linux-kernel - Re: [PATCH] sound: soc: fsl: Do not set DAI sysclk when it is equal to system freq

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Date:   Tue, 5 Sep 2017 00:52:48 -0700
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     Łukasz Majewski <lukma@...x.de>
Cc:     Timur Tabi <timur@...i.org>, Xiubo Li <Xiubo.Lee@...il.com>,
        Fabio Estevam <fabio.estevam@....com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>, alsa-devel@...a-project.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sound: soc: fsl: Do not set DAI sysclk when it is equal
 to system freq

On Tue, Sep 05, 2017 at 09:37:43AM +0200, Łukasz Majewski wrote:

> >>The last call is changing the bit clock (BCLK) frequency to SSI's IP
> >>block clock (ipg = 66 MHz) [1].
> >
> >I think a bigger question here is why the routine sets BCLK to 66MHz.
> 
> Yes, exactly.
> 
> In my case the bclk is set to ipg clock, which is the SSI IP block clock
> (ipg).

Can you elaborate why you set ipg clock as bclk? I don't remember SSI could
derive bitclock from ipg clock.

> >>This is wrong, since IMX SSI block requires the I2S BCLK to be less
> >>than 1/5 of [1].
> >>
> >>As a result the driver initialization passes without any errors, but the
> >>speaker-test test case breaks.
> >>
> >>This commit checks if the fsl_ssi_set_dai_sysclk() frequency passed is
> >>not equal to [1].
> >
> >I don't feel it's quite comprehensive...what if it's being set to 67MHz.
> 
> I think that this clock is not changing for the SoC. It should be 66 MHz
> fixed.

What I mean is that we cannot just look at this SoC. Today is 66MHz for this
SoC. Tomorrow could be 133MHz for another one. We should put a check that none
of these shall pass -- the 1/5 limit.