[Intel-gfx] [RFC PATCH] drm/i915: Prefault the entire object on first page fault

[Chris Wilson]

Inserting additional PTEs has no side-effect for us as the pfn are fixed
for the entire time the object is resident in the global GTT. The
downside is that we pay the entire cost of faulting the object upon the
first hit, for which we in return receive the benefit of removing the
per-page faulting overhead.

On an Ivybridge i7-3720qm with 1600MHz DDR3, with 32 fences,
Upload rate for 2 linear surfaces:	8127MiB/s -> 8134MiB/s
Upload rate for 2 tiled surfaces:	8607MiB/s -> 8625MiB/s
Upload rate for 4 linear surfaces:	8127MiB/s -> 8127MiB/s
Upload rate for 4 tiled surfaces:	8611MiB/s -> 8602MiB/s
Upload rate for 8 linear surfaces:	8114MiB/s -> 8124MiB/s
Upload rate for 8 tiled surfaces:	8601MiB/s -> 8603MiB/s
Upload rate for 16 linear surfaces:	8110MiB/s -> 8123MiB/s
Upload rate for 16 tiled surfaces:	8595MiB/s -> 8606MiB/s
Upload rate for 32 linear surfaces:	8104MiB/s -> 8121MiB/s
Upload rate for 32 tiled surfaces:	8589MiB/s -> 8605MiB/s
Upload rate for 64 linear surfaces:	8107MiB/s -> 8121MiB/s
Upload rate for 64 tiled surfaces:	2013MiB/s -> 3017MiB/s

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: "Goel, Akash" <akash.goel at intel.com>
---

It survived light testing without noticable performance degradation. Can
anyone think of how this will impact us negatively?
-Chris

---
 drivers/gpu/drm/i915/i915_gem.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 14722f48fcd8..e0b3cda19363 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1514,14 +1514,26 @@ int i915_gem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
 	if (ret)
 		goto unpin;
 
-	obj->fault_mappable = true;
-
+	/* Finally, remap it using the new GTT offset */
 	pfn = dev_priv->gtt.mappable_base + i915_gem_obj_ggtt_offset(obj);
 	pfn >>= PAGE_SHIFT;
-	pfn += page_offset;
 
-	/* Finally, remap it using the new GTT offset */
-	ret = vm_insert_pfn(vma, (unsigned long)vmf->virtual_address, pfn);
+	if (!obj->fault_mappable) {
+		int i;
+
+		for (i = 0; i < obj->base.size >> PAGE_SHIFT; i++) {
+			ret = vm_insert_pfn(vma,
+					    (unsigned long)vma->vm_start + i * PAGE_SIZE,
+					    pfn + i);
+			if (ret)
+				break;
+		}
+
+		obj->fault_mappable = true;
+	} else
+		ret = vm_insert_pfn(vma,
+				    (unsigned long)vmf->virtual_address,
+				    pfn + page_offset);
 unpin:
 	i915_gem_object_ggtt_unpin(obj);
 unlock:
-- 
1.9.rc1