linux-kernel - [PATCH v2 4/4] drm/connector: warn when cleaning up a refcounted connector

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Message-Id: <20241106-drm-small-improvements-v2-4-f6e2aef86719@bootlin.com>
Date: Wed, 06 Nov 2024 11:48:27 +0100
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>
Cc: Thomas Petazzoni <thomas.petazzoni@...tlin.com>, 
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
 Luca Ceresoli <luca.ceresoli@...tlin.com>
Subject: [PATCH v2 4/4] drm/connector: warn when cleaning up a refcounted
 connector

Calling drm_connector_cleanup() should only be done via the free_cb =>
.destroy path, which cleans up the struct drm_connector only when the
refcount drops to zero.

A cleanup done with a refcount higher than 0 can result from buggy code,
e.g. by doing cleanup directly in the drivers teardown code. Serious
trouble can happen if this happens, so warn about it.

Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>
---
 drivers/gpu/drm/drm_connector.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
index fc35f47e2849ed6786d6223ac9c69e1c359fc648..e0bf9c490af43055de4caaee1580a4befbd608c5 100644
--- a/drivers/gpu/drm/drm_connector.c
+++ b/drivers/gpu/drm/drm_connector.c
@@ -624,6 +624,12 @@ void drm_connector_cleanup(struct drm_connector *connector)
 	struct drm_device *dev = connector->dev;
 	struct drm_display_mode *mode, *t;
 
+	/*
+	 * Cleanup must happen when the last ref is put, via the
+	 * drm_connector_free() callback.
+	 */
+	WARN_ON(drm_mode_object_read_refcount(&connector->base) != 0);
+
 	/* The connector should have been removed from userspace long before
 	 * it is finally destroyed.
 	 */

-- 
2.34.1