linux-kernel - Re: [PATCH 2/6] ASoC: samsung: i2s: configure PSR from sound card

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Message-ID: <Y0lPz91gbovAub9D@sirena.org.uk>
Date:   Fri, 14 Oct 2022 13:02:23 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Padmanabhan Rajanbabu <p.rajanbabu@...sung.com>
Cc:     lgirdwood@...il.com, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, s.nawrocki@...sung.com,
        perex@...ex.cz, tiwai@...e.com, pankaj.dubey@...sung.com,
        alim.akhtar@...sung.com, rcsekar@...sung.com,
        aswani.reddy@...sung.com, alsa-devel@...a-project.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-samsung-soc@...r.kernel.org
Subject: Re: [PATCH 2/6] ASoC: samsung: i2s: configure PSR from sound card

On Fri, Oct 14, 2022 at 03:51:47PM +0530, Padmanabhan Rajanbabu wrote:
> Currently the prescaler value in samsung I2S dai is calculated by
> dividing the peripheral input clock frequency with frame clock
> frequency and root clock frequency divider. This prescaler value is
> used to divide the input clock to generate root clock (RCLK) from which
> frame clock is generated for I2S communication.
> 
> However for the platforms which does not have a dedicated audio PLL as
> an input clock source, the prescaler divider will not generate accurate
> root clock frequency, which inturn affects sampling frequency also.
> 
> To overcome this scenario, support has been added to let the sound card
> identify right prescaler divider value and configure the prescaler (PSR)
> divider directly the from the sound card to achieve near accurate sample
> frequencies

It's not clear to me why the solution here is to move the configuration
to the sound card rather than to improve the I2S driver to be able to
cope with whatever the restrictions are on the PSR in these systems - it
seems more cumbersome for system integrators, especially since you've
not documented the issues or how to configure it.  Could you expand on
what the constraints are here and why it's not possible for the driver
to figure things out (given some quirk information)?

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