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

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Michel Thierry <michel.thierry@intel.com> writes:

> On 17/08/17 10:32, 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@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@intel.com>
>> Fixes: 142bc7d99bcf ("drm/i915: Modify error handler for per engine hang recovery")
>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
>> Cc: Michel Thierry <michel.thierry@intel.com>
>> ---
>>   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);
>>   }
>>   
>>   /**
>> 
>
> Thanks for digging out the commit history.
>
> Reviewed-by Michel Thierry <michel.thierry@intel.com>

Nice find, possibly preventing quite amount of brain sprain.

Acked-by: Mika Kuoppala <mika.kuoppala@intel.com>

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