drm/bridge: ti-sn65dsi86: Read num lanes from the DP sink

At least one panel hooked up to the bridge (AUO B116XAK01) only
supports 1 lane of DP.  Let's read this information and stop
hardcoding 4 DP lanes.
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191218143416.v3.5.Idbd0051d0de53f7e9d18a291ea33011c0854fcc6@changeid

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

@@ -313,8 +313,7 @@ static int ti_sn_bridge_attach(struct drm_bridge *bridge)
 		goto err_dsi_host;
 	}
 
-	/* TODO: setting to 4 lanes always for now */
-	pdata->dp_lanes = 4;
+	/* TODO: setting to 4 MIPI lanes always for now */
 	dsi->lanes = 4;
 	dsi->format = MIPI_DSI_FMT_RGB888;
 	dsi->mode_flags = MIPI_DSI_MODE_VIDEO;
@@ -511,12 +510,41 @@ static void ti_sn_bridge_set_video_timings(struct ti_sn_bridge *pdata)
 	usleep_range(10000, 10500); /* 10ms delay recommended by spec */
 }
 
+static unsigned int ti_sn_get_max_lanes(struct ti_sn_bridge *pdata)
+{
+	u8 data;
+	int ret;
+
+	ret = drm_dp_dpcd_readb(&pdata->aux, DP_MAX_LANE_COUNT, &data);
+	if (ret != 1) {
+		DRM_DEV_ERROR(pdata->dev,
+			      "Can't read lane count (%d); assuming 4\n", ret);
+		return 4;
+	}
+
+	return data & DP_LANE_COUNT_MASK;
+}
+
 static void ti_sn_bridge_enable(struct drm_bridge *bridge)
 {
 	struct ti_sn_bridge *pdata = bridge_to_ti_sn_bridge(bridge);
 	unsigned int val;
 	int ret;
 
+	/*
+	 * Run with the maximum number of lanes that the DP sink supports.
+	 *
+	 * Depending use cases, we might want to revisit this later because:
+	 * - It's plausible that someone may have run fewer lines to the
+	 *   sink than the sink actually supports, assuming that the lines
+	 *   will just be driven at a higher rate.
+	 * - The DP spec seems to indicate that it's more important to minimize
+	 *   the number of lanes than the link rate.
+	 *
+	 * If we do revisit, it would be important to measure the power impact.
+	 */
+	pdata->dp_lanes = ti_sn_get_max_lanes(pdata);
+
 	/* DSI_A lane config */
 	val = CHA_DSI_LANES(4 - pdata->dsi->lanes);
 	regmap_update_bits(pdata->regmap, SN_DSI_LANES_REG,