Commit 752a62b2 authored by Mauro Carvalho Chehab's avatar Mauro Carvalho Chehab
Browse files

[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: default avatarHans-Peter Jansen <hpj@urpla.net>
Signed-off-by: default avatarMauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: default avatarMichael Krufky <mkrufky@linuxtv.org>
Signed-off-by: default avatarMauro Carvalho Chehab <mchehab@redhat.com>
parent 0562aef2

drivers/media/dvb-frontends/cx24123.c

+7 −21
Original line number Diff line number Diff line
@@ -26,6 +26,7 @@ 
#include <linux/kernel.h>

#include <linux/module.h>

#include <linux/init.h>

#include <asm/div64.h>



#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);