +114,15 @@ __queue_and_release_pm(struct i915_request *rq,
> > > >    	ENGINE_TRACE(engine, "parking\n");
> > > > +	/*
> > > > +	 * Open coded one half of intel_context_enter, which we have to omit
> > > > +	 * here (see the large comment below) and because the other part must
> > > > +	 * not be called due constructing directly with __i915_request_create
> > > > +	 * which increments active count via intel_context_mark_active.
> > > > +	 */
> > > > +	GEM_BUG_ON(rq->context->active_count != 1);
> > > > +	__intel_gt_pm_get(engine->gt);
> > >
> > > where is it's brother "put"?
> >
> > It's in request retire via intel_context_exit. Ie. request construction is
> > special here, while retirement is standard.
>
> Thank you!
> Andi