drm: xlnx: zynqmp_dp: Fix kernel dump messages when the DP cable is d…

…isconnected

Below error message is shown when the DP cable is disconnected.

[  108.780458] zynqmp-display fd4a0000.display: failed to write a byte to the DPCD: -19

This error message will confuse the user because the aux channel is not
present after unplugging the cable, so any read / write operation to the
DPCD register will fail.

So adding a drm_dp_dpcd_readb() before we write to the DPCD register. And
if the read fails, skip dpcd_writeb and continue in the function to power
down phy and disable the audio to avoid a false alarm message.

Signed-off-by: Hsuan-Yu Lin <[email protected]>
Reviewed-by: Portia Stephens <[email protected]>
Reviewed-by: Vishal Sagar <[email protected]>

drivers/gpu/drm/xlnx/zynqmp_dp.c

@@ -1649,15 +1649,20 @@ static void zynqmp_dp_encoder_disable(struct drm_encoder *encoder)
 	struct zynqmp_dp *dp = encoder_to_dp(encoder);
 	void __iomem *iomem = dp->iomem;
 	int ret;
+	u8 pwr;
 
 	dp->enabled = false;
 	cancel_delayed_work(&dp->hpd_work);
 	zynqmp_dp_write(iomem, ZYNQMP_DP_TX_ENABLE_MAIN_STREAM, 0);
-	ret = drm_dp_dpcd_writeb(&dp->aux, DP_SET_POWER, DP_SET_POWER_D3);
-	if (ret < 0) {
-		dev_err(dp->dev, "failed to write a byte to the DPCD: %d\n",
-			ret);
-		return;
+	ret = drm_dp_dpcd_readb(&dp->aux, DP_SET_POWER, &pwr);
+	/* Do write only if read succeeds */
+	if (ret >= 0) {
+		ret = drm_dp_dpcd_writeb(&dp->aux, DP_SET_POWER, DP_SET_POWER_D3);
+		if (ret < 0) {
+			dev_err(dp->dev, "failed to write a byte to the DPCD: %d\n",
+				ret);
+			return;
+		}
 	}
 	zynqmp_dp_write(iomem, ZYNQMP_DP_TX_PHY_POWER_DOWN,
 			ZYNQMP_DP_TX_PHY_POWER_DOWN_ALL);