[PATCH 4.18 62/88] drm/nouveau: Reset MST branching unit before enabling

[Greg Kroah-Hartman]

4.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Lyude Paul <lyude@xxxxxxxxxx>

commit fa3cdf8d0b092c4561f9f017dfac409eb7644737 upstream.

When probing a new MST device, it's not safe to make any assumptions
about it's current state. While most well mannered MST hubs will just
disable the branching unit on hotplug disconnects, this isn't enough to
save us from various other scenarios that might have resulted in
something writing to the MST branching unit before we got control of it.
This could happen if a previous probe we tried failed, if we're booting
in kexec context and the hub is still in the state the last kernel put
it in, etc.

Luckily; there is no reason we can't just reset the branching unit
every time we enable a new topology. So, fix this by resetting it on
enabling new topologies to ensure that we always start off with a clean,
unmodified topology state on MST sinks.

This fixes occasional hard-lockups on my P50's laptop dock (e.g. AUX
times out all DPCD trasactions) observed after multiple docks, undocks,
and module reloads.

Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: Karol Herbst <karolherbst@xxxxxxxxx>
Signed-off-by: Ben Skeggs <bskeggs@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/gpu/drm/nouveau/dispnv50/disp.c |   20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

--- a/drivers/gpu/drm/nouveau/dispnv50/disp.c
+++ b/drivers/gpu/drm/nouveau/dispnv50/disp.c
@@ -1098,17 +1098,21 @@ nv50_mstm_enable(struct nv50_mstm *mstm,
 	int ret;
 
 	if (dpcd >= 0x12) {
-		ret = drm_dp_dpcd_readb(mstm->mgr.aux, DP_MSTM_CTRL, &dpcd);
+		/* Even if we're enabling MST, start with disabling the
+		 * branching unit to clear any sink-side MST topology state
+		 * that wasn't set by us
+		 */
+		ret = drm_dp_dpcd_writeb(mstm->mgr.aux, DP_MSTM_CTRL, 0);
 		if (ret < 0)
 			return ret;
 
-		dpcd &= ~DP_MST_EN;
-		if (state)
-			dpcd |= DP_MST_EN;
-
-		ret = drm_dp_dpcd_writeb(mstm->mgr.aux, DP_MSTM_CTRL, dpcd);
-		if (ret < 0)
-			return ret;
+		if (state) {
+			/* Now, start initializing */
+			ret = drm_dp_dpcd_writeb(mstm->mgr.aux, DP_MSTM_CTRL,
+						 DP_MST_EN);
+			if (ret < 0)
+				return ret;
+		}
 	}
 
 	return nvif_mthd(disp, 0, &args, sizeof(args));