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

[Bo Shen <voice.shen@atmel.com>]

Hi Peter,

On 02/09/2015 03:35 PM, Peter Rosin wrote:
> Bo Shen wrote:
>> Hi Peter,
>
> Hi!
>
>> 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.
>
> No problem!
>
>>> off base, and the SSC is doing this completely differently?
>>
>> What you mean here? I am not sure I fully understand.
>
> The SSC spec list a maximum rate (which varies with the direction
> of various signals, ignoring that for the sake of this explanation). Lets
> assume that this maximum rate is 11MHz, derived from the peripheral
> clock which might be 66MHz. If you then try to input an 11MHz signal
> derived from some unrelated xtal you might think it should work. My
> theory was that the rate limit would be broken if the peripheral clock
> wasn't really 66MHz, but instead a few ppm lower than nominal, and
> the unrelated xtal was a few ppm higher than nominal.
>
> If this matters or not depends on how the SSC is implemented.

This is to let the user to know the clock limitation, am I right?

And at the same time deal with the un-precise clock which come to SSC? 
If this case, I think we should trust the clock come to SSC.

> There might be other reasons for not caring all that much about
> this fringe case, and just trust the nominal rates and limits.
>
>>> 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

Best Regards,
Bo Shen