[PATCH] drm/mediatek: Add error handling for old state CRTC in atomic_disable

[Jason-JH Lin]

From: Jason-jh Lin <jason-jh.lin at mediatek.corp-partner.google.com>

Introduce error handling to address an issue where, after a hotplug
event, the cursor continues to update. This situation can lead to a
kernel panic due to accessing the NULL `old_state->crtc`.

E,g.
Unable to handle kernel NULL pointer dereference at virtual address
Call trace:
 mtk_crtc_plane_disable+0x24/0x140
 mtk_plane_atomic_update+0x8c/0xa8
 drm_atomic_helper_commit_planes+0x114/0x2c8
 drm_atomic_helper_commit_tail_rpm+0x4c/0x158
 commit_tail+0xa0/0x168
 drm_atomic_helper_commit+0x110/0x120
 drm_atomic_commit+0x8c/0xe0
 drm_atomic_helper_update_plane+0xd4/0x128
 __setplane_atomic+0xcc/0x110
 drm_mode_cursor_common+0x250/0x440
 drm_mode_cursor_ioctl+0x44/0x70
 drm_ioctl+0x264/0x5d8
 __arm64_sys_ioctl+0xd8/0x510
 invoke_syscall+0x6c/0xe0
 do_el0_svc+0x68/0xe8
 el0_svc+0x34/0x60
 el0t_64_sync_handler+0x1c/0xf8
 el0t_64_sync+0x180/0x188

Adding NULL pointer checks to ensure stability by preventing operations
on an invalid CRTC state.

Fixes: d208261e9f7c ("drm/mediatek: Add wait_event_timeout when disabling plane")
Signed-off-by: Jason-jh Lin <jason-jh.lin at mediatek.corp-partner.google.com>
---
 drivers/gpu/drm/mediatek/mtk_plane.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/mediatek/mtk_plane.c b/drivers/gpu/drm/mediatek/mtk_plane.c
index cbc4f37da8ba..02349bd44001 100644
--- a/drivers/gpu/drm/mediatek/mtk_plane.c
+++ b/drivers/gpu/drm/mediatek/mtk_plane.c
@@ -292,7 +292,8 @@ static void mtk_plane_atomic_disable(struct drm_plane *plane,
 	wmb(); /* Make sure the above parameter is set before update */
 	mtk_plane_state->pending.dirty = true;
 
-	mtk_crtc_plane_disable(old_state->crtc, plane);
+	if (old_state && old_state->crtc)
+		mtk_crtc_plane_disable(old_state->crtc, plane);
 }
 
 static void mtk_plane_atomic_update(struct drm_plane *plane,
-- 
2.43.0