drm/radeon: only enable swiotlb path when need v2

swiotlb expands our card accessing range, but its path always is slower
than ttm pool allocation.
So add condition to use it.
v2: move a bit later

Signed-off-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Monk Liu <monk.liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180209024410.1469-3-david1.zhou@amd.com

drivers/gpu/drm/radeon/radeon.h

@@ -2387,6 +2387,7 @@ struct radeon_device {
 	struct radeon_dummy_page	dummy_page;
 	bool				shutdown;
 	bool				need_dma32;
+	bool				need_swiotlb;
 	bool				accel_working;
 	bool				fastfb_working; /* IGP feature*/
 	bool				needs_reset, in_reset;

drivers/gpu/drm/radeon/radeon_device.c

@@ -29,6 +29,7 @@
 #include <linux/slab.h>
 #include <drm/drmP.h>
 #include <drm/drm_crtc_helper.h>
+#include <drm/drm_cache.h>
 #include <drm/radeon_drm.h>
 #include <linux/pm_runtime.h>
 #include <linux/vgaarb.h>
@@ -1378,6 +1379,7 @@ int radeon_device_init(struct radeon_device *rdev,
 		pci_set_consistent_dma_mask(rdev->pdev, DMA_BIT_MASK(32));
 		pr_warn("radeon: No coherent DMA available\n");
 	}
+	rdev->need_swiotlb = drm_get_max_iomem() > ((u64)1 << dma_bits);
 
 	/* Registers mapping */
 	/* TODO: block userspace mapping of io register */

drivers/gpu/drm/radeon/radeon_ttm.c

@@ -756,7 +756,7 @@ static int radeon_ttm_tt_populate(struct ttm_tt *ttm,
 #endif
 
 #ifdef CONFIG_SWIOTLB
-	if (swiotlb_nr_tbl()) {
+	if (rdev->need_swiotlb && swiotlb_nr_tbl()) {
 		return ttm_dma_populate(&gtt->ttm, rdev->dev, ctx);
 	}
 #endif
@@ -788,7 +788,7 @@ static void radeon_ttm_tt_unpopulate(struct ttm_tt *ttm)
 #endif
 
 #ifdef CONFIG_SWIOTLB
-	if (swiotlb_nr_tbl()) {
+	if (rdev->need_swiotlb && swiotlb_nr_tbl()) {
 		ttm_dma_unpopulate(&gtt->ttm, rdev->dev);
 		return;
 	}
@@ -1155,7 +1155,7 @@ static int radeon_ttm_debugfs_init(struct radeon_device *rdev)
 	count = ARRAY_SIZE(radeon_ttm_debugfs_list);
 
 #ifdef CONFIG_SWIOTLB
-	if (!swiotlb_nr_tbl())
+	if (!(rdev->need_swiotlb && swiotlb_nr_tbl()))
 		--count;
 #endif