Re: [PATCH v2 1/3] ASoC: es8316: judge PCM rate at later timing

From: Katsuhiro Suzuki
Date: Mon Sep 02 2019 - 15:19:35 EST

Hello Mark,

Thanks a lot for your comments.

On 2019/09/02 21:02, Mark Brown wrote:
On Sun, Sep 01, 2019 at 01:26:48AM +0900, Katsuhiro Suzuki wrote:
This patch change the judge timing about playing/capturing PCM rate.

Original code set constraints list of PCM rate limits at set_sysclk.
This strategy works well if system is using fixed rate clock.

But some boards and SoC (such as RockPro64 and RockChip I2S) has
connected SoC MCLK out to ES8316 MCLK in. In this case, SoC side I2S
will choose suitable frequency of MCLK such as fs * mclk-fs when
user starts playing or capturing.

The best way to handle this is to try to support both fixed and variable
clock rates, some other drivers do this by setting constraints based on
MCLK only if the MCLK has been set to a non-zero value. They have the
machine drivers reset the clock rate to 0 when it goes idle so that no
constraints are applied then. This means that if the machine has a
fixed clock there will be constraints, and that constraints get applied
if one direction has started and fixed the clock, but still allows the
clock to be varied where possible.

In my understanding, fixed and variable clock both use set_sysclk() for telling their MCLK to codec driver. For fixed MCLK cases we need to
apply constraint but for variable MCLK cases we should not set
constraints at set_sysclk(). How can we identify these two cases...?

For example, if machine sets very low MCLK once, the driver applies low
Fs constraints which I2S driver cannot support to. After that this sound
card cannot play/capture any Fs rate. It seems set_sysclk() is not
called if Fs does not match constraints. So we have no chance to
reconfigure MCLK by set_sysclk().

Best Regards,
Katsuhiro Suzuki