[PATCH 4/6] drm/i915: Trim the object sg table

[Tvrtko Ursulin]

From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>

After we have built the coallesced sg table of object pages
we can discard the over-allocated entries by copying the
table to a new, most compact one.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/i915_gem.c | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index d2ad73d0b5b9..250bcc79d6e5 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2232,6 +2232,39 @@ static unsigned int swiotlb_max_size(void)
 #endif
 }
 
+static struct sg_table *i915_sg_trim(struct sg_table *orig_st)
+{
+	struct sg_table *new_st;
+	struct scatterlist *sg, *new_sg;
+	unsigned int i;
+
+	if (orig_st->nents == orig_st->orig_nents)
+		return orig_st;
+
+	new_st = kmalloc(sizeof(*new_st), GFP_KERNEL);
+	if (new_st == NULL)
+		return orig_st;
+
+	if (sg_alloc_table(new_st, orig_st->nents, GFP_KERNEL)) {
+		kfree(new_st);
+		return orig_st;
+	}
+
+	new_sg = new_st->sgl;
+	for_each_sg(orig_st->sgl, sg, orig_st->nents, i) {
+		sg_set_page(new_sg, sg_page(sg), sg->length, 0);
+		new_sg = sg_next(new_sg);
+	}
+
+	if (new_sg)
+		sg_mark_end(new_sg);
+
+	sg_free_table(orig_st);
+	kfree(orig_st);
+
+	return new_st;
+}
+
 static struct sg_table *
 i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
 {
@@ -2317,6 +2350,8 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
 	if (sg) /* loop terminated early; short sg table */
 		sg_mark_end(sg);
 
+	st = i915_sg_trim(st);
+
 	ret = i915_gem_gtt_prepare_pages(obj, st);
 	if (ret)
 		goto err_pages;
-- 
2.7.4