linux-kernel - Re: [PATCH] SoC: simple-card-utils: set 0Hz to sysclk when shutdown

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Date:   Mon, 9 Sep 2019 11:49:27 +0900
From:   Katsuhiro Suzuki <katsuhiro@...suster.net>
To:     Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>
Cc:     Mark Brown <broonie@...nel.org>, alsa-devel@...a-project.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] SoC: simple-card-utils: set 0Hz to sysclk when shutdown

On 2019/09/09 9:31, Kuninori Morimoto wrote:
> 
> Hi Katsuhiro
> 
>> This patch set 0Hz to sysclk when shutdown the card.
>>
>> Some codecs set rate constraints that derives from sysclk. This
>> mechanism works correctly if machine drivers give fixed frequency.
>>
>> But simple-audio and audio-graph card set variable clock rate if
>> 'mclk-fs' property exists. In this case, rate constraints will go
>> bad scenario. For example a codec accepts three limited rates
>> (mclk / 256, mclk / 384, mclk / 512).
>>
>> Bad scenario as follows (mclk-fs = 256):
>>     - Initialize sysclk by correct value (Ex. 12.288MHz)
>>       - Codec set constraints of PCM rate by sysclk
>>         48kHz (1/256), 32kHz (1/384), 24kHz (1/512)
>>     - Play 48kHz sound, it's acceptable
>>     - Sysclk is not changed
>>
>>     - Play 32kHz sound, it's acceptable
>>     - Set sysclk to 8.192MHz (= fs * mclk-fs = 32k * 256)
>>       - Codec set constraints of PCM rate by sysclk
>>         32kHz (1/256), 21.33kHz (1/384), 16kHz (1/512)
>>
>>     - Play 48kHz again, but it's NOT acceptable because constraints
>>       do not allow 48kHz
>>
>> So codecs treat 0Hz sysclk as signal of applying no constraints to
>> avoid this problem.
>>
>> Signed-off-by: Katsuhiro Suzuki <katsuhiro@...suster.net>
>> ---
> 
> I'm not 100% understand your issue.
> .hw_params (= set mclk/sysclk) is not called in bad case ??
> Or it is called but Codec driver ignores it somehow ??
> 

Ah, sorry for confusing. It's not either. hw_params() of machine
driver has been called even if constraints don't have a requested
PCM rate. But it's not expected.

For example, if constraints are 32k, 21.33k, 16k, hw_params() will
be called with 32k when an user requests to play 48k sounds.


> Thank you for your help !!
> Best regards
> ---
> Kuninori Morimoto
> 

Best Regards,
Katsuhiro Suzuki