drm/bridge: ti-sn65dsi86: Add retries for link training

On some panels hooked up to the ti-sn65dsi86 bridge chip we found that
link training was failing.  Specifically, we'd see:

  ti_sn65dsi86 2-002d: [drm:ti_sn_bridge_enable] *ERROR* Link training failed, link is off (-5)

The panel was hooked up to a logic analyzer and it was found that, as
part of link training, the bridge chip was writing a 0x1 to DPCD
address 00600h and the panel responded NACK.  As can be seen in header
files, the write of 0x1 to DPCD address 0x600h means we were trying to
write the value DP_SET_POWER_D0 to the register DP_SET_POWER.  The
panel vendor says that a NACK in this case is not unexpected and means
"not ready, try again".

In testing, we found that this panel would respond with a NACK in
about 1/25 times.  Adding the retry logic worked fine and the most
number of tries needed was 3.  Just to be safe, we'll add 10 tries
here and we'll add a little blurb to the logs if we ever need more
than 5.
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Tested-By: Steev Klimaszewski <steev@kali.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201002135920.1.I2adbc90b2db127763e2444bd5a4e5bf30e1db8e5@changeid

drivers/gpu/drm/bridge/ti-sn65dsi86.c

@@ -106,6 +106,8 @@
 #define SN_NUM_GPIOS			4
 #define SN_GPIO_PHYSICAL_OFFSET		1
 
+#define SN_LINK_TRAINING_TRIES		10
+
 /**
  * struct ti_sn_bridge - Platform data for ti-sn65dsi86 driver.
  * @dev:          Pointer to our device.
@@ -673,6 +675,7 @@ static int ti_sn_link_training(struct ti_sn_bridge *pdata, int dp_rate_idx,
 {
 	unsigned int val;
 	int ret;
+	int i;
 
 	/* set dp clk frequency value */
 	regmap_update_bits(pdata->regmap, SN_DATARATE_CONFIG_REG,
@@ -689,19 +692,34 @@ static int ti_sn_link_training(struct ti_sn_bridge *pdata, int dp_rate_idx,
 		goto exit;
 	}
 
-	/* Semi auto link training mode */
-	regmap_write(pdata->regmap, SN_ML_TX_MODE_REG, 0x0A);
-	ret = regmap_read_poll_timeout(pdata->regmap, SN_ML_TX_MODE_REG, val,
-				       val == ML_TX_MAIN_LINK_OFF ||
-				       val == ML_TX_NORMAL_MODE, 1000,
-				       500 * 1000);
-	if (ret) {
-		*last_err_str = "Training complete polling failed";
-	} else if (val == ML_TX_MAIN_LINK_OFF) {
-		*last_err_str = "Link training failed, link is off";
-		ret = -EIO;
+	/*
+	 * We'll try to link train several times.  As part of link training
+	 * the bridge chip will write DP_SET_POWER_D0 to DP_SET_POWER.  If
+	 * the panel isn't ready quite it might respond NAK here which means
+	 * we need to try again.
+	 */
+	for (i = 0; i < SN_LINK_TRAINING_TRIES; i++) {
+		/* Semi auto link training mode */
+		regmap_write(pdata->regmap, SN_ML_TX_MODE_REG, 0x0A);
+		ret = regmap_read_poll_timeout(pdata->regmap, SN_ML_TX_MODE_REG, val,
+					       val == ML_TX_MAIN_LINK_OFF ||
+					       val == ML_TX_NORMAL_MODE, 1000,
+					       500 * 1000);
+		if (ret) {
+			*last_err_str = "Training complete polling failed";
+		} else if (val == ML_TX_MAIN_LINK_OFF) {
+			*last_err_str = "Link training failed, link is off";
+			ret = -EIO;
+			continue;
+		}
+
+		break;
 	}
 
+	/* If we saw quite a few retries, add a note about it */
+	if (!ret && i > SN_LINK_TRAINING_TRIES / 2)
+		DRM_DEV_INFO(pdata->dev, "Link training needed %d retries\n", i);
+
 exit:
 	/* Disable the PLL if we failed */
 	if (ret)