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

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

Hi Peter,

On 02/09/2015 05:07 PM, Peter Rosin wrote:
> Bo Shen wrote:
>> Hi Peter,
>>
>> On 02/09/2015 04:09 PM, Peter Rosin wrote:
>>
>> [Snip]
>>
>>>>>
>>>>>     /*-------------------------------------------------------------------------*\
>>>>>      * DAI functions
>>>>> @@ -200,6 +290,7 @@ static int atmel_ssc_startup(struct
>> snd_pcm_substream *substream,
>>>>>     	struct atmel_ssc_info *ssc_p = &ssc_info[dai->id];
>>>>>     	struct atmel_pcm_dma_params *dma_params;
>>>>>     	int dir, dir_mask;
>>>>> +	int ret;
>>>>>
>>>>>     	pr_debug("atmel_ssc_startup: SSC_SR=0x%u\n",
>>>>>     		ssc_readl(ssc_p->ssc->regs, SR)); @@ -207,6 +298,7 @@
>> static
>>>>> int atmel_ssc_startup(struct snd_pcm_substream *substream,
>>>>>     	/* Enable PMC peripheral clock for this SSC */
>>>>>     	pr_debug("atmel_ssc_dai: Starting clock\n");
>>>>>     	clk_enable(ssc_p->ssc->clk);
>>>>> +	ssc_p->mck_rate = clk_get_rate(ssc_p->ssc->clk) * 2;
>>>>
>>>> Why the mck_rate is calculated in this form?
>>>
>>> What did you have in mind? Add another clock to the ssc node in the
>>> device tree?
>>>
>>> IIUC, the device tree (at least normally) has the ssc clk as the
>>> peripheral clock divided by 2, but the ssc specifies (when capturing
>>> in the CBM/CFS
>>> case) the rate limit as the peripheral clock divided by 3 (i.e. ssc clk / 1.5).
>>> Since the SSC spec expresses the rate limit in terms of the peripheral
>>> clock, this was what I came up with. I didn't want to require dt changes...
>>
>> You make a misunderstand for the mck for ssc peripheral. The mck here is
>> not the system mck, it only related with the ssc, it is the PMC output.
>> For example, in device tree, the ssc clock divided by 2, then the pmc output
>> for ssc is "system mck / 2", so the ssc mck is "system mck / 2".
>> If divided by 4, then the ssc mck is "system / 4"
>
> I think the reason for my misunderstanding might be that in the
> 3.10-at91 tree, the ssc clk is twice the rate compared to what it is
> in the 3.18-at91 tree. This made me assume that the ssc clk had

I think maybe you didn't use the latest linux-3.10-at91 code. In that 
code, we also divided by 2 in device tree.

> been changed to mean the rate after the fixed divider by two that
> is activated as soon as the ssc clock divider (given by SSC_CMR) is
> activated, and that it was a simple matter of multiplying by two to
> get to the MCK rate. I further assumed that "Master Clock" in the
> "Serial Clock Ratio Considerations" section was this MCK. Maybe
> the mistake was to involve the peripheral clock at all?
>
> Ok, so I may have misunderstood, but in that case what does that
> mean in terms of finding the "Master Clock" rate that is mentioned
> in the "Serial Clock Ratio Considerations" section? Is it perhaps the
> rate of the parent clock of the given ssc clk? Or, given the above
> explanation, is it correct to simply multiply by two as I have done?

The "Master Clock" actually is the same as "Peripheral clock".

Thanks for your information, I will send this to our datasheet team to 
update this part.

> [snip]
>
> Cheers,
> Peter
>

Best Regards,
Bo Shen