[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 <hpj@urpla.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

1 files changed, 7 insertions, 21 deletions

diff --git a/drivers/media/dvb-frontends/cx24123.c b/drivers/media/dvb-frontends/cx24123.c
index 68c88ab58e71..a771da3e9f99 100644
--- a/drivers/media/dvb-frontends/cx24123.c
+++ b/drivers/media/dvb-frontends/cx24123.c
@@ -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);