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

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Tvrtko Ursulin (2019-07-17 14:21:50)
> 
> 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.

The engines are still stopped. The functional change is to remove the
dangerous clearing of RING_HEAD/CTL.
-Chris
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