Re: [PATCH] si2157: Fix DVB-C bandwidth.

[Mauro Carvalho Chehab <m.chehab@samsung.com>]

Em Tue, 22 Jul 2014 17:28:07 +0100
Luis Alves <ljalvs@gmail.com> escreveu:

> That's right,
> A few days ago I also checked that with Antti. I've also had made some
> debugging and DVB core is in fact passing the correct bandwidth to the
> driver.
> 
> But the true is that it doesn't work...
> The sample I have is a dvb-c mux using QAM128 @ 6 Mbaud (which results
> in 7MHz bw) using 7MHz filter value will make the TS stream
> unwatchable (lots of continuity errors).
> 
> Can this be a hardware fault?
> All closed source drivers I've seen are hardcoding this value to 8MHz
> when working in dvb-c (easily seen on i2c sniffs).

Could be. Well, here, the DVB-C channel operators use 6MHz-spaced channels,
with symbol rate equal to 5,217 Kbaud. I'll see if I can test it latter
this week with a PCTV 292e.

Regards,
Mauro
> 
> 
> On Tue, Jul 22, 2014 at 5:10 PM, Mauro Carvalho Chehab
> <m.chehab@samsung.com> wrote:
> > Em Tue, 22 Jul 2014 12:09:48 +0100
> > Luis Alves <ljalvs@gmail.com> escreveu:
> >
> >> This patch fixes DVB-C reception.
> >> Without setting the bandwidth to 8MHz the received stream gets corrupted.
> >>
> >> Regards,
> >> Luis
> >>
> >> Signed-off-by: Luis Alves <ljalvs@gmail.com>
> >> ---
> >>  drivers/media/tuners/si2157.c | 1 +
> >>  1 file changed, 1 insertion(+)
> >>
> >> diff --git a/drivers/media/tuners/si2157.c b/drivers/media/tuners/si2157.c
> >> index 6c53edb..e2de428 100644
> >> --- a/drivers/media/tuners/si2157.c
> >> +++ b/drivers/media/tuners/si2157.c
> >> @@ -245,6 +245,7 @@ static int si2157_set_params(struct dvb_frontend *fe)
> >>                       break;
> >>       case SYS_DVBC_ANNEX_A:
> >>                       delivery_system = 0x30;
> >> +                     bandwidth = 0x08;
> >
> > Hmm... this patch looks wrong, as it will break DVB-C support where
> > the bandwidth is lower than 6MHz.
> >
> > The DVB core sets c->bandwidth_hz for DVB-C based on the rolloff and
> > the symbol rate. If this is not working for you, then something else
> > is likely wrong.
> >
> > I suggest you to add a printk() there to show what's the value set
> > at c->bandwidth_hz and what's the symbol rate that you're using.
> >
> > On DVB-C, the rolloff is fixed (1.15 for annex A and 1.13 for Annex C).
> > Not sure if DVB-C2 allows selecting a different rolloff factor, nor
> > if si2157 works with DVB-C2.
> >
> >>                       break;
> >>       default:
> >>                       ret = -EINVAL;