--- a/Documentation/gpu/todo.rst
+++ b/Documentation/gpu/todo.rst
@@ -560,6 +560,27 @@ Some of these date from the very introdu
 
 Level: Intermediate
 
+Remove automatic page mapping from dma-buf importing
+----------------------------------------------------
+
+When importing dma-bufs, the dma-buf and PRIME frameworks automatically map
+imported pages into the importer's DMA area. drm_gem_prime_fd_to_handle() and
+drm_gem_prime_handle_to_fd() require that importers call dma_buf_attach()
+even if they never do actual device DMA, but only CPU access through
+dma_buf_vmap(). This is a problem for USB devices, which do not support DMA
+operations.
+
+To fix the issue, automatic page mappings should be removed from the
+buffer-sharing code. Fixing this is a bit more involved, since the import/export
+cache is also tied to &drm_gem_object.import_attach. Meanwhile we paper over
+this problem for USB devices by fishing out the USB host controller device, as
+long as that supports DMA. Otherwise importing can still needlessly fail.
+
+Contact: Thomas Zimmermann <tzimmermann@suse.de>, Daniel Vetter
+
+Level: Advanced
+
+
 Better Testing
 ==============
 
--- a/drivers/gpu/drm/tiny/gm12u320.c
+++ b/drivers/gpu/drm/tiny/gm12u320.c
@@ -83,6 +83,7 @@ MODULE_PARM_DESC(eco_mode, "Turn on Eco
 
 struct gm12u320_device {
 	struct drm_device	         dev;
+	struct device                   *dmadev;
 	struct drm_simple_display_pipe   pipe;
 	struct drm_connector	         conn;
 	struct usb_device               *udev;
@@ -598,6 +599,22 @@ static const uint64_t gm12u320_pipe_modi
 	DRM_FORMAT_MOD_INVALID
 };
 
+/*
+ * FIXME: Dma-buf sharing requires DMA support by the importing device.
+ *        This function is a workaround to make USB devices work as well.
+ *        See todo.rst for how to fix the issue in the dma-buf framework.
+ */
+static struct drm_gem_object *gm12u320_gem_prime_import(struct drm_device *dev,
+							struct dma_buf *dma_buf)
+{
+	struct gm12u320_device *gm12u320 = to_gm12u320(dev);
+
+	if (!gm12u320->dmadev)
+		return ERR_PTR(-ENODEV);
+
+	return drm_gem_prime_import_dev(dev, dma_buf, gm12u320->dmadev);
+}
+
 DEFINE_DRM_GEM_FOPS(gm12u320_fops);
 
 static struct drm_driver gm12u320_drm_driver = {
@@ -611,6 +628,7 @@ static struct drm_driver gm12u320_drm_dr
 
 	.fops		 = &gm12u320_fops,
 	DRM_GEM_SHMEM_DRIVER_OPS,
+	.gem_prime_import = gm12u320_gem_prime_import,
 };
 
 static const struct drm_mode_config_funcs gm12u320_mode_config_funcs = {
@@ -637,16 +655,19 @@ static int gm12u320_usb_probe(struct usb
 				      struct gm12u320_device, dev);
 	if (IS_ERR(gm12u320))
 		return PTR_ERR(gm12u320);
+	dev = &gm12u320->dev;
+
+	gm12u320->dmadev = usb_intf_get_dma_device(to_usb_interface(dev->dev));
+	if (!gm12u320->dmadev)
+		drm_warn(dev, "buffer sharing not supported"); /* not an error */
 
 	gm12u320->udev = interface_to_usbdev(interface);
 	INIT_DELAYED_WORK(&gm12u320->fb_update.work, gm12u320_fb_update_work);
 	mutex_init(&gm12u320->fb_update.lock);
 
-	dev = &gm12u320->dev;
-
 	ret = drmm_mode_config_init(dev);
 	if (ret)
-		return ret;
+		goto err_put_device;
 
 	dev->mode_config.min_width = GM12U320_USER_WIDTH;
 	dev->mode_config.max_width = GM12U320_USER_WIDTH;
@@ -656,15 +677,15 @@ static int gm12u320_usb_probe(struct usb
 
 	ret = gm12u320_usb_alloc(gm12u320);
 	if (ret)
-		return ret;
+		goto err_put_device;
 
 	ret = gm12u320_set_ecomode(gm12u320);
 	if (ret)
-		return ret;
+		goto err_put_device;
 
 	ret = gm12u320_conn_init(gm12u320);
 	if (ret)
-		return ret;
+		goto err_put_device;
 
 	ret = drm_simple_display_pipe_init(&gm12u320->dev,
 					   &gm12u320->pipe,
@@ -674,24 +695,31 @@ static int gm12u320_usb_probe(struct usb
 					   gm12u320_pipe_modifiers,
 					   &gm12u320->conn);
 	if (ret)
-		return ret;
+		goto err_put_device;
 
 	drm_mode_config_reset(dev);
 
 	usb_set_intfdata(interface, dev);
 	ret = drm_dev_register(dev, 0);
 	if (ret)
-		return ret;
+		goto err_put_device;
 
 	drm_fbdev_generic_setup(dev, 0);
 
 	return 0;
+
+err_put_device:
+	put_device(gm12u320->dmadev);
+	return ret;
 }
 
 static void gm12u320_usb_disconnect(struct usb_interface *interface)
 {
 	struct drm_device *dev = usb_get_intfdata(interface);
+	struct gm12u320_device *gm12u320 = to_gm12u320(dev);
 
