drm/amdgpu: align GTT start to 4GB v2

For VCE to work properly the start of the GTT space must be aligned to a
4GB boundary.

v2: add comment why we do this

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_device.c

@@ -622,7 +622,10 @@ void amdgpu_gart_location(struct amdgpu_device *adev, struct amdgpu_mc *mc)
 			dev_warn(adev->dev, "limiting GTT\n");
 			mc->gart_size = size_af;
 		}
-		mc->gart_start = mc->vram_end + 1;
+		/* VCE doesn't like it when BOs cross a 4GB segment, so align
+		 * the GART base on a 4GB boundary as well.
+		 */
+		mc->gart_start = ALIGN(mc->vram_end + 1, 0x100000000ULL);
 	}
 	mc->gart_end = mc->gart_start + mc->gart_size - 1;
 	dev_info(adev->dev, "GTT: %lluM 0x%016llX - 0x%016llX\n",