Re: [alsa-devel] ASoC: sun4i-i2s: Restricting the MCLK

[Chen-Yu Tsai <wens-jdAy2FN1RRM@public.gmane.org>]

Hi,

On Tue, Aug 2, 2016 at 11:05 PM, Thomas Niederprüm
<niederp-PRwTpj6vllL6m1q2wEL4bg@public.gmane.org> wrote:
> I am using the new sun4i-i2s driver in linux-next to run a sta326 codec
> via the pin header on a cubietruck (this involves some soldering as
> described in [1]). The codec can only be configured as slave and needs
> a master clock for its internal PLL. It is therefore great that, unlike
> many other SOCs, the a20 provides a MCLK that can be used to sync the
> codec. However, in my case there is a restriction imposed on the MCLK
> by the codec. As stated in the datasheet [2], the master clock should
> be at maximum 12.288 MHz/11.2896 MHz or otherwise the codec will not
> work (confirmed by experiment!).
>
> Since the mod clock of the a20's dai block is running at twice this
> frequency, I need to assure that the MLCK divider is at least 2.
> Actually it is rather straight forward to achieve this by changing the
> breaking condition in the divider calculation, as shown by the
> following one-line patch:
>
> diff --git a/sound/soc/sunxi/sun4i-i2s.c b/sound/soc/sunxi/sun4i-i2s.c
> index 687a8f8..8920844 100644
> --- a/sound/soc/sunxi/sun4i-i2s.c
> +++ b/sound/soc/sunxi/sun4i-i2s.c
> @@ -206,7 +206,7 @@ static int sun4i_i2s_set_clk_rate(struct sun4i_i2s
> *i2s,
>                                                   clk_rate,
>                                                   rate);
>
> -               if ((bclk_div >= 0) && (mclk_div >= 0))
> +               if ((bclk_div >= 0) && (mclk_div >= 1))
>                         break;
>         }
>
> Of course, this is just a hack to fix my use case and I am wondering
> what would be the best way to handle my corner case more generally? I
> see one option in introducing a DT property mlck_mindiv, or similar,
> that defaults to 1 and can be overwritten at the board level. I'm
> willing to prepare a proper patch to tackle this problem but right now
> I'm wondering if there a major objections to this approach or whether
> there is any better solution to the problem.

I think the proper way would be to have sun4i-i2s support the .set_sysclk
callback in .dai_ops. simple-card calls snd_soc_dai_set_sysclk if you
have the "mclk-fs" property set in the DT.

Regards
ChenYu

>
> Best regards,
>
> Thomas
>
> [1] https://hifiduino.wordpress.com/2014/03/07/cubieboard-for-audio/
> [2] www.st.com/resource/en/datasheet/sta326.pdf
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