[PATCH] drm/client: Fix: drm_client_new: Don't require DRM to be registered

[Noralf Trønnes]

Commit 894a677f4b3e ("drm/cma-helper: Use the generic fbdev emulation")
broke almost all drivers that use the CMA helper.

The reason is that drm_client_new() requires that the DRM device has
been registered, but the drivers register fbdev before registering DRM.

Remove the requirement that DRM should be registered when creating a
new client.

This creates a theoretical race condition where drm_client_new() races
with drm_dev_unregister() resulting in the .unregister hook not being
called.

Case:
User loads module which calls drm_client_new().
Device is unplugged and drm_dev_unregister() is called.

If drm_dev_unregister() gets the clientlist_mutex first, the new client
will never get it's unregister hook called.

Fixes: c76f0f7cb546 ("drm: Begin an API for in-kernel clients")
Cc: Maxime Ripard <maxime.ripard at bootlin.com>
Cc: Icenowy Zheng <icenowy at aosc.io>
Cc: Chen-Yu Tsai <wens at csie.org>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Signed-off-by: Noralf Trønnes <noralf at tronnes.org>
---
 drivers/gpu/drm/drm_client.c | 11 +----------
 1 file changed, 1 insertion(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/drm_client.c b/drivers/gpu/drm/drm_client.c
index 4039a4d103a8..9b142f58d489 100644
--- a/drivers/gpu/drm/drm_client.c
+++ b/drivers/gpu/drm/drm_client.c
@@ -78,7 +78,6 @@ EXPORT_SYMBOL(drm_client_close);
 int drm_client_new(struct drm_device *dev, struct drm_client_dev *client,
 		   const char *name, const struct drm_client_funcs *funcs)
 {
-	bool registered;
 	int ret;
 
 	if (!drm_core_check_feature(dev, DRIVER_MODESET) ||
@@ -97,21 +96,13 @@ int drm_client_new(struct drm_device *dev, struct drm_client_dev *client,
 		goto err_put_module;
 
 	mutex_lock(&dev->clientlist_mutex);
-	registered = dev->registered;
-	if (registered)
-		list_add(&client->list, &dev->clientlist);
+	list_add(&client->list, &dev->clientlist);
 	mutex_unlock(&dev->clientlist_mutex);
-	if (!registered) {
-		ret = -ENODEV;
-		goto err_close;
-	}
 
 	drm_dev_get(dev);
 
 	return 0;
 
-err_close:
-	drm_client_close(client);
 err_put_module:
 	if (funcs)
 		module_put(funcs->owner);
-- 
2.15.1