drm/shmem-helper: Import dmabuf without mapping its sg_table

[WHY]
1. Drivers using DRM_GEM_SHADOW_PLANE_HELPER_FUNCS and
   DRM_GEM_SHMEM_DRIVER_OPS (e.g., udl, ast) do not require
   sg_table import.
   They only need dma_buf_vmap() to access the shared buffer's
   kernel virtual address.

2. On certain Aspeed-based boards, a dma_mask of 0xffff_ffff may
   trigger SWIOTLB during dmabuf import. However, IO_TLB_SEGSIZE
   restricts the maximum DMA streaming mapping memory, resulting in
   errors like:

   ast 0000:07:00.0: swiotlb buffer is full (sz: 3145728 bytes), total 32768 (slots), used 0 (slots)

[HOW]
Provide a gem_prime_import implementation without sg_table mapping
to avoid issues (e.g., "swiotlb buffer is full"). Drivers that do not
require sg_table can adopt this.

Signed-off-by: Shixiong Ou <oushixiong@kylinos.cn>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20250522070714.439824-1-oushixiong1025@163.com

drivers/gpu/drm/drm_gem_shmem_helper.c

@@ -804,6 +804,63 @@ drm_gem_shmem_prime_import_sg_table(struct drm_device *dev,
 }
 EXPORT_SYMBOL_GPL(drm_gem_shmem_prime_import_sg_table);
 
+/**
+ * drm_gem_shmem_prime_import_no_map - Import dmabuf without mapping its sg_table
+ * @dev: Device to import into
+ * @dma_buf: dma-buf object to import
+ *
+ * Drivers that use the shmem helpers but also wants to import dmabuf without
+ * mapping its sg_table can use this as their &drm_driver.gem_prime_import
+ * implementation.
+ */
+struct drm_gem_object *drm_gem_shmem_prime_import_no_map(struct drm_device *dev,
+							 struct dma_buf *dma_buf)
+{
+	struct dma_buf_attachment *attach;
+	struct drm_gem_shmem_object *shmem;
+	struct drm_gem_object *obj;
+	size_t size;
+	int ret;
+
+	if (drm_gem_is_prime_exported_dma_buf(dev, dma_buf)) {
+		/*
+		 * Importing dmabuf exported from our own gem increases
+		 * refcount on gem itself instead of f_count of dmabuf.
+		 */
+		obj = dma_buf->priv;
+		drm_gem_object_get(obj);
+		return obj;
+	}
+
+	attach = dma_buf_attach(dma_buf, dev->dev);
+	if (IS_ERR(attach))
+		return ERR_CAST(attach);
+
+	get_dma_buf(dma_buf);
+
+	size = PAGE_ALIGN(attach->dmabuf->size);
+
+	shmem = __drm_gem_shmem_create(dev, size, true, NULL);
+	if (IS_ERR(shmem)) {
+		ret = PTR_ERR(shmem);
+		goto fail_detach;
+	}
+
+	drm_dbg_prime(dev, "size = %zu\n", size);
+
+	shmem->base.import_attach = attach;
+	shmem->base.resv = dma_buf->resv;
+
+	return &shmem->base;
+
+fail_detach:
+	dma_buf_detach(dma_buf, attach);
+	dma_buf_put(dma_buf);
+
+	return ERR_PTR(ret);
+}
+EXPORT_SYMBOL_GPL(drm_gem_shmem_prime_import_no_map);
+
 MODULE_DESCRIPTION("DRM SHMEM memory-management helpers");
 MODULE_IMPORT_NS("DMA_BUF");
 MODULE_LICENSE("GPL v2");

drivers/gpu/drm/drm_prime.c

@@ -928,6 +928,26 @@ struct dma_buf *drm_gem_prime_export(struct drm_gem_object *obj,
 }
 EXPORT_SYMBOL(drm_gem_prime_export);
 
