Re: [PATCH v2] sound/soc/adi/axi-spdif.c: Support programmable master clock

[Mike Looijmans <mike.looijmans@topic.nl>]

On 12/10/2014 10:34 AM, Lars-Peter Clausen wrote:
> On 12/05/2014 01:37 PM, Mike Looijmans wrote:
>> If the master clock supports programmable rates, program it to generate
>> the desired frequency. Only apply constraints when the clock is fixed.
>> This allows proper clock generation for both 44100 and 48000 Hz based
>> sampling rates if the platform supports it.
>>
>> The clock frequency must be set before enabling it. Enabling the clock
>> was done in "startup", but that occurs before "hw_params" where the rate
>> is known. Enabling a programmable clock without first setting a valid
>> frequency may harm the system. Move the clock start to the hw_params
>> routine, and keep track of whether the clock has been started, because
>> shutdown may be called without having called hw_params first.
>> Starting the clock and enabling the SPDIF output AFTER programming the
>> dividers is a more logical order anyway.
>>
>> To detect if the source clock is fixed, the driver calls clk_round_rate
>> for two frequencies. If the results are equal, or if the call returns
>> an error, the driver assumes the clock is fixed.
>>
>> Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
>
> Hi,
>
> Sorry for the delay.
>
> [...]
>>
>> +    /* Try to set the master clock */
>> +    clk_set_rate(spdif->clk_ref, rate * 128);
>> +
>>       clkdiv = DIV_ROUND_CLOSEST(clk_get_rate(spdif->clk_ref),
>>               rate * 64 * 2) - 1;
>>       clkdiv <<= AXI_SPDIF_CTRL_CLKDIV_OFFSET;
>> @@ -103,6 +108,14 @@ static int axi_spdif_hw_params(struct snd_pcm_substream
>> *substream,
>>       regmap_update_bits(spdif->regmap, AXI_SPDIF_REG_CTRL,
>>           AXI_SPDIF_CTRL_CLKDIV_MASK, clkdiv);
>>
>> +    ret = clk_prepare_enable(spdif->clk_ref);
>
> I'm still not convinced this is the right place. I do see your point. But it
> just feels wrong to enable the clock in hw_params. It's a bit of a dilemma.
> the startup callback is to early, hw_params is the wrong place and we can't
> put it in the trigger callback as the trigger callback can not sleep.

The clock interface suggests that we should do the clk_prepare in hw_params 
and the clk_enable in trigger then, since clk_prepare may sleep but clk_enable 
cannot. Which would complicate the clock state housekeeping because I'd have 
to keep track of prepare and enable states separately.

(Imagine the housekeeping on a board that could have two codecs running on one 
DAI using the clock generated by a third codec... I never even bothered to 
submit that...)

> But in any way hwparmas can be called multiple times, so you need to handle
> the case where the clock is already enabled.

A simple "if (spdif->clk_ref_running)" check would fix that. I wasn't aware of 
that though, I thought there'd be a shutdown before another hw_params.

>> +    if (ret)
>> +        return ret;
>> +    spdif->clk_ref_running = true;
>> +
>> +    regmap_update_bits(spdif->regmap, AXI_SPDIF_REG_CTRL,
>> +        AXI_SPDIF_CTRL_TXEN, AXI_SPDIF_CTRL_TXEN);
>
> This should probably be moved to the trigger callback though.


I'll create a v3.




Met vriendelijke groet / kind regards,

Mike Looijmans
System Expert


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