drm/nouveau: Make use of TTM busy_placements.

Previously we were filling it the same as "placements", but in some
cases there're valid alternatives that we were ignoring completely.
Keeping a back-up memory type helps on several low-mem situations.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
diff --git a/drivers/gpu/drm/nouveau/nouveau_gem.c b/drivers/gpu/drm/nouveau/nouveau_gem.c
index 0d22f66..1bc0b38 100644
--- a/drivers/gpu/drm/nouveau/nouveau_gem.c
+++ b/drivers/gpu/drm/nouveau/nouveau_gem.c
@@ -180,40 +180,35 @@
 {
 	struct nouveau_bo *nvbo = gem->driver_private;
 	struct ttm_buffer_object *bo = &nvbo->bo;
-	uint64_t flags;
+	uint32_t domains = valid_domains &
+		(write_domains ? write_domains : read_domains);
+	uint32_t pref_flags = 0, valid_flags = 0;
 
-	if (!valid_domains || (!read_domains && !write_domains))
+	if (!domains)
 		return -EINVAL;
 
-	if (write_domains) {
-		if ((valid_domains & NOUVEAU_GEM_DOMAIN_VRAM) &&
-		    (write_domains & NOUVEAU_GEM_DOMAIN_VRAM))
-			flags = TTM_PL_FLAG_VRAM;
-		else
-		if ((valid_domains & NOUVEAU_GEM_DOMAIN_GART) &&
-		    (write_domains & NOUVEAU_GEM_DOMAIN_GART))
-			flags = TTM_PL_FLAG_TT;
-		else
-			return -EINVAL;
-	} else {
-		if ((valid_domains & NOUVEAU_GEM_DOMAIN_VRAM) &&
-		    (read_domains & NOUVEAU_GEM_DOMAIN_VRAM) &&
-		    bo->mem.mem_type == TTM_PL_VRAM)
-			flags = TTM_PL_FLAG_VRAM;
-		else
-		if ((valid_domains & NOUVEAU_GEM_DOMAIN_GART) &&
-		    (read_domains & NOUVEAU_GEM_DOMAIN_GART) &&
-		    bo->mem.mem_type == TTM_PL_TT)
-			flags = TTM_PL_FLAG_TT;
-		else
-		if ((valid_domains & NOUVEAU_GEM_DOMAIN_VRAM) &&
-		    (read_domains & NOUVEAU_GEM_DOMAIN_VRAM))
-			flags = TTM_PL_FLAG_VRAM;
-		else
-			flags = TTM_PL_FLAG_TT;
-	}
+	if (valid_domains & NOUVEAU_GEM_DOMAIN_VRAM)
+		valid_flags |= TTM_PL_FLAG_VRAM;
 
-	nouveau_bo_placement_set(nvbo, flags);
+	if (valid_domains & NOUVEAU_GEM_DOMAIN_GART)
+		valid_flags |= TTM_PL_FLAG_TT;
+
+	if ((domains & NOUVEAU_GEM_DOMAIN_VRAM) &&
+	    bo->mem.mem_type == TTM_PL_VRAM)
+		pref_flags |= TTM_PL_FLAG_VRAM;
+
+	else if ((domains & NOUVEAU_GEM_DOMAIN_GART) &&
+		 bo->mem.mem_type == TTM_PL_TT)
+		pref_flags |= TTM_PL_FLAG_TT;
+
+	else if (domains & NOUVEAU_GEM_DOMAIN_VRAM)
+		pref_flags |= TTM_PL_FLAG_VRAM;
+
+	else
+		pref_flags |= TTM_PL_FLAG_TT;
+
+	nouveau_bo_placement_set(nvbo, pref_flags, valid_flags);
+
 	return 0;
 }