drm/ttm: fix caching problem on non-PAT systems.

http://bugzilla.kernel.org/show_bug.cgi?id=15328

This fixes a serious regression on AGP/non-PAT systems, where
pages were ending up in the wrong state and slowing down the
whole system.

[airlied: taken this from the bug as the other option is to revert
the change which caused it].

Tested-by: John W. Linville (in bug).
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/ttm/ttm_tt.c

@@ -196,14 +196,15 @@ EXPORT_SYMBOL(ttm_tt_populate);
 
 #ifdef CONFIG_X86
 static inline int ttm_tt_set_page_caching(struct page *p,
-					  enum ttm_caching_state c_state)
+					  enum ttm_caching_state c_old,
+					  enum ttm_caching_state c_new)
 {
 	int ret = 0;
 
 	if (PageHighMem(p))
 		return 0;
 
-	if (get_page_memtype(p) != -1) {
+	if (c_old != tt_cached) {
 		/* p isn't in the default caching state, set it to
 		 * writeback first to free its current memtype. */
 
@@ -212,16 +213,17 @@ static inline int ttm_tt_set_page_caching(struct page *p,
 			return ret;
 	}
 
-	if (c_state == tt_wc)
+	if (c_new == tt_wc)
 		ret = set_memory_wc((unsigned long) page_address(p), 1);
-	else if (c_state == tt_uncached)
+	else if (c_new == tt_uncached)
 		ret = set_pages_uc(p, 1);
 
 	return ret;
 }
 #else /* CONFIG_X86 */
 static inline int ttm_tt_set_page_caching(struct page *p,
-					  enum ttm_caching_state c_state)
+					  enum ttm_caching_state c_old,
+					  enum ttm_caching_state c_new)
 {
 	return 0;
 }
@@ -254,7 +256,9 @@ static int ttm_tt_set_caching(struct ttm_tt *ttm,
 	for (i = 0; i < ttm->num_pages; ++i) {
 		cur_page = ttm->pages[i];
 		if (likely(cur_page != NULL)) {
-			ret = ttm_tt_set_page_caching(cur_page, c_state);
+			ret = ttm_tt_set_page_caching(cur_page,
+						      ttm->caching_state,
+						      c_state);
 			if (unlikely(ret != 0))
 				goto out_err;
 		}
@@ -268,7 +272,7 @@ out_err:
 	for (j = 0; j < i; ++j) {
 		cur_page = ttm->pages[j];
 		if (likely(cur_page != NULL)) {
-			(void)ttm_tt_set_page_caching(cur_page,
+			(void)ttm_tt_set_page_caching(cur_page, c_state,
 						      ttm->caching_state);
 		}
 	}