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

Tvrtko Ursulin
2023-04-04T16:22:48Z

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