ACK/Cmnt: [SRU][Groovy][PATCH] drm/i915: Mark ininitial fb obj as WT on eLLC machines to avoid rcu lockup during fbdev init

[Stefan Bader]

On 20.11.20 18:21, Kamal Mostafa wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> BugLink: https://bugs.launchpad.net/bugs/1903397
> 
> 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>
> Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Link: https://patchwork.freedesktop.org/patch/msgid/20201007120329.17076-1-ville.syrjala@linux.intel.com
> Link: https://patchwork.freedesktop.org/patch/msgid/20201015122138.30161-1-chris@chris-wilson.co.uk
> (cherry picked from commit d46b60a2e8d246f1f0faa38e52f4f5a73858c338)
> Signed-off-by: Rodrigo Vivi <rodrigo.vivi at intel.com>
> (cherry picked from commit 1664ffee760a5d98952318fdd9b198fae396d660)
> Signed-off-by: Kamal Mostafa <kamal at canonical.com>
Acked-by: Stefan Bader <stefan.bader at canonical.com>
> ---

Maybe the bug report (at least the justification) could be clarified to the fact
that this is something appearing to be a hang but in reality it is no video output.

-Stefan

>  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 26996e1839e2..4bd383ff0880 100644
> --- a/drivers/gpu/drm/i915/display/intel_display.c
> +++ b/drivers/gpu/drm/i915/display/intel_display.c
> @@ -3431,6 +3431,14 @@ 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);
> +
>  	switch (plane_config->tiling) {
>  	case I915_TILING_NONE:
>  		break;
> 


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