Re: [PATCH v3 2/3] drm/i915: Do not WARN_ON in i915_vm_to_ppgtt

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On 01/04/16 08:46, Joonas Lahtinen wrote:
According to Chris, use of i915_vm_to_ppgtt is visible in benchmark
unless WARN_ON is removed, so lets get rid of it.

Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Reported-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
---
  drivers/gpu/drm/i915/i915_drv.h | 1 -
  1 file changed, 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index d3ebb2f..0035dc3 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -3157,7 +3157,6 @@ bool i915_gem_obj_is_pinned(struct drm_i915_gem_object *obj);
  static inline struct i915_hw_ppgtt *
  i915_vm_to_ppgtt(struct i915_address_space *vm)
  {
-	WARN_ON(i915_is_ggtt(vm));
  	return container_of(vm, struct i915_hw_ppgtt, base);
  }

I shouldn't think the actual check (i915_is_ggtt()) is slowing things down, so it must be the overhead of an unactivated WARN_ON(), which probably generates quite a bit of code and may disable optimisations, even when the code block isn't entered. As an alternative (if you *want* things to break if this is used incorrectly) you could have it return NULL for that case:

  static inline struct i915_hw_ppgtt *
  i915_vm_to_ppgtt(struct i915_address_space *vm)
  {
    return i915_is_ggtt(vm) ? NULL :
           container_of(vm, struct i915_hw_ppgtt, base);
  }

Caller will then very likely get an OOPS!

.Dave.
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