[Intel-gfx] [PATCH] drm/i915: Trim the object sg table

[Tvrtko Ursulin]

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

At the moment we allocate enough sg table entries assuming we
will not be able to do any coallescing. But since in practice
we most often can, and more so very effectively, this ends up
wasting a lot of memory.

A simple and effective way of trimming the over-allocated
entries is to copy the table over to a new one allocated to the
exact size.

Experiment on my freshly logged and idle desktop (KDE) showed
that by doing this we can save approximately 1 MiB of RAM, or
when running a typical benchmark like gl_manhattan I have
even seen a 6 MiB saving.

v2:
 * Update commit message.
 * Use temporary sg_table on stack. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index d2ad73d0b5b9..411aae535abe 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2232,6 +2232,28 @@ static unsigned int swiotlb_max_size(void)
 #endif
 }
 
+static void 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;
+
+	if (sg_alloc_table(&new_st, orig_st->nents, GFP_KERNEL))
+		return;
+
+	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);
+	}
+
+	sg_free_table(orig_st);
+	memcpy(orig_st, &new_st, sizeof(*orig_st));
+}
+
 static struct sg_table *
 i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
 {
@@ -2317,6 +2339,9 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
 	if (sg) /* loop terminated early; short sg table */
 		sg_mark_end(sg);
 
+	/* Trim unused sg entries to avoid wasting memory. */
+	i915_sg_trim(st);
+
 	ret = i915_gem_gtt_prepare_pages(obj, st);
 	if (ret)
 		goto err_pages;
-- 
2.7.4