[PATCH 6/6] drm/rockchip: fix race with kms hotplug and fbdev

[Mark Yao]

Since fb_helper is not a pointer on rockchip_drm_private, it's no
need to check pointer.

Kms hotplug event may race into fbdev helper initial, and fb_helper->dev
may be NULL pointer, that would cause the bug:

[    0.735411] [00000200] *pgd=00000000f6ffe003, *pud=00000000f6ffe003, *pmd=0000000000000000
[    0.736156] Internal error: Oops: 96000005 [#1] PREEMPT SMP
[    0.736648] Modules linked in:
[    0.736930] CPU: 2 PID: 20 Comm: kworker/2:0 Not tainted 4.4.41 #20
[    0.737480] Hardware name: Rockchip RK3399 Board rev2 (BOX) (DT)
[    0.738020] Workqueue: events cdn_dp_pd_event_work
[    0.738447] task: ffffffc0f21f3100 ti: ffffffc0f2218000 task.ti: ffffffc0f2218000
[    0.739109] PC is at mutex_lock+0x14/0x44
[    0.739469] LR is at drm_fb_helper_hotplug_event+0x30/0x114
[    0.756253] [<ffffff8008a344f4>] mutex_lock+0x14/0x44
[    0.756260] [<ffffff8008445708>] drm_fb_helper_hotplug_event+0x30/0x114
[    0.756271] [<ffffff8008473c84>] rockchip_drm_output_poll_changed+0x18/0x20
[    0.756280] [<ffffff8008439fcc>] drm_kms_helper_hotplug_event+0x28/0x34
[    0.756286] [<ffffff800846c444>] cdn_dp_pd_event_work+0x394/0x3c4
[    0.756295] [<ffffff80080b2b38>] process_one_work+0x218/0x3e0
[    0.756302] [<ffffff80080b3538>] worker_thread+0x2e8/0x404
[    0.756308] [<ffffff80080b7e70>] kthread+0xe8/0xf0
[    0.756316] [<ffffff8008082690>] ret_from_fork+0x10/0x40

Signed-off-by: Mark Yao <mark.yao at rock-chips.com>
---
 drivers/gpu/drm/rockchip/rockchip_drm_fb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/rockchip/rockchip_drm_fb.c b/drivers/gpu/drm/rockchip/rockchip_drm_fb.c
index 81f9548..e6bd0f4 100644
--- a/drivers/gpu/drm/rockchip/rockchip_drm_fb.c
+++ b/drivers/gpu/drm/rockchip/rockchip_drm_fb.c
@@ -170,7 +170,7 @@ static void rockchip_drm_output_poll_changed(struct drm_device *dev)
 	struct rockchip_drm_private *private = dev->dev_private;
 	struct drm_fb_helper *fb_helper = &private->fbdev_helper;
 
-	if (fb_helper)
+	if (fb_helper->dev)
 		drm_fb_helper_hotplug_event(fb_helper);
 }
 
-- 
1.9.1