Quoting Mika Kuoppala (2018-08-10 15:00:36)
> If engine reports that it is not ready for reset, we
> give up. Evidence shows that forcing a per engine reset
> on an engine which is not reporting to be ready for reset,
> can bring it back into a working order. There is risk that
> we corrupt the context image currently executing on that
> engine. But that is a risk worth taking as if we unblock
> the engine, we prevent a whole device wedging in a case
> of full gpu reset.
> 
> Reset individual engine even if it reports that it is not
> prepared for reset, but only if we aim for full gpu reset
> and not on first reset attempt.
> 
> v2: force reset only on later attempts, readability (Chris)
> 
> Cc: Chris Wilson <ch...@chris-wilson.co.uk>
> Signed-off-by: Mika Kuoppala <mika.kuopp...@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/intel_uncore.c | 49 +++++++++++++++++++++++------
>  1 file changed, 39 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_uncore.c 
> b/drivers/gpu/drm/i915/intel_uncore.c
> index 027d14574bfa..d24026839b17 100644
> --- a/drivers/gpu/drm/i915/intel_uncore.c
> +++ b/drivers/gpu/drm/i915/intel_uncore.c
> @@ -2099,9 +2099,6 @@ static int gen8_reset_engine_start(struct 
> intel_engine_cs *engine)
>                                            RESET_CTL_READY_TO_RESET,
>                                            700, 0,
>                                            NULL);
> -       if (ret)
> -               DRM_ERROR("%s: reset request timeout\n", engine->name);
> -
>         return ret;
>  }
>  
> @@ -2113,6 +2110,42 @@ static void gen8_reset_engine_cancel(struct 
> intel_engine_cs *engine)
>                       _MASKED_BIT_DISABLE(RESET_CTL_REQUEST_RESET));
>  }
>  
> +static int reset_engines(struct drm_i915_private *i915,
> +                        unsigned int engine_mask,
> +                        unsigned int retry)
> +{
> +       if (INTEL_GEN(i915) >= 11)
> +               return gen11_reset_engines(i915, engine_mask);
> +       else
> +               return gen6_reset_engines(i915, engine_mask, retry);
> +}
> +
> +static int gen8_prepare_engine_for_reset(struct intel_engine_cs *engine,
> +                                        unsigned int retry)
> +{
> +       const bool force_reset_on_non_ready = retry >= 1;
> +       int ret;
> +
> +       ret = gen8_reset_engine_start(engine);
> +
> +       if (ret && force_reset_on_non_ready) {
> +               /*
> +                * Try to unblock a single non-ready engine by risking
> +                * context corruption.
> +                */
> +               ret = reset_engines(engine->i915,
> +                                   intel_engine_flag(engine),
> +                                   retry);
> +               if (!ret)
> +                       ret = gen8_reset_engine_start(engine);
> +
> +               DRM_ERROR("%s: reset request timeout, forcing reset (%d)\n",
> +                         engine->name, ret);

This looks dubious now ;)

After the force you then do a reset in the caller. Twice the reset for
twice the unpreparedness.

> +       }
> +
> +       return ret;
> +}
> +
>  static int gen8_reset_engines(struct drm_i915_private *dev_priv,
>                               unsigned int engine_mask,
>                               unsigned int retry)
> @@ -2122,16 +2155,12 @@ static int gen8_reset_engines(struct drm_i915_private 
> *dev_priv,
>         int ret;
>  
>         for_each_engine_masked(engine, dev_priv, engine_mask, tmp) {
> -               if (gen8_reset_engine_start(engine)) {
> -                       ret = -EIO;
> +               ret = gen8_prepare_engine_for_reset(engine, retry);
> +               if (ret)

if (ret && !force_reset_unread)

>                         goto not_ready;
> -               }
>         }
>  
> -       if (INTEL_GEN(dev_priv) >= 11)
> -               ret = gen11_reset_engines(dev_priv, engine_mask);
> -       else
> -               ret = gen6_reset_engines(dev_priv, engine_mask, retry);
> +       ret = reset_engines(dev_priv, engine_mask, retry);
>  
>  not_ready:
>         for_each_engine_masked(engine, dev_priv, engine_mask, tmp)
> -- 
> 2.17.1
> 
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