Re: [PATCH 6/9] libdvbv5/dvb-v5-std.c: add DTV_BANDWIDTH_HZ where possible

[Mauro Carvalho Chehab <mchehab+huawei@kernel.org>]

Em Thu,  5 Jun 2025 12:58:29 +0200
Hans Verkuil <hverkuil@xs4all.nl> escreveu:

> Several systems support DTV_BANDWIDTH_HZ, add it.
> 
> This fixes a dvbv5-scan error message about missing support for
> DTV_BANDWIDTH_HZ.
> 
> Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
> ---
>  lib/libdvbv5/dvb-v5-std.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/lib/libdvbv5/dvb-v5-std.c b/lib/libdvbv5/dvb-v5-std.c
> index c0a14175..74e2e4fe 100644
> --- a/lib/libdvbv5/dvb-v5-std.c
> +++ b/lib/libdvbv5/dvb-v5-std.c
> @@ -81,6 +81,7 @@ const unsigned int sys_isdbt_props[] = {
>  const unsigned int sys_atsc_props[] = {
>  	DTV_FREQUENCY,
>  	DTV_MODULATION,
> +	DTV_BANDWIDTH_HZ,
>  	0
>  };

Indeed, ISDB-T could have a bandwidth. In the beginning, only
Japan and Brazil were using it, with a 6MHz bandwidth. As far as
I remember, all drivers we currently have are for devices with
such limit. Yet, the spec allows other bandwidths as well. Not sure
if any Country is using a different bandwidth in practice, though.

> @@ -111,12 +112,14 @@ const unsigned int sys_dvbc_annex_ac_props[] = {
>  	DTV_INVERSION,
>  	DTV_SYMBOL_RATE,
>  	DTV_INNER_FEC,
> +	DTV_BANDWIDTH_HZ,
>  	0
>  };
>  
>  const unsigned int sys_dvbc_annex_b_props[] = {
>  	DTV_FREQUENCY,
>  	DTV_MODULATION,
> +	DTV_BANDWIDTH_HZ,
>  	0
>  };
>  
> @@ -126,6 +129,7 @@ const unsigned int sys_dvbs_props[] = {
>  	DTV_SYMBOL_RATE,
>  	DTV_INNER_FEC,
>  	DTV_POLARIZATION,
> +	DTV_BANDWIDTH_HZ,
>  	0
>  };
>  
> @@ -139,6 +143,7 @@ const unsigned int sys_dvbs2_props[] = {
>  	DTV_ROLLOFF,
>  	DTV_POLARIZATION,
>  	DTV_STREAM_ID,
> +	DTV_BANDWIDTH_HZ,
>  	0
>  };
>  
> @@ -149,12 +154,14 @@ const unsigned int sys_turbo_props[] = {
>  	DTV_INNER_FEC,
>  	DTV_MODULATION,
>  	DTV_POLARIZATION,
> +	DTV_BANDWIDTH_HZ,
>  	0
>  };
>  
>  const unsigned int sys_isdbs_props[] = {
>  	DTV_FREQUENCY,
>  	DTV_STREAM_ID,
> +	DTV_BANDWIDTH_HZ,
>  	0
>  };

The above are not right: Satellite and Cable don't use bandwidth.
Instead, the bandwidth is indirectly calculated from the symbol
rate and rolloff, using something like this:

	float rolloff = 1.35;	/* DVB-S rolloff */

	int bandwidth_hz = int(symbol_rate * rolloff);



For DVB-C Annex A and B, and for DVB-S (and, afaikt, for DVB-TURBO), the
rolloff is fixed. DVB-S2 is the only one that supports different
rolloff factors.

In any case, DVB core calculates it. See this code snippet:

        switch (c->delivery_system) {
        case SYS_ATSC:
        case SYS_DVBC_ANNEX_B:
                c->bandwidth_hz = 6000000;
                break;
        case SYS_DVBC_ANNEX_A:
                rolloff = 115;
                break;
        case SYS_DVBC_ANNEX_C:
                rolloff = 113;
                break;
        case SYS_DSS:
                rolloff = 120;
                break;
        case SYS_DVBS:
        case SYS_TURBO:
        case SYS_ISDBS:
                rolloff = 135;
                break;
        case SYS_DVBS2:
                switch (c->rolloff) {
                case ROLLOFF_20:
                        rolloff = 120;
                        break;
                case ROLLOFF_25:
                        rolloff = 125;
                        break;
                default:
                case ROLLOFF_35:
                        rolloff = 135;
                }
                break;
        default:
                break;
        }
        if (rolloff)
                c->bandwidth_hz = mult_frac(c->symbol_rate, rolloff, 100);

The Kernel calculates the bandwidth and may return it, but the opposite
is not true: any set operation for a TV standard that has DTV_SYMBOL_RATE
will simply discard/ignore what is there at DTV_BANDWIDTH_HZ.

Currently, the logic inside libdvbv5 assumes that all parameters are
read/write.

Thanks,
Mauro