[Intel-gfx] [PATCH 1/3] drm/i915: Mark ininitial fb obj as WT on eLLC machines to avoid rcu lockup during fbdev init

[Chris Wilson]

See subject, s/ininitial/iniital/

Quoting Ville Syrjala (2020-10-07 13:03:27)
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> Currently we leave the cache_level of the initial fb obj
> set to NONE. This means on eLLC machines the first pin_to_display()
> will try to switch it to WT which requires a vma unbind+bind.
> If that happens during the fbdev initialization rcu does not
> seem operational which causes the unbind to get stuck. To
> most appearances this looks like a dead machine on boot.
> 
> Avoid the unbind by already marking the object cache_level
> as WT when creating it. We still do an excplicit ggtt pin
> which will rewrite the PTEs anyway, so they will match whatever
> cache level we set.
> 
> Cc: <stable at vger.kernel.org> # v5.7+
> Suggested-by: Chris Wilson <chris at chris-wilson.co.uk>
> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2381
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_display.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
> index 907e1d155443..00c08600c60a 100644
> --- a/drivers/gpu/drm/i915/display/intel_display.c
> +++ b/drivers/gpu/drm/i915/display/intel_display.c
> @@ -3445,6 +3445,14 @@ initial_plane_vma(struct drm_i915_private *i915,
>         if (IS_ERR(obj))
>                 return NULL;
>  
> +       /*
> +        * Mark it WT ahead of time to avoid changing the
> +        * cache_level during fbdev initialization. The
> +        * unbind there would get stuck waiting for rcu.
> +        */
> +       i915_gem_object_set_cache_coherency(obj, HAS_WT(i915) ?
> +                                           I915_CACHE_WT : I915_CACHE_NONE);

Ok, I've been worrying about whether there were any more side-effects,
but I think it all comes out in the wash. The proof is definitely in the
eating, and we will know soon enough if we break someone's virtual
terminal.

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris