Re: [Intel-gfx] [PATCH 3/4] drm/i915/gt: Perform an arbitration check before busywaiting

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

On 11/01/2021 10:57, Chris Wilson wrote:
> 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.

In other words MI_SEMAPHORE_POLL is not a preemption point? Can't 
remember if I knew that or not..

1)
Why not the same handling in !gen12 version?

2)
Failed reset leads to busy-hang in following request _tail_? But there 
is an arb check at the start of following request as well. Or in cases 
where we context switch into the middle of a previously executing request?

But why would that busy hang? Hasn't the failed request unpaused the ring?

Regards,

Tvrtko

> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/i915/gt/gen8_engine_cs.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/gen8_engine_cs.c b/drivers/gpu/drm/i915/gt/gen8_engine_cs.c
> index 9a182652a35e..e9ac281644cc 100644
> --- a/drivers/gpu/drm/i915/gt/gen8_engine_cs.c
> +++ b/drivers/gpu/drm/i915/gt/gen8_engine_cs.c
> @@ -567,6 +567,7 @@ u32 *gen11_emit_fini_breadcrumb_rcs(struct i915_request *rq, u32 *cs)
>   
>   static u32 *gen12_emit_preempt_busywait(struct i915_request *rq, u32 *cs)
>   {
> +	*cs++ = MI_ARB_CHECK;
>   	*cs++ = MI_SEMAPHORE_WAIT_TOKEN |
>   		MI_SEMAPHORE_GLOBAL_GTT |
>   		MI_SEMAPHORE_POLL |
> @@ -575,7 +576,6 @@ static u32 *gen12_emit_preempt_busywait(struct i915_request *rq, u32 *cs)
>   	*cs++ = preempt_address(rq->engine);
>   	*cs++ = 0;
>   	*cs++ = 0;
> -	*cs++ = MI_NOOP;
>   
>   	return cs;
>   }
> 
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