[media] dib8000: estimate strength in dBm

Better to have Signal strength in dB.
This takes a very rough estimation for the signal strength,
that was calibrated using a Dektec DTA-2111 Gold RF generator
and a Pixelview dib8076 stick.

It estimates the signal strength using a linear equation where:
	- the max is -22.5 dBm, with returns 55953
	- the min is -35.0 dBm, with returns 50110

With -22dBm, the signal strengh is returned as 65535.
Unfortunately, the min strength generated with DTA-2111 is
-35dBm.

It should be noticed that approximating it by a linear equation
is not right. I should probably be splitting it into 0.5 dB
linear segments, in order to get a higher precision, just like
it is done on mb86a20s, but that would force me to add some
attenuators, in order to get dB levels below -35dBm, which is,
btw, strong enough to get signal lock.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Acked-by: Patrick Boettcher <pboettcher@kernellabs.com>

drivers/media/dvb-frontends/dib8000.c

@@ -11,6 +11,7 @@
 #include <linux/slab.h>
 #include <linux/i2c.h>
 #include <linux/mutex.h>
+#include <asm/div64.h>
 
 #include "dvb_math.h"
 
@@ -1002,7 +1003,7 @@ static void dib8000_reset_stats(struct dvb_frontend *fe)
 	c->post_bit_error.len = 1;
 	c->post_bit_count.len = 1;
 
-	c->strength.stat[0].scale = FE_SCALE_RELATIVE;
+	c->strength.stat[0].scale = FE_SCALE_DECIBEL;
 	c->strength.stat[0].uvalue = 0;
 
 	c->cnr.stat[0].scale = FE_SCALE_NOT_AVAILABLE;
@@ -3847,12 +3848,30 @@ static int dib8000_get_stats(struct dvb_frontend *fe, fe_status_t stat)
 	struct dib8000_state *state = fe->demodulator_priv;
 	struct dtv_frontend_properties *c = &state->fe[0]->dtv_property_cache;
 	int i, lock;
+	u64 tmp;
 	u32 snr, val;
 	u16 strength;
 
 	/* Get Signal strength */
 	dib8000_read_signal_strength(fe, &strength);
-	c->strength.stat[0].uvalue = strength;
+
+	/*
+	 * Estimate it in dBm
+	 * This calculus was empirically determinated by measuring the signal
+	 * strength generated by a DTA-2111 RF generator directly connected into
+	 * a dib8076 device. The real value can actually be different on other
+	 * devices, depending if LNA is enabled or not, if diversity is enabled,
+	 * etc.
+	 */
+	if (strength == 65535) {
+		c->strength.stat[0].svalue = -22000;
+	} else {
+		tmp = strength * 25000L;
+		do_div(tmp, 11646);
+		c->strength.stat[0].svalue = tmp - 142569;
+		if (c->strength.stat[0].svalue > -22000)
+			c->strength.stat[0].svalue = -22000;
+	}
 
 	/* Check if 1 second was elapsed */
 	if (!time_after(jiffies, state->get_stats_time))