From 992d38cc787d353aba8b0e13d37a719a3431db79 Mon Sep 17 00:00:00 2001
From: Chris Wilson <chris@chris-wilson.co.uk>
Date: Sat, 13 May 2017 11:52:00 +0100
Subject: [PATCH] drm/dp: Read the tx msg state once after checking for an event
Git-commit: 992d38cc787d353aba8b0e13d37a719a3431db79
Patch-mainline: v4.13-rc1
References: FATE#322643 bsc#1055900

Both as an exercise to document that we are reading the state outside of
the appropriate mutex and to ensure that we only read the value once
before the multiple comparisons, use READ_ONCE.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170513105201.17658-1-chris@chris-wilson.co.uk
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 drivers/gpu/drm/drm_dp_mst_topology.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -744,16 +744,16 @@ static void drm_dp_mst_put_payload_id(st
 static bool check_txmsg_state(struct drm_dp_mst_topology_mgr *mgr,
 			      struct drm_dp_sideband_msg_tx *txmsg)
 {
-	bool ret;
+	unsigned int state;
 
 	/*
 	 * All updates to txmsg->state are protected by mgr->qlock, and the two
 	 * cases we check here are terminal states. For those the barriers
 	 * provided by the wake_up/wait_event pair are enough.
 	 */
-	ret = (txmsg->state == DRM_DP_SIDEBAND_TX_RX ||
-	       txmsg->state == DRM_DP_SIDEBAND_TX_TIMEOUT);
-	return ret;
+	state = READ_ONCE(txmsg->state);
+	return (state == DRM_DP_SIDEBAND_TX_RX ||
+		state == DRM_DP_SIDEBAND_TX_TIMEOUT);
 }
 
 static int drm_dp_mst_wait_tx_reply(struct drm_dp_mst_branch *mstb,