drm/nouveau/disp: fix DP disable race

If a HPD pulse signalling the need to retrain the link occurs between
the KMS driver releasing the output and the supervisor interrupt that
finishes the teardown, it was possible get a NULL-ptr deref.

Avoid this by marking the link as inactive earlier.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

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

@@ -413,14 +413,10 @@ nvkm_dp_train(struct nvkm_dp *dp, u32 dataKBps)
 }
 
 static void
-nvkm_dp_release(struct nvkm_outp *outp, struct nvkm_ior *ior)
+nvkm_dp_disable(struct nvkm_outp *outp, struct nvkm_ior *ior)
 {
 	struct nvkm_dp *dp = nvkm_dp(outp);
 
-	/* Prevent link from being retrained if sink sends an IRQ. */
-	atomic_set(&dp->lt.done, 0);
-	ior->dp.nr = 0;
-
 	/* Execute DisableLT script from DP Info Table. */
 	nvbios_init(&ior->disp->engine.subdev, dp->info.script[4],
 		init.outp = &dp->outp.info;
@@ -429,6 +425,16 @@ nvkm_dp_release(struct nvkm_outp *outp, struct nvkm_ior *ior)
 	);
 }
 
+static void
+nvkm_dp_release(struct nvkm_outp *outp)
+{
+	struct nvkm_dp *dp = nvkm_dp(outp);
+
+	/* Prevent link from being retrained if sink sends an IRQ. */
+	atomic_set(&dp->lt.done, 0);
+	dp->outp.ior->dp.nr = 0;
+}
+
 static int
 nvkm_dp_acquire(struct nvkm_outp *outp)
 {
@@ -607,6 +613,7 @@ nvkm_dp_func = {
 	.fini = nvkm_dp_fini,
 	.acquire = nvkm_dp_acquire,
 	.release = nvkm_dp_release,
+	.disable = nvkm_dp_disable,
 };
 
 static int

drivers/gpu/drm/nouveau/nvkm/engine/disp/nv50.c

@@ -501,11 +501,11 @@ nv50_disp_super_2_0(struct nv50_disp *disp, struct nvkm_head *head)
 	nv50_disp_super_ied_off(head, ior, 2);
 
 	/* If we're shutting down the OR's only active head, execute
-	 * the output path's release function.
+	 * the output path's disable function.
 	 */
 	if (ior->arm.head == (1 << head->id)) {
-		if ((outp = ior->arm.outp) && outp->func->release)
-			outp->func->release(outp, ior);
+		if ((outp = ior->arm.outp) && outp->func->disable)
+			outp->func->disable(outp, ior);
 	}
 }
 

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

@@ -93,6 +93,8 @@ nvkm_outp_release(struct nvkm_outp *outp, u8 user)
 	if (ior) {
 		outp->acquired &= ~user;
 		if (!outp->acquired) {
+			if (outp->func->release && outp->ior)
+				outp->func->release(outp);
 			outp->ior->asy.outp = NULL;
 			outp->ior = NULL;
 		}

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

@@ -40,7 +40,8 @@ struct nvkm_outp_func {
 	void (*init)(struct nvkm_outp *);
 	void (*fini)(struct nvkm_outp *);
 	int (*acquire)(struct nvkm_outp *);
-	void (*release)(struct nvkm_outp *, struct nvkm_ior *);
+	void (*release)(struct nvkm_outp *);
+	void (*disable)(struct nvkm_outp *, struct nvkm_ior *);
 };
 
 #define OUTP_MSG(o,l,f,a...) do {                                              \