Re: [PATCH 03/14] drm/i915/execlists: Flush the tasklet on parking

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 02/05/2019 15:21, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2019-05-02 15:14:08)
>>
>> On 02/05/2019 14:53, Chris Wilson wrote:
>>> Quoting Tvrtko Ursulin (2019-05-02 14:48:18)
>>>>
>>>> On 01/05/2019 12:45, Chris Wilson wrote:
>>>>> Tidy up the cleanup sequence by always ensure that the tasklet is
>>>>> flushed on parking (before we cleanup). The parking provides a
>>>>> convenient point to ensure that the backend is truly idle.
>>>>>
>>>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>>>>> ---
>>>>>     drivers/gpu/drm/i915/gt/intel_lrc.c         | 7 ++++++-
>>>>>     drivers/gpu/drm/i915/intel_guc_submission.c | 1 +
>>>>>     2 files changed, 7 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
>>>>> index 851e62ddcb87..7be54b868d8e 100644
>>>>> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
>>>>> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
>>>>> @@ -2331,6 +2331,11 @@ static int gen8_init_rcs_context(struct i915_request *rq)
>>>>>         return i915_gem_render_state_emit(rq);
>>>>>     }
>>>>>     
>>>>> +static void execlists_park(struct intel_engine_cs *engine)
>>>>> +{
>>>>> +     tasklet_kill(&engine->execlists.tasklet);
>>>>
>>>> Isn't it actually a problem if tasklet is scheduled and unstarted, or
>>>> even in progress at the point of engine getting parked?
>>>
>>> That would be a broken driver. :|
>>>
>>> We must be quite sure that engine isn't going to send an interrupt as we
>>> are just about to drop the wakeref we need to service that interrupt.
>>>
>>> tasklet_kill()
>>> GEM_BUG_ON(engine->execlists.active);
>>
>> Or instead of both:
>>
>> /* Tasklet must not be running or scheduled at this point. */
>> GEM_BUG_ON(engine->execlists.tasklet.state);
> 
> There's the dilemma that we start parking based on retirement not
> final CS event.

But engine->park() is called once the last engine pm reference is 
dropped. Are we dropping the last reference with a CS event pending?

Regards,

Tvrtko
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