drm/nouveau/disp/dp: ensure sink is powered up before attempting link training

This can happen under some annoying circumstances, and is a quick fix
until more substantial changes can be made.

Fixed eDP mode changes on (at least) the Lenovo P50.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/nouveau/nvkm/engine/disp/dport.c

@@ -328,6 +328,7 @@ nvkm_dp_train(struct work_struct *w)
 		.outp = outp,
 	}, *dp = &_dp;
 	u32 datarate = 0;
+	u8  pwr;
 	int ret;
 
 	if (!outp->base.info.location && disp->func->sor.magic)
@@ -355,6 +356,15 @@ nvkm_dp_train(struct work_struct *w)
 	/* disable link interrupt handling during link training */
 	nvkm_notify_put(&outp->irq);
 
+	/* ensure sink is not in a low-power state */
+	if (!nvkm_rdaux(outp->aux, DPCD_SC00, &pwr, 1)) {
+		if ((pwr & DPCD_SC00_SET_POWER) != DPCD_SC00_SET_POWER_D0) {
+			pwr &= ~DPCD_SC00_SET_POWER;
+			pwr |=  DPCD_SC00_SET_POWER_D0;
+			nvkm_wraux(outp->aux, DPCD_SC00, &pwr, 1);
+		}
+	}
+
 	/* enable down-spreading and execute pre-train script from vbios */
 	dp_link_train_init(dp, outp->dpcd[3] & 0x01);
 

drivers/gpu/drm/nouveau/nvkm/engine/disp/dport.h

@@ -71,5 +71,11 @@
 #define DPCD_LS0C_LANE1_POST_CURSOR2                                       0x0c
 #define DPCD_LS0C_LANE0_POST_CURSOR2                                       0x03
 
+/* DPCD Sink Control */
+#define DPCD_SC00                                                       0x00600
+#define DPCD_SC00_SET_POWER                                                0x03
+#define DPCD_SC00_SET_POWER_D0                                             0x01
+#define DPCD_SC00_SET_POWER_D3                                             0x03
+
 void nvkm_dp_train(struct work_struct *);
 #endif