Re: [PATCH v3 1/4] ASoC: es8316: judge PCM rate at later timing

[Katsuhiro Suzuki <katsuhiro@katsuster.net>]

Hello Mark,

On 2019/09/04 2:48, Mark Brown wrote:
> On Wed, Sep 04, 2019 at 01:53:19AM +0900, Katsuhiro Suzuki wrote:
> 
>> Root cause of this strange behavior is changing constraints list at
>> set_sysclk timing. It seems that is too early to determine. So this
>> patch does not use constraints list and check PCM rate limit more
>> later timing at hw_params.
> 
> hw_params is a bit late to impose constraints, you want them to be
> available to be available to the application before it gets as far as
> picking the parameters it wants so that you don't get hw_params failing
> due to an invalid configuration.  That makes everything run more
> smoothly, applications should be able to trust the constraints they got
> and some will not handle failures well.
> 
> The way this works with the variable MCLKs is that you end up in one of
> two cases (wm8731 and wm8741 do this):
> 
>     1. The system is idle, MCLK is set to 0.  In this case no constraints
>        are set and we just set MCLK to whatever is required in hw_params()
>        in the machine driver.
>     2. One direction is active, MCLK is set to whatever that needed.  In
>        this case startup() sets constraints derived from the MCLK.
> 
> There are races in this if streams are being started and torn down
> simultaneously, there's not much we can do about them with the API the
> way it is so we do have to validate in hw_params() anyway but it should
> be validation not constraint imposition.
> 
> If the system has a fixed MCLK it just sets that on probe then we always
> get the constraints applied on startup through the same code that
> handles case 2.
> 

Thank you for explanation. I agree with apply no constraints if MCLK is
set to 0, and suitable constraints if MCLK is set to other values like
as wm8731 and wm8741 drivers.

I'll change my patch set and send.


Would you tell me one more thing. I don't understand who sets MCLK to 0.
Is it needed original machine driver instead of audio-graph-card?

On my test environment (audio-graph-card + Rockchip I2S + ES8316), it
seems audio-graph-card has never called set_sysclk() with freq = 0 after
stop play/capture sound. So my env will go to bad scenario as I 
described in this patch.

Best Regards,
Katsuhiro Suzuki