Re: [PATCH v2] ASoC: atmel_ssc_dai: Allow more rates

[Bo Shen]

Hi Peter,

On 02/07/2015 06:51 PM, Peter Rosin wrote:
> Mark Brown wrote:
> > On Wed, Feb 04, 2015 at 12:52:25PM +0100, Peter Rosin wrote:
> >
> > > One thing remains a bit unclear, and that is the 500ppm deduction. Is
> > > that really warranted? The number was just pulled out of my hat...
> >
> > I don't really get what this is supposed to be protecting against.
> >
> > > +	case SND_SOC_DAIFMT_CBM_CFS:
> > > +	case SND_SOC_DAIFMT_CBM_CFM:
> > > +		t.min = 8000;
> > > +		t.max = ssc_p->mck_rate / mck_div / frame_size;
> > > +		/* Take away 500ppm, just to be on the safe side. */
> > > +		t.max -= t.max / 2000;
> > > +		t.openmin = t.openmax = 0;
> > > +		t.integer = 0;
> > > +		ret = snd_interval_refine(i, &t);
> >
> > As I understand it this is a straight divider rather than something that's doing
> > dithering or anything else more fancy.  Given that it seems as well just to
> > trust the clock rate we've got - we don't do any error tracking with the clock
> > API (perhaps we should) and for many applications some degree of
> > divergence from the nominal rate is not
> > *too* bad for audio systems (for application specific values of "some"
> > and "too" of course).  If it is just dividers I'm not sure the situation is really
> > improved materially by knocking off the top frequency.
> >
> > If we are doing something more fancy than divididing my analysis is off base
> > of course.
>
> I'm thinking that the SSC samples the selected BCK pin using the (possibly
> divided) peripheral clock. Getting too near the theoretical rate limit would
> be bad, if these two independent clocks drift the wrong way. At least that
> is my take on it, but I don't know the internal workings of the SSC, so...
>
> I was hoping that someone from Atmel could chime in? Maybe I'm totally

Sorry for late response.

> off base, and the SSC is doing this completely differently?

What you mean here? I am not sure I fully understand.

> In our application, we're not near the limit. Therefore, it really doesn't
> matter much to us.
>
> Should I resend w/o the 500ppm deduction?
>
> Cheers,
> Peter

Best Regards,
Bo Shen
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