[Intel-gfx] [PATCH] drm/i915: Stop touching forcewake following a gen6+ engine reset

[Daniel Vetter]

On Thu, Aug 17, 2017 at 06:32:29PM +0100, Chris Wilson wrote:
> Forcewake is not affected by the engine reset on gen6+. Indeed the
> reason why we added intel_uncore_forcewake_reset() to
> gen6_reset_engines() was to keep the bookkeeping intact because the
> reset did not touch the forcewake bit (yet we cancelled the forcewake
> consumers)!  This was done in commit 521198a2e7095:
>     Author: Mika Kuoppala <mika.kuoppala at linux.intel.com>
>     Date:   Fri Aug 23 16:52:30 2013 +0300
> 
> 	drm/i915: sanitize forcewake registers on reset
> 
> 	In reset we try to restore the forcewake state to
> 	pre reset state, using forcewake_count. The reset
> 	doesn't seem to clear the forcewake bits so we
> 	get warn on forcewake ack register not clearing.
> 
> That futzing of the forcewake bookkeeping was dropped in commit
> 0294ae7b44bb ("drm/i915: Consolidate forcewake resetting to a single
> function"), but it did not make the realisation that the remaining
> intel_uncore_forcewake_reset() was redundant.
> 
> The new danger with using intel_uncore_forcewake_reset() with per-engine
> resets is that the driver and hw are still in an active state as we
> perform the reset. We may be using the forcewake to read protected
> registers elsewhere and those results may be clobbered by the concurrent
> dropping of forcewake.
> 
> Reported-by: Michel Thierry <michel.thierry at intel.com>
> Fixes: 142bc7d99bcf ("drm/i915: Modify error handler for per engine hang recovery")
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> Cc: Michel Thierry <michel.thierry at intel.com>

Could this explain some of the random unclaimed register failures we're
seeing on hsw shard CI?

https://bugs.freedesktop.org/show_bug.cgi?id=102249

Otoh this is in the display well, but maybe we're just racing and it's
just a stale bit in the unclaimed mmio debugging stuff. Worth a
References: I think.
-Daniel
> ---
>  drivers/gpu/drm/i915/intel_uncore.c | 7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_uncore.c b/drivers/gpu/drm/i915/intel_uncore.c
> index deb4430541cf..1d7b879cc68c 100644
> --- a/drivers/gpu/drm/i915/intel_uncore.c
> +++ b/drivers/gpu/drm/i915/intel_uncore.c
> @@ -1497,7 +1497,6 @@ static int gen6_reset_engines(struct drm_i915_private *dev_priv,
>  		[VECS] = GEN6_GRDOM_VECS,
>  	};
>  	u32 hw_mask;
> -	int ret;
>  
>  	if (engine_mask == ALL_ENGINES) {
>  		hw_mask = GEN6_GRDOM_FULL;
> @@ -1509,11 +1508,7 @@ static int gen6_reset_engines(struct drm_i915_private *dev_priv,
>  			hw_mask |= hw_engine_mask[engine->id];
>  	}
>  
> -	ret = gen6_hw_domain_reset(dev_priv, hw_mask);
> -
> -	intel_uncore_forcewake_reset(dev_priv, true);
> -
> -	return ret;
> +	return gen6_hw_domain_reset(dev_priv, hw_mask);
>  }
>  
>  /**
> -- 
> 2.14.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch