Quoting Mika Kuoppala (2017-08-18 09:56:23)
> 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.
Indeed it was.
> Acked-by: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
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
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