[media] cxd2841er: Fix signal strengh for DVB-T/T2 and show it in dBm

The signal strength value is reversed: the bigger the number,
the weaker is the signal.

Fix the logic and present it in dBm. Please notice that the
dBm measure is actually an estimation, as the ratio is not
fully linear. It also varies with the frequency.

Yet, the estimation should be good enough for programs like
Kaffeine to indicate when the signal is good or bad.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Abylay Ospan <aospan@netup.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/dvb-frontends/cxd2841er.c

@@ -1727,32 +1727,39 @@ static int cxd2841er_read_ber(struct dvb_frontend *fe, u32 *ber)
 	return 0;
 }
 
-static int cxd2841er_read_signal_strength(struct dvb_frontend *fe,
-					  u16 *strength)
+static void cxd2841er_read_signal_strength(struct dvb_frontend *fe)
 {
 	struct dtv_frontend_properties *p = &fe->dtv_property_cache;
 	struct cxd2841er_priv *priv = fe->demodulator_priv;
+	u32 strength;
 
 	dev_dbg(&priv->i2c->dev, "%s()\n", __func__);
 	switch (p->delivery_system) {
 	case SYS_DVBT:
 	case SYS_DVBT2:
-		*strength = 65535 - cxd2841er_read_agc_gain_t_t2(
-			priv, p->delivery_system);
-		break;
+		strength = cxd2841er_read_agc_gain_t_t2(priv,
+							p->delivery_system);
+		p->strength.stat[0].scale = FE_SCALE_DECIBEL;
+		/* Formula was empirically determinated @ 410 MHz */
+		p->strength.stat[0].uvalue = ((s32)strength) * 366 / 100 - 89520;
+		break;	/* Code moved out of the function */
 	case SYS_ISDBT:
-		*strength = 65535 - cxd2841er_read_agc_gain_i(
+		strength = 65535 - cxd2841er_read_agc_gain_i(
 				priv, p->delivery_system);
+		p->strength.stat[0].scale = FE_SCALE_RELATIVE;
+		p->strength.stat[0].uvalue = strength;
 		break;
 	case SYS_DVBS:
 	case SYS_DVBS2:
-		*strength = 65535 - cxd2841er_read_agc_gain_s(priv);
+		strength = 65535 - cxd2841er_read_agc_gain_s(priv);
+		p->strength.stat[0].scale = FE_SCALE_RELATIVE;
+		p->strength.stat[0].uvalue = strength;
 		break;
 	default:
-		*strength = 0;
+		p->strength.stat[0].scale = FE_SCALE_RELATIVE;
+		p->strength.stat[0].uvalue = 0;
 		break;
 	}
-	return 0;
 }
 
 static int cxd2841er_read_snr(struct dvb_frontend *fe, u16 *snr)
@@ -2926,7 +2933,7 @@ static int cxd2841er_get_frontend(struct dvb_frontend *fe,
 				  struct dtv_frontend_properties *p)
 {
 	enum fe_status status = 0;
-	u16 strength = 0, snr = 0;
+	u16 snr = 0;
 	u32 errors = 0, ber = 0;
 	struct cxd2841er_priv *priv = fe->demodulator_priv;
 
@@ -2936,9 +2943,7 @@ static int cxd2841er_get_frontend(struct dvb_frontend *fe,
 	else if (priv->state == STATE_ACTIVE_TC)
 		cxd2841er_read_status_tc(fe, &status);
 
-	cxd2841er_read_signal_strength(fe, &strength);
-	p->strength.stat[0].scale = FE_SCALE_RELATIVE;
-	p->strength.stat[0].uvalue = strength;
+	cxd2841er_read_signal_strength(fe);
 
 	if (status & FE_HAS_LOCK) {
 		cxd2841er_read_snr(fe, &snr);