media: i2c: max9286: Make channel amplitude programmable

Instrument the function that configures the reverse channel with a
programmable amplitude value.

This change serves to prepare to adjust the reverse channel amplitude
depending on the remote end high-threshold configuration.
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

@@ -336,19 +336,28 @@ static void max9286_configure_i2c(struct max9286_priv *priv, bool localack)
 	usleep_range(3000, 5000);
 }
 
-static void max9286_reverse_channel_setup(struct max9286_priv *priv)
+static void max9286_reverse_channel_setup(struct max9286_priv *priv,
+					  unsigned int chan_amplitude)
 {
+	/* Reverse channel transmission time: default to 1. */
+	u8 chan_config = MAX9286_REV_TRF(1);
+
 	/*
 	 * Reverse channel setup.
 	 *
 	 * - Enable custom reverse channel configuration (through register 0x3f)
 	 *   and set the first pulse length to 35 clock cycles.
-	 * - Increase the reverse channel amplitude to 170mV to accommodate the
-	 *   high threshold enabled by the serializer driver.
+	 * - Adjust reverse channel amplitude: values > 130 are programmed
+	 *   using the additional +100mV REV_AMP_X boost flag
 	 */
 	max9286_write(priv, 0x3f, MAX9286_EN_REV_CFG | MAX9286_REV_FLEN(35));
-	max9286_write(priv, 0x3b, MAX9286_REV_TRF(1) | MAX9286_REV_AMP(70) |
-		      MAX9286_REV_AMP_X);
+
+	if (chan_amplitude > 100) {
+		/* It is not possible to express values (100 < x < 130) */
+		chan_amplitude = max(30U, chan_amplitude - 100);
+		chan_config |= MAX9286_REV_AMP_X;
+	}
+	max9286_write(priv, 0x3b, chan_config | MAX9286_REV_AMP(chan_amplitude));
 	usleep_range(2000, 2500);
 }
 
@@ -957,7 +966,7 @@ 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);
+	max9286_reverse_channel_setup(priv, 170);
 
 	/*
 	 * Enable GMSL links, mask unused ones and autodetect link