linux-kernel - Re: [PATCH v2 2/2] ASoC: fsl_asrc: always use internal ratio

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Date:   Mon, 20 Jul 2020 18:20:44 +0800
From:   Shengjiu Wang <shengjiu.wang@...il.com>
To:     Arnaud Ferraris <arnaud.ferraris@...labora.com>
Cc:     Nicolin Chen <nicoleotsuka@...il.com>,
        Linux-ALSA <alsa-devel@...a-project.org>,
        Timur Tabi <timur@...nel.org>, Xiubo Li <Xiubo.Lee@...il.com>,
        Fabio Estevam <festevam@...il.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>, kernel@...labora.com
Subject: Re: [PATCH v2 2/2] ASoC: fsl_asrc: always use internal ratio

On Fri, Jul 17, 2020 at 5:58 PM Arnaud Ferraris
<arnaud.ferraris@...labora.com> wrote:
>
>
>
> Le 17/07/2020 à 01:37, Nicolin Chen a écrit :
> >> @@ -507,8 +507,7 @@ static int fsl_asrc_config_pair(struct fsl_asrc_pair *pair, bool use_ideal_rate)
> >>
> >>      /* Enable Ideal Ratio mode */
> >
> > The code is against the comments now -- need to update this line.
>
> It isn't, the following code still enables "Ideal Ratio" mode (see below)
>
> >>      regmap_update_bits(asrc->regmap, REG_ASRCTR,
> >> -                       ASRCTR_IDRi_MASK(index) | ASRCTR_USRi_MASK(index),
> >> -                       ASRCTR_IDR(index) | ASRCTR_USR(index));
> >> +                       ASRCTR_IDRi_MASK(index), ASRCTR_IDR(index);
> >
> > The driver falls back to ideal ratio mode if there is no matched
> > clock source. Your change seems to apply internal ratio mode any
> > way? Probably would break the fallback routine.
>
> Strictly speaking, internal ratio is only enabled when we have matched
> clock sources, and is used in addition to the calculated dividers
> (allows the ASRC to better adjust to drifting/inaccurate physical
> clocks). "Ideal Ratio" mode is different, and still enabled as a
> fallback when no clock source is matched.
>
> Ideal ratio requires both USRi and IDRi bits to be set, and that would
> still be the case if there is no matched clock source.
>
> The only difference my patch introduces is that USRi is always set (was
> previously cleared for "normal" mode), and therefore only IDRi needs to
> be set in order to enable ideal ratio mode.
>

In my experience, the USRi = 0, no matter the value of IDRi, it is
internal ratio mode.  USRi=1, IDRi=0, it is also internal ratio mode.
So original code should be ok for internal ratio mode,  no need
to add this change.
could you please double check it?

best regards
wang shengjiu