drm/shmem-helper: Add .gem_create_object helper that sets map_cached flag

The helper drm_gem_shmem_create_object_cached() allocates an GEM SHMEM
object and sets the map_cached flag. Useful for drivers that want cached
mappings.

v3:
* style fixes

Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Emil Velikov <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_gem_shmem_helper.c

···
 EXPORT_SYMBOL(drm_gem_shmem_purge);
 
 /**
+ * drm_gem_shmem_create_object_cached - Create a shmem buffer object with
+ *                                      cached mappings
+ * @dev: DRM device
+ * @size: Size of the object to allocate
+ *
+ * By default, shmem buffer objects use writecombine mappings. This
+ * function implements struct drm_driver.gem_create_object for shmem
+ * buffer objects with cached mappings.
+ *
+ * Returns:
+ * A struct drm_gem_shmem_object * on success or NULL negative on failure.
+ */
+struct drm_gem_object *
+drm_gem_shmem_create_object_cached(struct drm_device *dev, size_t size)
+{
+	struct drm_gem_shmem_object *shmem;
+
+	shmem = kzalloc(sizeof(*shmem), GFP_KERNEL);
+	if (!shmem)
+		return NULL;
+	shmem->map_cached = true;
+
+	return &shmem->base;
+}
+EXPORT_SYMBOL(drm_gem_shmem_create_object_cached);
+
+/**
  * drm_gem_shmem_dumb_create - Create a dumb shmem buffer object
  * @file: DRM file structure to create the dumb buffer for
  * @dev: DRM device

include/drm/drm_gem_shmem_helper.h

···
 drm_gem_shmem_create_with_handle(struct drm_file *file_priv,
 				 struct drm_device *dev, size_t size,
 				 uint32_t *handle);
+
+struct drm_gem_object *
+drm_gem_shmem_create_object_cached(struct drm_device *dev, size_t size);
+
 int drm_gem_shmem_dumb_create(struct drm_file *file, struct drm_device *dev,
 			      struct drm_mode_create_dumb *args);