Michel Thierry <michel.thierry@xxxxxxxxx> 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@xxxxxxxxxxxxxxx>
>> 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@xxxxxxxxx>
>> Fixes: 142bc7d99bcf ("drm/i915: Modify error handler for per engine hang recovery")
>> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
>> Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
>> Cc: Michel Thierry <michel.thierry@xxxxxxxxx>
>> ---
>> 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@xxxxxxxxx>
Nice find, possibly preventing quite amount of brain sprain.
Acked-by: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
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