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

[Chris Wilson]

Quoting Mika Kuoppala (2017-08-18 09:56:23)
> Michel Thierry <michel.thierry at 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 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>
> >> ---
> >>   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 at intel.com>
> 
> Nice find, possibly preventing quite amount of brain sprain.

Indeed it was.
 
> Acked-by: Mika Kuoppala <mika.kuoppala at intel.com>

I've pushed, but without the added references. I added the link to the
bugzilla just in case I am wrong.

Thanks for finding this bug Michel!
-Chris