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@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>
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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> Intel-gfx@xxxxxxxxxxxxxxxxxxxxx
> https://lists.freedesktop.org/mailman/listinfo/intel-gfx
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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