[PATCH 18/25] ASoC: sun8i-codec: Automatically set the system sample rate

[Maxime Ripard]

On Wed, Sep 30, 2020 at 09:11:41PM -0500, Samuel Holland wrote:
> The sun8i codec has three clock/sample rate domains:
>  - The AIF1 domain, with a sample rate equal to AIF1 LRCK
>  - The AIF2 domain, with a sample rate equal to AIF2 LRCK
>  - The SYSCLK domain, containing the ADC, DAC, and effects (AGC/DRC),
>    with a sample rate given by a divisor from SYSCLK. The divisor is
>    controlled by the AIF1_FS or AIF2_FS field in SYS_SR_CTRL, depending
>    on if SYSCLK's source is AIF1CLK or AIF2CLK, respectively. The exact
>    sample rate depends on if SYSCLK is running at 22.6 MHz or 24.6 MHz.
> 
> When an AIF (currently only AIF1) is active, the ADC and DAC should run
> at that sample rate to avoid artifacting. Sample rate conversion is only
> available when multiple AIFs are active and are routed to each other;
> this means the sample rate conversion hardware usually cannot be used.
> 
> Only attach the event hook to the channel 0 AIF widgets, since we only
> need one event when a DAI stream starts or stops. Channel 0 is always
> brought up with a DAI stream, regardless of the number of channels in
> the stream.
> 
> The ADC and DAC (along with their effects blocks) can be used even if
> no AIFs are in use. In that case, we should select an appropriate sample
> rate divisor, instead of keeping the last-used AIF sample rate.
> 44.1/48 kHz was chosen to balance audio quality and power consumption.
> 
> Since the sample rate is tied to active AIF paths, disabling pmdown_time
> allows switching to the optimal sample rate immediately, instead of
> after a 5 second delay.
> 
> Signed-off-by: Samuel Holland <samuel at sholland.org>

Acked-by: Maxime Ripard <mripard at kernel.org>

Maxime
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