drm/bridge: tc358767: Add configurable default preemphasis

Make the default DP port preemphasis configurable via new DT property
"toshiba,pre-emphasis". This is useful in case the DP link properties
are known and starting link training from preemphasis setting of 0 dB
is not useful. The preemphasis can be set separately for both DP lanes
in range 0=0dB, 1=3.5dB, 2=6dB .

Acked-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240708150130.54484-2-marex@denx.de

drivers/gpu/drm/bridge/tc358767.c

@@ -241,6 +241,10 @@
 
 /* Link Training */
 #define DP0_SRCCTRL		0x06a0
+#define DP0_SRCCTRL_PRE1		GENMASK(29, 28)
+#define DP0_SRCCTRL_SWG1		GENMASK(25, 24)
+#define DP0_SRCCTRL_PRE0		GENMASK(21, 20)
+#define DP0_SRCCTRL_SWG0		GENMASK(17, 16)
 #define DP0_SRCCTRL_SCRMBLDIS		BIT(13)
 #define DP0_SRCCTRL_EN810B		BIT(12)
 #define DP0_SRCCTRL_NOTP		(0 << 8)
@@ -278,6 +282,8 @@
 #define AUDIFDATA6		0x0720	/* DP0 Audio Info Frame Bytes 27 to 24 */
 
 #define DP1_SRCCTRL		0x07a0	/* DP1 Control Register */
+#define DP1_SRCCTRL_PRE			GENMASK(21, 20)
+#define DP1_SRCCTRL_SWG			GENMASK(17, 16)
 
 /* PHY */
 #define DP_PHY_CTRL		0x0800
@@ -369,6 +375,7 @@ struct tc_data {
 
 	u32			rev;
 	u8			assr;
+	u8			pre_emphasis[2];
 
 	struct gpio_desc	*sd_gpio;
 	struct gpio_desc	*reset_gpio;
@@ -1090,13 +1097,17 @@ static int tc_main_link_enable(struct tc_data *tc)
 			return ret;
 	}
 
-	ret = regmap_write(tc->regmap, DP0_SRCCTRL, tc_srcctrl(tc));
+	ret = regmap_write(tc->regmap, DP0_SRCCTRL,
+			   tc_srcctrl(tc) |
+			   FIELD_PREP(DP0_SRCCTRL_PRE0, tc->pre_emphasis[0]) |
+			   FIELD_PREP(DP0_SRCCTRL_PRE1, tc->pre_emphasis[1]));
 	if (ret)
 		return ret;
 	/* SSCG and BW27 on DP1 must be set to the same as on DP0 */
 	ret = regmap_write(tc->regmap, DP1_SRCCTRL,
 		 (tc->link.spread ? DP0_SRCCTRL_SSCG : 0) |
-		 ((tc->link.rate != 162000) ? DP0_SRCCTRL_BW27 : 0));
+		 ((tc->link.rate != 162000) ? DP0_SRCCTRL_BW27 : 0) |
+		 FIELD_PREP(DP1_SRCCTRL_PRE, tc->pre_emphasis[1]));
 	if (ret)
 		return ret;
 
@@ -1188,8 +1199,10 @@ static int tc_main_link_enable(struct tc_data *tc)
 		goto err_dpcd_write;
 
 	/* Reset voltage-swing & pre-emphasis */
-	tmp[0] = tmp[1] = DP_TRAIN_VOLTAGE_SWING_LEVEL_0 |
-			  DP_TRAIN_PRE_EMPH_LEVEL_0;
+	tmp[0] = DP_TRAIN_VOLTAGE_SWING_LEVEL_0 |
+		 FIELD_PREP(DP_TRAIN_PRE_EMPHASIS_MASK, tc->pre_emphasis[0]);
+	tmp[1] = DP_TRAIN_VOLTAGE_SWING_LEVEL_0 |
+		 FIELD_PREP(DP_TRAIN_PRE_EMPHASIS_MASK, tc->pre_emphasis[1]);
 	ret = drm_dp_dpcd_write(aux, DP_TRAINING_LANE0_SET, tmp, 2);
 	if (ret < 0)
 		goto err_dpcd_write;
@@ -1213,7 +1226,9 @@ static int tc_main_link_enable(struct tc_data *tc)
 	ret = regmap_write(tc->regmap, DP0_SRCCTRL,
 			   tc_srcctrl(tc) | DP0_SRCCTRL_SCRMBLDIS |
 			   DP0_SRCCTRL_AUTOCORRECT |
-			   DP0_SRCCTRL_TP1);
+			   DP0_SRCCTRL_TP1 |
+			   FIELD_PREP(DP0_SRCCTRL_PRE0, tc->pre_emphasis[0]) |
+			   FIELD_PREP(DP0_SRCCTRL_PRE1, tc->pre_emphasis[1]));
 	if (ret)
 		return ret;
 
@@ -1248,7 +1263,9 @@ static int tc_main_link_enable(struct tc_data *tc)
 	ret = regmap_write(tc->regmap, DP0_SRCCTRL,
 			   tc_srcctrl(tc) | DP0_SRCCTRL_SCRMBLDIS |
 			   DP0_SRCCTRL_AUTOCORRECT |
-			   DP0_SRCCTRL_TP2);
+			   DP0_SRCCTRL_TP2 |
+			   FIELD_PREP(DP0_SRCCTRL_PRE0, tc->pre_emphasis[0]) |
+			   FIELD_PREP(DP0_SRCCTRL_PRE1, tc->pre_emphasis[1]));
 	if (ret)
 		return ret;
 
@@ -1274,7 +1291,9 @@ static int tc_main_link_enable(struct tc_data *tc)
 
 	/* Clear Training Pattern, set AutoCorrect Mode = 1 */
 	ret = regmap_write(tc->regmap, DP0_SRCCTRL, tc_srcctrl(tc) |
-			   DP0_SRCCTRL_AUTOCORRECT);
+			   DP0_SRCCTRL_AUTOCORRECT |
+			   FIELD_PREP(DP0_SRCCTRL_PRE0, tc->pre_emphasis[0]) |
+			   FIELD_PREP(DP0_SRCCTRL_PRE1, tc->pre_emphasis[1]));
 	if (ret)
 		return ret;
 
@@ -2363,6 +2382,18 @@ static int tc_probe_bridge_endpoint(struct tc_data *tc)
 			return -EINVAL;
 		}
 		mode |= BIT(endpoint.port);
+
+		if (endpoint.port == 2) {
+			of_property_read_u8_array(node, "toshiba,pre-emphasis",
+						  tc->pre_emphasis,
+						  ARRAY_SIZE(tc->pre_emphasis));
+
+			if (tc->pre_emphasis[0] < 0 || tc->pre_emphasis[0] > 2 ||
+			    tc->pre_emphasis[1] < 0 || tc->pre_emphasis[1] > 2) {
+				dev_err(dev, "Incorrect Pre-Emphasis setting, use either 0=0dB 1=3.5dB 2=6dB\n");
+				return -EINVAL;
+			}
+		}
 	}
 
 	if (mode == mode_dpi_to_edp || mode == mode_dpi_to_dp) {