[Intel-gfx] [PATCH] drm/i915: Skip unbinding large unmappable global buffers

[Chris Wilson]

On Thu, Oct 13, 2016 at 12:29:44PM +0100, Tvrtko Ursulin wrote:
> 
> On 13/10/2016 09:55, Chris Wilson wrote:
> >If the user requests a mappable binding to the global GTT, we will first
> >unbind an existing mapping if it doesn't match. We will unbind even if
> >there is no possibility that the object can fit in the mappable
> >aperture. This may lead to a ping-pong migration of the object, for
> >example igt/gem_exec_big.
> >
> >Testcase: igt/gem_exec_big
> >Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> >---
> >  drivers/gpu/drm/i915/i915_gem.c | 19 ++++++++++++++++++-
> >  1 file changed, 18 insertions(+), 1 deletion(-)
> >
> >diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> >index 2f939f90984b..f33aa9bb95c0 100644
> >--- a/drivers/gpu/drm/i915/i915_gem.c
> >+++ b/drivers/gpu/drm/i915/i915_gem.c
> >@@ -3144,6 +3144,9 @@ search_free:
> >  			goto err_unpin;
> >  		}
> >+
> >+		GEM_BUG_ON(vma->node.start < start);
> >+		GEM_BUG_ON(vma->node.start + vma->node.size > end);
> >  	}
> >  	GEM_BUG_ON(!i915_gem_valid_gtt_space(vma, obj->cache_level));
> >@@ -3843,7 +3846,8 @@ i915_gem_object_ggtt_pin(struct drm_i915_gem_object *obj,
> >  			 u64 alignment,
> >  			 u64 flags)
> >  {
> >-	struct i915_address_space *vm = &to_i915(obj->base.dev)->ggtt.base;
> >+	struct drm_i915_private *dev_priv = to_i915(obj->base.dev);
> >+	struct i915_address_space *vm = &dev_priv->ggtt.base;
> >  	struct i915_vma *vma;
> >  	int ret;
> >@@ -3858,6 +3862,19 @@ i915_gem_object_ggtt_pin(struct drm_i915_gem_object *obj,
> >  		    (i915_vma_is_pinned(vma) || i915_vma_is_active(vma)))
> >  			return ERR_PTR(-ENOSPC);
> >+		if (flags & PIN_MAPPABLE) {
> >+			u32 fence_size;
> >+
> >+			fence_size = i915_gem_get_ggtt_size(dev_priv, vma->size,
> >+							    i915_gem_object_get_tiling(obj));
> >+			if (fence_size > dev_priv->ggtt.mappable_end)
> >+				return ERR_PTR(-E2BIG);
> >+
> >+			if (flags & PIN_NONBLOCK &&
> >+			    fence_size > dev_priv->ggtt.mappable_end / 2)
> >+				return ERR_PTR(-ENOSPC);
> 
> One half of aperture is a magic number or something else?

Magic number. NONBLOCK means try, we have a fallback planned.

It more or less comes from the old habit of having to assume that all
fences take up twice the space we expect due to alignment. And given
NONBLOCK means that we should have a better fallback than evicting the
majority of the aperture on a whim.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre