media: si2157: add on-demand rf strength func

Add get_rf_strength callback to get RSSI from the tuner. DVBv5
stat cache is updated. get_rf_strength is called by tuner_core
for analog tuners and is also used by some bridge drivers to
obtain RSSI directly from the tuner.
Signed-off-by: Brad Love <brad@nextdimension.cc>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/tuners/si2157.c

@@ -726,6 +726,42 @@ static int si2157_get_if_frequency(struct dvb_frontend *fe, u32 *frequency)
 	return 0;
 }
 
+static int si2157_get_rf_strength(struct dvb_frontend *fe, u16 *rssi)
+{
+	struct i2c_client *client = fe->tuner_priv;
+	struct dtv_frontend_properties *c = &fe->dtv_property_cache;
+	struct si2157_cmd cmd;
+	int ret;
+	int strength;
+
+	dev_dbg(&client->dev, "\n");
+
+	memcpy(cmd.args, "\x42\x00", 2);
+	cmd.wlen = 2;
+	cmd.rlen = 12;
+	ret = si2157_cmd_execute(client, &cmd);
+	if (ret)
+		goto err;
+
+	c->strength.stat[0].scale = FE_SCALE_DECIBEL;
+	c->strength.stat[0].svalue = (s8)cmd.args[3] * 1000;
+
+	/* normalize values based on Silicon Labs reference
+	 * add 100, then anything > 80 is 100% signal
+	 */
+	strength = (s8)cmd.args[3] + 100;
+	strength = clamp_val(strength, 0, 80);
+	*rssi = (u16)(strength * 0xffff / 80);
+
+	dev_dbg(&client->dev, "strength=%d rssi=%u\n",
+		(s8)cmd.args[3], *rssi);
+
+	return 0;
+err:
+	dev_dbg(&client->dev, "failed=%d\n", ret);
+	return ret;
+}
+
 static const struct dvb_tuner_ops si2157_ops = {
 	.info = {
 		.name             = "Silicon Labs Si2141/Si2146/2147/2148/2157/2158",
@@ -739,7 +775,9 @@ static const struct dvb_tuner_ops si2157_ops = {
 	.set_analog_params = si2157_set_analog_params,
 	.get_frequency     = si2157_get_frequency,
 	.get_bandwidth     = si2157_get_bandwidth,
-	.get_if_frequency = si2157_get_if_frequency,
+	.get_if_frequency  = si2157_get_if_frequency,
+
+	.get_rf_strength   = si2157_get_rf_strength,
 };
 
 static void si2157_stat_work(struct work_struct *work)