drm: xlnx: zynqmp: release reset to DP controller before accessing DP registers

When insmod zynqmp-dpsub.ko after rmmod it, system will hang with the
error log as below:

root@xilinx-zynqmp:~# insmod zynqmp-dpsub.ko
[ 88.391289] [drm] Initialized zynqmp-dpsub 1.0.0 20130509 for fd4a0000.display on minor 0
[ 88.529906] Console: switching to colour frame buffer device 128x48
[ 88.549402] zynqmp-dpsub fd4a0000.display: [drm] fb0: zynqmp-dpsubdrm frame buffer device
[ 88.571624] zynqmp-dpsub fd4a0000.display: ZynqMP DisplayPort Subsystem driver probed
root@xilinx-zynqmp:~# rmmod zynqmp_dpsub
[ 94.023404] Console: switching to colour dummy device 80x25
root@xilinx-zynqmp:~# insmod zynqmp-dpsub.ko
<hang here>

This is because that in zynqmp_dp_probe it tries to access some DP
registers while the DP controller is still in the reset state. When
running "rmmod zynqmp_dpsub", zynqmp_dp_reset(dp, true) in
zynqmp_dp_phy_exit is called to force the DP controller into the reset
state. Then insmod will call zynqmp_dp_probe to program the DP registers,
but at this moment the DP controller hasn't been brought out of the reset
state yet since the function zynqmp_dp_reset(dp, false) is called later and
this will result the system hang.

Releasing the reset to DP controller before any read/write operation to it
will fix this issue. And for symmetry, move zynqmp_dp_reset() call from
zynqmp_dp_phy_exit() to zynqmp_dp_remove().

Signed-off-by: Quanyang Wang <[e-mail]>
Reviewed-by: Laurent Pinchart <[e-mail]>
Signed-off-by: Laurent Pinchart <[e-mail]>

drivers/gpu/drm/xlnx/zynqmp_dp.c

···
 		}
 	}
 
-	ret = zynqmp_dp_reset(dp, false);
-	if (ret < 0)
-		return ret;
-
 	zynqmp_dp_clr(dp, ZYNQMP_DP_PHY_RESET, ZYNQMP_DP_PHY_RESET_ALL_RESET);
 
 	/*
···
 			dev_err(dp->dev, "failed to power off phy(%d) %d\n", i,
 				ret);
 	}
-
-	zynqmp_dp_reset(dp, true);
 
 	for (i = 0; i < dp->num_lanes; i++) {
 		ret = phy_exit(dp->phy[i]);
···
 		return PTR_ERR(dp->reset);
 	}
 
+	ret = zynqmp_dp_reset(dp, false);
+	if (ret < 0)
+		return ret;
+
 	ret = zynqmp_dp_phy_probe(dp);
 	if (ret)
-		return ret;
+		goto err_reset;
 
 	/* Initialize the hardware. */
 	zynqmp_dp_write(dp, ZYNQMP_DP_TX_PHY_POWER_DOWN,
···
 
 	ret = zynqmp_dp_phy_init(dp);
 	if (ret)
-		return ret;
+		goto err_reset;
 
 	zynqmp_dp_write(dp, ZYNQMP_DP_TRANSMITTER_ENABLE, 1);
 
···
 					zynqmp_dp_irq_handler, IRQF_ONESHOT,
 					dev_name(dp->dev), dp);
 	if (ret < 0)
-		goto error;
+		goto err_phy_exit;
 
 	dev_dbg(dp->dev, "ZynqMP DisplayPort Tx probed with %u lanes\n",
 		dp->num_lanes);
 
 	return 0;
 
-error:
+err_phy_exit:
 	zynqmp_dp_phy_exit(dp);
+err_reset:
+	zynqmp_dp_reset(dp, true);
+
 	return ret;
 }
 
···
 	zynqmp_dp_write(dp, ZYNQMP_DP_INT_DS, 0xffffffff);
 
 	zynqmp_dp_phy_exit(dp);
+	zynqmp_dp_reset(dp, true);
 }