Re: [PATCH 2/5] drm/i915/gt: Push engine stopping into reset-prepare

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

On 17/07/2019 14:08, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2019-07-17 14:04:34)
>>
>> On 16/07/2019 13:49, Chris Wilson wrote:
>>> Push the engine stop into the back reset_prepare (where it already was!)
>>> This allows us to avoid dangerously setting the RING registers to 0 for
>>> logical contexts. If we clear the register on a live context, those
>>> invalid register values are recorded in the logical context state and
>>> replayed (with hilarious results).
>>
>> So essentially statement is gen3_stop_engine is not needed and even
>> dangerous with execlists?
> 
> Yes. It has been a nuisance in the past, which is why we try to avoid
> it. I have come to conclusion that it serves no purpose for execlists
> and only makes recovery worse.
> 
>>
>>>
>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>>> ---
>>>    drivers/gpu/drm/i915/gt/intel_lrc.c        | 16 +++++-
>>>    drivers/gpu/drm/i915/gt/intel_reset.c      | 58 ----------------------
>>>    drivers/gpu/drm/i915/gt/intel_ringbuffer.c | 40 ++++++++++++++-
>>>    3 files changed, 53 insertions(+), 61 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
>>> index 9e0992498087..9b87a2fc186c 100644
>>> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
>>> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
>>> @@ -2173,11 +2173,23 @@ static void execlists_reset_prepare(struct intel_engine_cs *engine)
>>>        __tasklet_disable_sync_once(&execlists->tasklet);
>>>        GEM_BUG_ON(!reset_in_progress(execlists));
>>>    
>>> -     intel_engine_stop_cs(engine);
>>> -
>>>        /* And flush any current direct submission. */
>>>        spin_lock_irqsave(&engine->active.lock, flags);
>>>        spin_unlock_irqrestore(&engine->active.lock, flags);
>>> +
>>> +     /*
>>> +      * We stop engines, otherwise we might get failed reset and a
>>> +      * dead gpu (on elk). Also as modern gpu as kbl can suffer
>>> +      * from system hang if batchbuffer is progressing when
>>> +      * the reset is issued, regardless of READY_TO_RESET ack.
>>> +      * Thus assume it is best to stop engines on all gens
>>> +      * where we have a gpu reset.
>>> +      *
>>> +      * WaKBLVECSSemaphoreWaitPoll:kbl (on ALL_ENGINES)
>>> +      *
>>> +      * FIXME: Wa for more modern gens needs to be validated
>>> +      */
>>> +     intel_engine_stop_cs(engine);
>>>    }
>>>    
>>>    static void reset_csb_pointers(struct intel_engine_cs *engine)
>>> diff --git a/drivers/gpu/drm/i915/gt/intel_reset.c b/drivers/gpu/drm/i915/gt/intel_reset.c
>>> index 7ddedfb16aa2..55e2ddcbd215 100644
>>> --- a/drivers/gpu/drm/i915/gt/intel_reset.c
>>> +++ b/drivers/gpu/drm/i915/gt/intel_reset.c
>>> @@ -135,47 +135,6 @@ void __i915_request_reset(struct i915_request *rq, bool guilty)
>>>        }
>>>    }
>>>    
>>> -static void gen3_stop_engine(struct intel_engine_cs *engine)
>>> -{
>>> -     struct intel_uncore *uncore = engine->uncore;
>>> -     const u32 base = engine->mmio_base;
>>> -
>>> -     GEM_TRACE("%s\n", engine->name);
>>> -
>>> -     if (intel_engine_stop_cs(engine))
>>> -             GEM_TRACE("%s: timed out on STOP_RING\n", engine->name);
>>> -
>>> -     intel_uncore_write_fw(uncore,
>>> -                           RING_HEAD(base),
>>> -                           intel_uncore_read_fw(uncore, RING_TAIL(base)));
>>> -     intel_uncore_posting_read_fw(uncore, RING_HEAD(base)); /* paranoia */
>>> -
>>> -     intel_uncore_write_fw(uncore, RING_HEAD(base), 0);
>>> -     intel_uncore_write_fw(uncore, RING_TAIL(base), 0);
>>> -     intel_uncore_posting_read_fw(uncore, RING_TAIL(base));
>>> -
>>> -     /* The ring must be empty before it is disabled */
>>> -     intel_uncore_write_fw(uncore, RING_CTL(base), 0);
>>> -
>>> -     /* Check acts as a post */
>>> -     if (intel_uncore_read_fw(uncore, RING_HEAD(base)))
>>> -             GEM_TRACE("%s: ring head [%x] not parked\n",
>>> -                       engine->name,
>>> -                       intel_uncore_read_fw(uncore, RING_HEAD(base)));
>>> -}
>>> -
>>> -static void stop_engines(struct intel_gt *gt, intel_engine_mask_t engine_mask)
>>> -{
>>> -     struct intel_engine_cs *engine;
>>> -     intel_engine_mask_t tmp;
>>> -
>>> -     if (INTEL_GEN(gt->i915) < 3)
>>> -             return;
>>> -
>>> -     for_each_engine_masked(engine, gt->i915, engine_mask, tmp)
>>> -             gen3_stop_engine(engine);
>>> -}
>>> -
>>>    static bool i915_in_reset(struct pci_dev *pdev)
>>>    {
>>>        u8 gdrst;
>>> @@ -607,23 +566,6 @@ int __intel_gt_reset(struct intel_gt *gt, intel_engine_mask_t engine_mask)
>>>         */
>>>        intel_uncore_forcewake_get(gt->uncore, FORCEWAKE_ALL);
>>>        for (retry = 0; ret == -ETIMEDOUT && retry < retries; retry++) {
>>> -             /*
>>> -              * We stop engines, otherwise we might get failed reset and a
>>> -              * dead gpu (on elk). Also as modern gpu as kbl can suffer
>>> -              * from system hang if batchbuffer is progressing when
>>> -              * the reset is issued, regardless of READY_TO_RESET ack.
>>> -              * Thus assume it is best to stop engines on all gens
>>> -              * where we have a gpu reset.
>>> -              *
>>> -              * WaKBLVECSSemaphoreWaitPoll:kbl (on ALL_ENGINES)
>>> -              *
>>> -              * WaMediaResetMainRingCleanup:ctg,elk (presumably)
>>> -              *
>>> -              * FIXME: Wa for more modern gens needs to be validated
>>> -              */
>>> -             if (retry)
>>> -                     stop_engines(gt, engine_mask);
>>> -
>>
>> Only other functional change I see is that we stop retrying to stop the
>> engines before reset attempts. I don't know if that is a concern or not.
> 
> Ah, but we do stop the engine before resets in *reset_prepare. The other
> path to arrive is in sanitize where we don't know enough state to safely
> tweak the engines. For those, I claim it shouldn't matter as the engines
> should be idle and we only need the reset to clear stale context state.

Yes I know that we do call stop in prepare, just not on the reset retry 
path. It's the above loop, if reset was failing and needed retries 
before we would re-retried stopping engines and now we would not.

Regards,

Tvrtko



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