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 <[e-mail]>
Reviewed-by: Monk Liu <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/radeon/radeon.h

···
 	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

···
 #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>
···
 		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

···
 #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
···
 #endif
 
 #ifdef CONFIG_SWIOTLB
-	if (swiotlb_nr_tbl()) {
+	if (rdev->need_swiotlb && swiotlb_nr_tbl()) {
 		ttm_dma_unpopulate(&gtt->ttm, rdev->dev);
 		return;
 	}
···
 	count = ARRAY_SIZE(radeon_ttm_debugfs_list);
 
 #ifdef CONFIG_SWIOTLB
-	if (!swiotlb_nr_tbl())
+	if (!(rdev->need_swiotlb && swiotlb_nr_tbl()))
 		--count;
 #endif