From 752a62b2ec9c21931dce869f2ae2c921c83a1164 Mon Sep 17 00:00:00 2001 From: Mauro Carvalho Chehab Date: Sun, 7 Apr 2013 21:11:53 -0300 Subject: [media] cx24123: improve precision when calculating symbol rate ratio Symbol rate ratio were using a rough calculus, as the code was limited to 32 bits arithmetic. Change it to 64 bits, in order to better estimate the bandwidth low-pass filter on the demod. This should reduce the noise and improve reception. Reported-by: Hans-Peter Jansen Signed-off-by: Mauro Carvalho Chehab Signed-off-by: Michael Krufky Signed-off-by: Mauro Carvalho Chehab diff --git a/drivers/media/dvb-frontends/cx24123.c b/drivers/media/dvb-frontends/cx24123.c index 68c88ab..a771da3 100644 --- a/drivers/media/dvb-frontends/cx24123.c +++ b/drivers/media/dvb-frontends/cx24123.c @@ -26,6 +26,7 @@ #include #include #include +#include #include "dvb_frontend.h" #include "cx24123.h" @@ -452,7 +453,8 @@ static u32 cx24123_int_log2(u32 a, u32 b) static int cx24123_set_symbolrate(struct cx24123_state *state, u32 srate) { - u32 tmp, sample_rate, ratio, sample_gain; + u64 tmp; + u32 sample_rate, ratio, sample_gain; u8 pll_mult; /* check if symbol rate is within limits */ @@ -482,27 +484,11 @@ static int cx24123_set_symbolrate(struct cx24123_state *state, u32 srate) sample_rate = pll_mult * XTAL; - /* - SYSSymbolRate[21:0] = (srate << 23) / sample_rate - - We have to use 32 bit unsigned arithmetic without precision loss. - The maximum srate is 45000000 or 0x02AEA540. This number has - only 6 clear bits on top, hence we can shift it left only 6 bits - at a time. Borrowed from cx24110.c - */ - - tmp = srate << 6; - ratio = tmp / sample_rate; - - tmp = (tmp % sample_rate) << 6; - ratio = (ratio << 6) + (tmp / sample_rate); - - tmp = (tmp % sample_rate) << 6; - ratio = (ratio << 6) + (tmp / sample_rate); - - tmp = (tmp % sample_rate) << 5; - ratio = (ratio << 5) + (tmp / sample_rate); + /* SYSSymbolRate[21:0] = (srate << 23) / sample_rate */ + tmp = ((u64)srate) << 23; + do_div(tmp, sample_rate); + ratio = (u32) tmp; cx24123_writereg(state, 0x01, pll_mult * 6); -- cgit v0.10.2