+	put_device(gm12u320->dmadev);
+	gm12u320->dmadev = NULL;
 	drm_dev_unplug(dev);
 	drm_atomic_helper_shutdown(dev);
 }
--- a/drivers/gpu/drm/udl/udl_drv.c
+++ b/drivers/gpu/drm/udl/udl_drv.c
@@ -32,6 +32,22 @@ static int udl_usb_resume(struct usb_int
 	return drm_mode_config_helper_resume(dev);
 }
 
+/*
+ * FIXME: Dma-buf sharing requires DMA support by the importing device.
+ *        This function is a workaround to make USB devices work as well.
+ *        See todo.rst for how to fix the issue in the dma-buf framework.
+ */
+static struct drm_gem_object *udl_driver_gem_prime_import(struct drm_device *dev,
+							  struct dma_buf *dma_buf)
+{
+	struct udl_device *udl = to_udl(dev);
+
+	if (!udl->dmadev)
+		return ERR_PTR(-ENODEV);
+
+	return drm_gem_prime_import_dev(dev, dma_buf, udl->dmadev);
+}
+
 DEFINE_DRM_GEM_FOPS(udl_driver_fops);
 
 static struct drm_driver driver = {
@@ -42,6 +58,7 @@ static struct drm_driver driver = {
 
 	.fops = &udl_driver_fops,
 	DRM_GEM_SHMEM_DRIVER_OPS,
+	.gem_prime_import = udl_driver_gem_prime_import,
 
 	.name = DRIVER_NAME,
 	.desc = DRIVER_DESC,
--- a/drivers/gpu/drm/udl/udl_drv.h
+++ b/drivers/gpu/drm/udl/udl_drv.h
@@ -50,6 +50,7 @@ struct urb_list {
 struct udl_device {
 	struct drm_device drm;
 	struct device *dev;
+	struct device *dmadev;
 	struct usb_device *udev;
 
 	struct drm_simple_display_pipe display_pipe;
--- a/drivers/gpu/drm/udl/udl_main.c
+++ b/drivers/gpu/drm/udl/udl_main.c
@@ -314,6 +314,10 @@ int udl_init(struct udl_device *udl)
 
 	DRM_DEBUG("\n");
 
+	udl->dmadev = usb_intf_get_dma_device(to_usb_interface(dev->dev));
+	if (!udl->dmadev)
+		drm_warn(dev, "buffer sharing not supported"); /* not an error */
+
 	mutex_init(&udl->gem_lock);
 
 	if (!udl_parse_vendor_descriptor(dev, udl->udev)) {
@@ -342,12 +346,18 @@ int udl_init(struct udl_device *udl)
 err:
 	if (udl->urbs.count)
 		udl_free_urb_list(dev);
+	put_device(udl->dmadev);
 	DRM_ERROR("%d\n", ret);
 	return ret;
 }
 
 int udl_drop_usb(struct drm_device *dev)
 {
+	struct udl_device *udl = to_udl(dev);
+
 	udl_free_urb_list(dev);
+	put_device(udl->dmadev);
+	udl->dmadev = NULL;
+
 	return 0;
 }
--- a/drivers/usb/core/usb.c
+++ b/drivers/usb/core/usb.c
@@ -748,6 +748,38 @@ void usb_put_intf(struct usb_interface *
 }
 EXPORT_SYMBOL_GPL(usb_put_intf);
 
+/**
+ * usb_intf_get_dma_device - acquire a reference on the usb interface's DMA endpoint
+ * @intf: the usb interface
+ *
+ * While a USB device cannot perform DMA operations by itself, many USB
+ * controllers can. A call to usb_intf_get_dma_device() returns the DMA endpoint
+ * for the given USB interface, if any. The returned device structure must be
+ * released with put_device().
+ *
+ * See also usb_get_dma_device().
+ *
+ * Returns: A reference to the usb interface's DMA endpoint; or NULL if none
+ *          exists.
+ */
+struct device *usb_intf_get_dma_device(struct usb_interface *intf)
+{
+	struct usb_device *udev = interface_to_usbdev(intf);
+	struct device *dmadev;
+
+	if (!udev->bus)
+		return NULL;
+
+	dmadev = get_device(udev->bus->sysdev);
+	if (!dmadev || !dmadev->dma_mask) {
+		put_device(dmadev);
+		return NULL;
+	}
+
+	return dmadev;
+}
+EXPORT_SYMBOL_GPL(usb_intf_get_dma_device);
+
 /*			USB device locking
  *
  * USB devices and interfaces are locked using the semaphore in their
--- a/include/linux/usb.h
+++ b/include/linux/usb.h
@@ -746,6 +746,8 @@ extern int usb_lock_device_for_reset(str
 extern int usb_reset_device(struct usb_device *dev);
 extern void usb_queue_reset_device(struct usb_interface *dev);
 
+extern struct device *usb_intf_get_dma_device(struct usb_interface *intf);
+
 #ifdef CONFIG_ACPI
 extern int usb_acpi_set_power_state(struct usb_device *hdev, int index,
 	bool enable);
