[Intel-gfx] [PATCH] drm/i915: Prevent TLB error on first execution on SNB

[Daniel Vetter]

On Fri, Feb 13, 2015 at 12:59:45PM +0000, Chris Wilson wrote:
> Long ago I found that I was getting sporadic errors when booting SNB,
> with the symptom being that the first batch died with IPEHR != *ACTHD,
> typically caused by the TLB being invalid. These magically disappeared
> if I held the forcewake during the entire ring initialisation sequence.
> (It can probably be shortened to a short critical section, but the whole
> initialisation is full of register writes and so we would be taking and
> releasing forcewake almost continually, and so holding it over the
> entire sequence will probably be a net win!)
> 
> Note some of the kernels I encounted the issue already had the deferred
> forcewake release, so it is still relevant.
> 
> I know that there have been a few other reports with similar failure
> conditions on SNB, I think such as
> References: https://bugs.freedesktop.org/show_bug.cgi?id=80913
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>

Given that we've already added a forcewake critical section around
individual ring inits this makes maybe a bit too much sense. But I do
wonder whether we don't need the same for resume and gpu resets?

With the split into hw/sw setup we could get that by pusing the
forcewake_get/put inti i915_gem_init_hw. Does the magic still work with
that? And if we put it there there fw_get/put in init_ring_common is fully
redundant and could be remove.
-Daniel

> ---
>  drivers/gpu/drm/i915/i915_gem.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 8d15c8110962..2426f6d9b5a5 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -4868,6 +4868,7 @@ int i915_gem_init(struct drm_device *dev)
>  		dev_priv->gt.stop_ring = intel_logical_ring_stop;
>  	}
>  
> +	intel_uncore_forcewake_get(dev_priv, FORCEWAKE_ALL);
>  	ret = i915_gem_init_userptr(dev);
>  	if (ret)
>  		goto out_unlock;
> @@ -4894,6 +4895,7 @@ int i915_gem_init(struct drm_device *dev)
>  	}
>  
>  out_unlock:
> +	intel_uncore_forcewake_put(dev_priv, FORCEWAKE_ALL);
>  	mutex_unlock(&dev->struct_mutex);
>  
>  	return ret;
> -- 
> 2.1.4
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch