drm/i915: Cache last obj->pages location for i915_gem_object_get_page()

The biggest user of i915_gem_object_get_page() is the relocation
processing during execbuffer. Typically userspace passes in a set of
relocations in sorted order. Sadly, we alternate between relocations
increasing from the start of the buffers, and relocations decreasing
from the end. However the majority of consecutive lookups will still be
in the same page. We could cache the start of the last sg chain, however
for most callers, the entire sgl is inside a single chain and so we see
no improve from the extra layer of caching.

v2: Avoid the double increment inside unlikely()

References: https://bugs.freedesktop.org/show_bug.cgi?id=88308
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -1986,6 +1986,10 @@ struct drm_i915_gem_object {
 
 	struct sg_table *pages;
 	int pages_pin_count;
+	struct get_page {
+		struct scatterlist *sg;
+		int last;
+	} get_page;
 
 	/* prime dma-buf support */
 	void *dma_buf_vmapping;
@@ -2656,15 +2660,32 @@ int i915_gem_obj_prepare_shmem_read(struct drm_i915_gem_object *obj,
 				    int *needs_clflush);
 
 int __must_check i915_gem_object_get_pages(struct drm_i915_gem_object *obj);
-static inline struct page *i915_gem_object_get_page(struct drm_i915_gem_object *obj, int n)
+
+static inline int __sg_page_count(struct scatterlist *sg)
 {
-	struct sg_page_iter sg_iter;
+	return sg->length >> PAGE_SHIFT;
-
-	for_each_sg_page(obj->pages->sgl, &sg_iter, obj->pages->nents, n)
-		return sg_page_iter_page(&sg_iter);
-
-	return NULL;
 }
+
+static inline struct page *
+i915_gem_object_get_page(struct drm_i915_gem_object *obj, int n)
+{
+	if (WARN_ON(n >= obj->base.size >> PAGE_SHIFT))
+		return NULL;
+
+	if (n < obj->get_page.last) {
+		obj->get_page.sg = obj->pages->sgl;
+		obj->get_page.last = 0;
+	}
+
+	while (obj->get_page.last + __sg_page_count(obj->get_page.sg) <= n) {
+		obj->get_page.last += __sg_page_count(obj->get_page.sg++);
+		if (unlikely(sg_is_chain(obj->get_page.sg)))
+			obj->get_page.sg = sg_chain_ptr(obj->get_page.sg);
+	}
+
+	return nth_page(sg_page(obj->get_page.sg), n - obj->get_page.last);
+}
+
 static inline void i915_gem_object_pin_pages(struct drm_i915_gem_object *obj)
 {
 	BUG_ON(obj->pages == NULL);

drivers/gpu/drm/i915/i915_gem.c

@@ -2178,6 +2178,10 @@ i915_gem_object_get_pages(struct drm_i915_gem_object *obj)
 		return ret;
 
 	list_add_tail(&obj->global_list, &dev_priv->mm.unbound_list);
+
+	obj->get_page.sg = obj->pages->sgl;
+	obj->get_page.last = 0;
+
 	return 0;
 }