drm/bridge: tc358767: use more reliable seq when finishing LT

At the end of the link training, two steps have to be taken: 1)
tc358767's LT mode is disabled by a write to DP0_SRCCTRL, and 2) Remove
LT flag in DPCD 0x102.

Toshiba's documentation tells to first write the DPCD, then modify
DP0_SRCCTRL. In my testing this often causes issues, and the link
disconnects right after those steps.

If I reverse the steps, it works every time. There's a chance that this
is DP sink specific, though, but as my testing shows this sequence to be
much more reliable, let's change it.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190528082747.3631-15-tomi.valkeinen@ti.com

drivers/gpu/drm/bridge/tc358767.c

@@ -961,6 +961,18 @@ static int tc_main_link_enable(struct tc_data *tc)
 	if (ret)
 		goto err;
 
+	/*
+	 * Toshiba's documentation suggests to first clear DPCD 0x102, then
+	 * clear the training pattern bit in DP0_SRCCTRL. Testing shows
+	 * that the link sometimes drops if those steps are done in that order,
+	 * but if the steps are done in reverse order, the link stays up.
+	 *
+	 * So we do the steps differently than documented here.
+	 */
+
+	/* Clear Training Pattern, set AutoCorrect Mode = 1 */
+	tc_write(DP0_SRCCTRL, tc_srcctrl(tc) | DP0_SRCCTRL_AUTOCORRECT);
+
 	/* Clear DPCD 0x102 */
 	/* Note: Can Not use DP0_SNKLTCTRL (0x06E4) short cut */
 	tmp[0] = tc->link.scrambler_dis ? DP_LINK_SCRAMBLING_DISABLE : 0x00;
@@ -968,9 +980,6 @@ static int tc_main_link_enable(struct tc_data *tc)
 	if (ret < 0)
 		goto err_dpcd_write;
 
-	/* Clear Training Pattern, set AutoCorrect Mode = 1 */
-	tc_write(DP0_SRCCTRL, tc_srcctrl(tc) | DP0_SRCCTRL_AUTOCORRECT);
-
 	/* Wait */
 	timeout = 100;
 	do {