drm: Expose a method for creating anonymous struct file around drm_minor

Sometimes we need to create a struct file to wrap a drm_device, as it
the user were to have opened /dev/dri/card0 but to do so anonymously
(i.e. for internal use). Provide a utility method to create a struct
file with the drm_device->driver.fops, that wrap the drm_device.

v2: Restrict usage to selftests
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107180601.30815-2-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_file.c

@@ -31,7 +31,9 @@
  * OTHER DEALINGS IN THE SOFTWARE.
  */
 
+#include <linux/anon_inodes.h>
 #include <linux/dma-fence.h>
+#include <linux/file.h>
 #include <linux/module.h>
 #include <linux/pci.h>
 #include <linux/poll.h>
@@ -754,3 +756,43 @@ void drm_send_event(struct drm_device *dev, struct drm_pending_event *e)
 	spin_unlock_irqrestore(&dev->event_lock, irqflags);
 }
 EXPORT_SYMBOL(drm_send_event);
+
+/**
+ * mock_drm_getfile - Create a new struct file for the drm device
+ * @minor: drm minor to wrap (e.g. #drm_device.primary)
+ * @flags: file creation mode (O_RDWR etc)
+ *
+ * This create a new struct file that wraps a DRM file context around a
+ * DRM minor. This mimicks userspace opening e.g. /dev/dri/card0, but without
+ * invoking userspace. The struct file may be operated on using its f_op
+ * (the drm_device.driver.fops) to mimick userspace operations, or be supplied
+ * to userspace facing functions as an internal/anonymous client.
+ *
+ * RETURNS:
+ * Pointer to newly created struct file, ERR_PTR on failure.
+ */
+struct file *mock_drm_getfile(struct drm_minor *minor, unsigned int flags)
+{
+	struct drm_device *dev = minor->dev;
+	struct drm_file *priv;
+	struct file *file;
+
+	priv = drm_file_alloc(minor);
+	if (IS_ERR(priv))
+		return ERR_CAST(priv);
+
+	file = anon_inode_getfile("drm", dev->driver->fops, priv, flags);
+	if (IS_ERR(file)) {
+		drm_file_free(priv);
+		return file;
+	}
+
+	/* Everyone shares a single global address space */
+	file->f_mapping = dev->anon_inode->i_mapping;
+
+	drm_dev_get(dev);
+	priv->filp = file;
+
+	return file;
+}
+EXPORT_SYMBOL_FOR_TESTS_ONLY(mock_drm_getfile);

include/drm/drm_file.h

@@ -42,6 +42,7 @@ struct dma_fence;
 struct drm_file;
 struct drm_device;
 struct device;
+struct file;
 
 /*
  * FIXME: Not sure we want to have drm_minor here in the end, but to avoid
@@ -387,4 +388,6 @@ void drm_event_cancel_free(struct drm_device *dev,
 void drm_send_event_locked(struct drm_device *dev, struct drm_pending_event *e);
 void drm_send_event(struct drm_device *dev, struct drm_pending_event *e);
 
+struct file *mock_drm_getfile(struct drm_minor *minor, unsigned int flags);
+
 #endif /* _DRM_FILE_H_ */