[media] cxd2841er: adjust the dB scale for DVB-C

Instead of using a relative frequency range, calibrate it to
show the results in dB. The callibration was done getting
samples with a signal generated from -50dBm to -12dBm,
incremented in steps of 0.5 dB, using 3 frequencies:
175 MHz, 410 MHz and 800 MHz. The modulated signal was
using QAM64, and it was used a linear interpolation of all
the results.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/drivers/media/dvb-frontends/cxd2841er.c b/drivers/media/dvb-frontends/cxd2841er.c
index 2a698711d25b..95d82c4445e2 100644
--- a/drivers/media/dvb-frontends/cxd2841er.c
+++ b/drivers/media/dvb-frontends/cxd2841er.c
@@ -1768,8 +1768,13 @@ static void cxd2841er_read_signal_strength(struct dvb_frontend *fe)
         case SYS_DVBC_ANNEX_A:
                 strength = cxd2841er_read_agc_gain_t_t2(priv,
                                                         p->delivery_system);
-                p->strength.stat[0].scale = FE_SCALE_RELATIVE;
-                p->strength.stat[0].uvalue = strength;
+                p->strength.stat[0].scale = FE_SCALE_DECIBEL;
+                /*
+                 * Formula was empirically determinated via linear regression,
+                 * using frequencies: 175 MHz, 410 MHz and 800 MHz, and a
+                 * stream modulated with QAM64
+                 */
+                p->strength.stat[0].uvalue = ((s32)strength) * 4045 / 1000 - 85224;
                 break;
         case SYS_ISDBT:
                 strength = 65535 - cxd2841er_read_agc_gain_i(