drm/amd/display: Prevent using DMUB rptr that is out-of-bounds

[Why]
Running into bugchecks during stress test where rptr is 0xFFFFFFFF.
Typically this is caused by a hard hang, and can come from HW outside
of DCN.

[How]
To prevent bugchecks when writing the DMUB rptr, fist check that the
rptr is valid.
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Wyatt Wood <wyatt.wood@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dmub/dmub_srv.h

@@ -84,6 +84,7 @@ enum dmub_status {
 	DMUB_STATUS_QUEUE_FULL,
 	DMUB_STATUS_TIMEOUT,
 	DMUB_STATUS_INVALID,
+	DMUB_STATUS_HW_FAILURE,
 };
 
 /* enum dmub_asic - dmub asic identifier */

drivers/gpu/drm/amd/display/dmub/src/dmub_srv.c

@@ -655,13 +655,19 @@ enum dmub_status dmub_srv_wait_for_phy_init(struct dmub_srv *dmub,
 enum dmub_status dmub_srv_wait_for_idle(struct dmub_srv *dmub,
 					uint32_t timeout_us)
 {
-	uint32_t i;
+	uint32_t i, rptr;
 
 	if (!dmub->hw_init)
 		return DMUB_STATUS_INVALID;
 
 	for (i = 0; i <= timeout_us; ++i) {
-			dmub->inbox1_rb.rptr = dmub->hw_funcs.get_inbox1_rptr(dmub);
+		rptr = dmub->hw_funcs.get_inbox1_rptr(dmub);
+
+		if (rptr > dmub->inbox1_rb.capacity)
+			return DMUB_STATUS_HW_FAILURE;
+
+		dmub->inbox1_rb.rptr = rptr;
+
 		if (dmub_rb_empty(&dmub->inbox1_rb))
 			return DMUB_STATUS_OK;