[PATCH] drm/vc4: fix NULL dereference in vc4_hvs_unbind

[Thorsten Blum <thorsten.blum@linux.dev>]

With 'dtoverlay=vc4-kms-v3d,noaudio' and 'hdmi=off' on Raspberry Pi,
unloading the vc4 module calls vc4_hvs_unbind() with
dev_get_drvdata(master) returning NULL.

Return early when 'drm' is NULL before converting it to 'vc4' and before
dereferencing 'vc4->hvs', preventing a kernel oops.

Fixes: c8b75bca92cb ("drm/vc4: Add KMS support for Raspberry Pi.")
Cc: stable@vger.kernel.org
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
---
 drivers/gpu/drm/vc4/vc4_hvs.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/vc4/vc4_hvs.c b/drivers/gpu/drm/vc4/vc4_hvs.c
index ee8d0738501b..9cb66f696fc7 100644
--- a/drivers/gpu/drm/vc4/vc4_hvs.c
+++ b/drivers/gpu/drm/vc4/vc4_hvs.c
@@ -1753,10 +1753,16 @@ static void vc4_hvs_unbind(struct device *dev, struct device *master,
 			   void *data)
 {
 	struct drm_device *drm = dev_get_drvdata(master);
-	struct vc4_dev *vc4 = to_vc4_dev(drm);
-	struct vc4_hvs *hvs = vc4->hvs;
+	struct vc4_dev *vc4;
+	struct vc4_hvs *hvs;
 	struct drm_mm_node *node, *next;
 
+	if (!drm)
+		return;
+
+	vc4 = to_vc4_dev(drm);
+	hvs = vc4->hvs;
+
 	if (drm_mm_node_allocated(&vc4->hvs->mitchell_netravali_filter))
 		drm_mm_remove_node(&vc4->hvs->mitchell_netravali_filter);