media: i2c: max9286: Configure reverse channel amplitude

Adjust the initial reverse channel amplitude parsing from
firmware interface the 'maxim,reverse-channel-microvolt'
property.

This change is required for both rdacm20 and rdacm21 camera
modules to be correctly probed when used in combination with
the max9286 deserializer.
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/i2c/max9286.c

@@ -163,6 +163,8 @@ struct max9286_priv {
 	unsigned int mux_channel;
 	bool mux_open;
 
+	u32 reverse_channel_mv;
+
 	struct v4l2_ctrl_handler ctrls;
 	struct v4l2_ctrl *pixelrate;
 
@@ -556,10 +558,14 @@ static int max9286_notify_bound(struct v4l2_async_notifier *notifier,
 	 * All enabled sources have probed and enabled their reverse control
 	 * channels:
 	 *
+	 * - Increase the reverse channel amplitude to compensate for the
+	 *   remote ends high threshold, if not done already
 	 * - Verify all configuration links are properly detected
 	 * - Disable auto-ack as communication on the control channel are now
 	 *   stable.
 	 */
+	if (priv->reverse_channel_mv < 170)
+		max9286_reverse_channel_setup(priv, 170);
 	max9286_check_config_link(priv, priv->source_mask);
 
 	/*
@@ -966,7 +972,7 @@ static int max9286_setup(struct max9286_priv *priv)
 	 * only. This should be disabled after the mux is initialised.
 	 */
 	max9286_configure_i2c(priv, true);
-	max9286_reverse_channel_setup(priv, 170);
+	max9286_reverse_channel_setup(priv, priv->reverse_channel_mv);
 
 	/*
 	 * Enable GMSL links, mask unused ones and autodetect link
@@ -1130,6 +1136,7 @@ static int max9286_parse_dt(struct max9286_priv *priv)
 	struct device_node *i2c_mux;
 	struct device_node *node = NULL;
 	unsigned int i2c_mux_mask = 0;
+	u32 reverse_channel_microvolt;
 
 	/* Balance the of_node_put() performed by of_find_node_by_name(). */
 	of_node_get(dev->of_node);
@@ -1220,6 +1227,20 @@ static int max9286_parse_dt(struct max9286_priv *priv)
 	}
 	of_node_put(node);
 
+	/*
+	 * Parse the initial value of the reverse channel amplitude from
+	 * the firmware interface and convert it to millivolts.
+	 *
+	 * Default it to 170mV for backward compatibility with DTBs that do not
+	 * provide the property.
+	 */
+	if (of_property_read_u32(dev->of_node,
+				 "maxim,reverse-channel-microvolt",
+				 &reverse_channel_microvolt))
+		priv->reverse_channel_mv = 170;
+	else
+		priv->reverse_channel_mv = reverse_channel_microvolt / 1000U;
+
 	priv->route_mask = priv->source_mask;
 
 	return 0;