[PATCH] drm/rockchip: analogix_dp: Ensure that the bridge is powered before poking it

[Marc Zyngier]

Stopping the X display manager on a kevin platform results in the
following crash:

[  674.833536] Synchronous External Abort: synchronous external abort (0x96000010) at 0xffff00000c970640
[  674.843886] Internal error: : 96000010 [#1] PREEMPT SMP
[  674.849744] Modules linked in:
[  674.849755] CPU: 1 PID: 86 Comm: kworker/1:1 Not tainted 4.15.0-rc3-00057-gff24f8cf492d-dirty #3
[  674.849760] detected fb_set_par error, error code: -16
[  674.849761] Hardware name: Google Kevin (DT)
[  674.849773] Workqueue: events analogix_dp_psr_work
[  674.849778] pstate: 60000005 (nZCv daif -PAN -UAO)
[  674.849784] pc : analogix_dp_send_psr_spd+0x8/0x168
[  674.849788] lr : analogix_dp_enable_psr+0x54/0x60
[  674.849789] sp : ffff000009b2bd60
[  674.849790] x29: ffff000009b2bd60 x28: 0000000000000000
[  674.849794] x27: ffff000009913d20 x26: ffff00000900fbf0
[  674.849797] x25: ffff8000f1b30000 x24: ffff8000f0c21d98
[  674.849800] x23: 0000000000000000 x22: ffff8000f7d3aa00
[  674.849803] x21: ffff8000f7d36980 x20: ffff8000f0c21c18
[  674.849806] x19: ffff8000f0c21db8 x18: 0000000000000001
[  674.849809] x17: 0000ffff89f2ed58 x16: ffff000008222908
[  674.849812] x15: 0000000000000000 x14: 0000000000000400
[  674.849815] x13: 0000000000000400 x12: 0000000000000000
[  674.849817] x11: 0000000000001414 x10: 0000000000000a00
[  674.849820] x9 : ffff000009b2bbb0 x8 : ffff8000f1b30a60
[  674.849823] x7 : 0000000000080000 x6 : 0000000000000001
[  674.849826] x5 : 0000000000000010 x4 : 0000000000000007
[  674.849829] x3 : 0000000000000002 x2 : ffff00000c970640
[  674.849832] x1 : ffff000009b2bd78 x0 : ffff8000f1624018
[  674.849836] Process kworker/1:1 (pid: 86, stack limit = 0x0000000083e5f7c3)
[  674.849838] Call trace:
[  674.849842]  analogix_dp_send_psr_spd+0x8/0x168
[  674.849844]  analogix_dp_psr_work+0x9c/0xa0
[  674.849849]  process_one_work+0x1cc/0x328
[  674.849852]  worker_thread+0x50/0x450
[  674.849856]  kthread+0xf8/0x128
[  674.849860]  ret_from_fork+0x10/0x18
[  674.849864] Code: b9000001 d65f03c0 f9445802 91190042 (b9400042)

Further investigation show that this happens because the the workqueue
races with the analogix_dp_bridge_disable() call from the core DRM code,
and end up trying to write to the DP bridge that has already been powered
down. This result is a very black screen, and a hard reset.

Instead of counting on luck to keep the bridge alive, let's use the
pm_runtime framework and take a reference on the device when we're about
to poke it. That is a fairly big hammer, but one that allows the system
to stay alive across dozens of X start/stop sequences.

Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
---
 drivers/gpu/drm/rockchip/analogix_dp-rockchip.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/rockchip/analogix_dp-rockchip.c b/drivers/gpu/drm/rockchip/analogix_dp-rockchip.c
index 08743ad96cb9..7f2c190f75e7 100644
--- a/drivers/gpu/drm/rockchip/analogix_dp-rockchip.c
+++ b/drivers/gpu/drm/rockchip/analogix_dp-rockchip.c
@@ -14,6 +14,7 @@
 
 #include <linux/component.h>
 #include <linux/mfd/syscon.h>
+#include <linux/pm_runtime.h>
 #include <linux/of_device.h>
 #include <linux/of_graph.h>
 #include <linux/regmap.h>
@@ -113,10 +114,12 @@ static void analogix_dp_psr_work(struct work_struct *work)
 	}
 
 	mutex_lock(&dp->psr_lock);
+	pm_runtime_get_sync(dp->dev);
 	if (dp->psr_state == EDP_VSC_PSR_STATE_ACTIVE)
 		analogix_dp_enable_psr(dp->dev);
 	else
 		analogix_dp_disable_psr(dp->dev);
+	pm_runtime_put_sync(dp->dev);
 	mutex_unlock(&dp->psr_lock);
 }
 
-- 
2.14.2