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drm/i915/gt: Perform an arbitration check before busywaiting
During igt_reset_nop_engine, it was observed that an unexpected failed engine reset lead to us busywaiting on the stop-ring semaphore (set during the reset preparations) on the first request afterwards. There was no explicit MI_ARB_CHECK in this sequence as the presumption was that the failed MI_SEMAPHORE_WAIT would itself act as an arbitration point. It did not in this circumstance, so force it. This patch is based on the assumption that the MI_SEMAPHORE_WAIT failure to arbitrate is a rare Tigerlake bug, similar to the lite-restore vs semaphore issues previously seen in the CS. The explicit MI_ARB_CHECK should always ensure that there is at least one arbitration point in the request before the MI_SEMAPHORE_WAIT to trigger the IDLE->ACTIVE event. Upon processing that event, we will clear the stop-ring flag and release the semaphore from its busywait. Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20210112100759.32698-2-chris@chris-wilson.co.uk
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@ -578,6 +578,7 @@ u32 *gen11_emit_fini_breadcrumb_rcs(struct i915_request *rq, u32 *cs)
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static u32 *gen12_emit_preempt_busywait(struct i915_request *rq, u32 *cs)
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{
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*cs++ = MI_ARB_CHECK; /* trigger IDLE->ACTIVE first */
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*cs++ = MI_SEMAPHORE_WAIT_TOKEN |
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MI_SEMAPHORE_GLOBAL_GTT |
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MI_SEMAPHORE_POLL |
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@ -586,7 +587,6 @@ static u32 *gen12_emit_preempt_busywait(struct i915_request *rq, u32 *cs)
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*cs++ = preempt_address(rq->engine);
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*cs++ = 0;
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*cs++ = 0;
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*cs++ = MI_NOOP;
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return cs;
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}
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