+
+/**
+ * drm_gem_is_prime_exported_dma_buf -
+ * checks if the DMA-BUF was exported from a GEM object belonging to @dev.
+ * @dev: drm_device to check against
+ * @dma_buf: dma-buf object to import
+ *
+ * Return: true if the DMA-BUF was exported from a GEM object belonging
+ * to @dev, false otherwise.
+ */
+
+bool drm_gem_is_prime_exported_dma_buf(struct drm_device *dev,
+				       struct dma_buf *dma_buf)
+{
+	struct drm_gem_object *obj = dma_buf->priv;
+
+	return (dma_buf->ops == &drm_gem_prime_dmabuf_ops) && (obj->dev == dev);
+}
+EXPORT_SYMBOL(drm_gem_is_prime_exported_dma_buf);
+
 /**
  * drm_gem_prime_import_dev - core implementation of the import callback
  * @dev: drm_device to import into
@@ -951,16 +971,14 @@ struct drm_gem_object *drm_gem_prime_import_dev(struct drm_device *dev,
 	struct drm_gem_object *obj;
 	int ret;
 
-	if (dma_buf->ops == &drm_gem_prime_dmabuf_ops) {
+	if (drm_gem_is_prime_exported_dma_buf(dev, dma_buf)) {
+		/*
+		 * Importing dmabuf exported from our own gem increases
+		 * refcount on gem itself instead of f_count of dmabuf.
+		 */
 		obj = dma_buf->priv;
-		if (obj->dev == dev) {
-			/*
-			 * Importing dmabuf exported from our own gem increases
-			 * refcount on gem itself instead of f_count of dmabuf.
-			 */
-			drm_gem_object_get(obj);
-			return obj;
-		}
+		drm_gem_object_get(obj);
+		return obj;
 	}
 
 	if (!dev->driver->gem_prime_import_sg_table)

include/drm/drm_gem_shmem_helper.h

@@ -287,6 +287,8 @@ drm_gem_shmem_prime_import_sg_table(struct drm_device *dev,
 				    struct sg_table *sgt);
 int drm_gem_shmem_dumb_create(struct drm_file *file, struct drm_device *dev,
 			      struct drm_mode_create_dumb *args);
+struct drm_gem_object *drm_gem_shmem_prime_import_no_map(struct drm_device *dev,
+							 struct dma_buf *buf);
 
 /**
  * DRM_GEM_SHMEM_DRIVER_OPS - Default shmem GEM operations
@@ -298,4 +300,17 @@ int drm_gem_shmem_dumb_create(struct drm_file *file, struct drm_device *dev,
 	.gem_prime_import_sg_table = drm_gem_shmem_prime_import_sg_table, \
 	.dumb_create		   = drm_gem_shmem_dumb_create
 
+/**
+ * DRM_GEM_SHMEM_DRIVER_OPS_NO_MAP_SGT - shmem GEM operations
+ *                                       without mapping sg_table on
+ *                                       imported buffer.
+ *
+ * This macro provides a shortcut for setting the shmem GEM operations in
+ * the &drm_driver structure for drivers that do not require a sg_table on
+ * imported buffers.
+ */
+#define DRM_GEM_SHMEM_DRIVER_OPS_NO_MAP_SGT \
+	.gem_prime_import       = drm_gem_shmem_prime_import_no_map, \
+	.dumb_create            = drm_gem_shmem_dumb_create
+
 #endif /* __DRM_GEM_SHMEM_HELPER_H__ */

include/drm/drm_prime.h

@@ -100,6 +100,9 @@ struct dma_buf *drm_gem_prime_export(struct drm_gem_object *obj,
 unsigned long drm_prime_get_contiguous_size(struct sg_table *sgt);
 
 /* helper functions for importing */
+bool drm_gem_is_prime_exported_dma_buf(struct drm_device *dev,
+				       struct dma_buf *dma_buf);
+
 struct drm_gem_object *drm_gem_prime_import_dev(struct drm_device *dev,
 						struct dma_buf *dma_buf,
 						struct device *attach_dev);